How Many Things Can You Put On One Chimney? Or Multiple Appliance Venting

Let’s start with a few definitions.

chimney flue ventingAppliances include fireplaces, woodstoves, furnaces, boilers, pellet stoves, hot water heaters, etc. They’re all individual appliances.

A chimney is a structure that has one or more flues in it.

A flue is simply the chimney passageway that vents the fumes from whatever is attached to it. (A flue is not the same as a damper either; a damper is something that can block the flue.)is simply the chimney passageway that vents the fumes from whatever is attached to it. (A flue is not the same as a damper either; a damper is something that can block the flue.)

How many appliances can you have per chimney?

And even though the question always comes across as “how many on one chimney?” let’s make sure to discuss “how many on one flue?” The answer to the question is: “It Depends”.

The rules are found in various NFPA standards and in the IRC (International Residential Code.) This article is general in nature but for those who want to drill down into the details, most of the information can be found in IRC chapters 10, 13, 18 and 24.

Solid fuel burning appliances.

Solid fuel includes coal or corn or cherry pits, but for most of us that means cord wood or pellets. The rule here is easy and clear.

IRC M1801.12 (PDF Here) Multiple solid fuel prohibited. A solid-fuel-burning appliance or fireplace shall not connect to a chimney passageway venting another appliance.

In other words, only one appliance per flue, period. It goes without saying, I hope, that gas or oil appliances cannot be vented into a flue which also vents a solid fuel appliance. EVERY SOLID FUEL APPLANCE GETS ITS OWN VENT!

How about hooking up a woodstove into an existing masonry fireplace flue?

That’s OK as long as:

  • The fireplace has been blocked off. Remember, only one appliance per flue!
  • The liner for the woodstove has to be properly sized, which generally means the same size as the collar-size coming from the appliance.
  • Make sure the chimney is clear of combustible materials before inserting the smaller liner.

Gas and Oil Appliance Venting

Gas fireplaces are factory-built systems. The manufacturer’s listing and instructions will preclude attaching any other appliances to it.

Multiple gas or oil furnaces or boilers, as well as hot water heaters, can be vented into one flue. There are a few rules to mention:

  • The rules apply to listed appliances. While I have never seen an unlisted gas or oil furnace in my life, if you have one, you are referred back to the rules for solid fuel burning appliances- one per flue.
  • If venting two or more appliances on the same flue, you have to know the flue can handle it, as determined but the BTU input and other factors.
  • Both or all appliances have to be on the same floor. So, no furnaces in the basement or room heaters on the second level of your home.
  • The connectors for the appliances have to be offset. They can’t come into the flue at the same height, and especially never directly across from each other.
  • The smaller of the two connectors go into the flue above the larger one (usually meaning the hot water heater).
  • As a general rule, don’t mix “natural draft” appliances and “fan assisted” appliances on the same flue. This rule is more complicated than this, but if this is your case, be sure you refer to the manufacturer’s instructions. Call an HVAC company and make them show you to your satisfaction it’s right. Don’t take anyone’s word for it, see it in writing.

The NFPA 54 (Gas) and the NFPA 31 (Oil) show diagrams in great detail, and cover sizing the connectors as well (connectors are the smoke pipes that carry the fumes from the appliance to the chimney flue.)

Read another helpful article by the American Society of Home Inspectors.

85 thoughts on "How Many Things Can You Put On One Chimney? Or Multiple Appliance Venting"

C. August says:

Great resource! Thanks!

I have a home from 1885 with a very large, unlined chimney (at least 18″ square) that currently serves my oil furnace and gas water heater. I would like to put in a wood stove and was hoping to use some of the space in this oversized chimney for a flexi-steel liner. I have seen conflicting statements about whether I could just add one liner for the stove only, leaving the other two appliances as they are, or whether I need to get a separate liner for each one.

Obviously, I’d like to pay for only one liner, but I don’t want to do it if it’s unsafe. I can’t, however, figure out why I would need three liners.

Any thoughts?

Paul Bancroft says:

It is a code violation to exhaust multiple heat sources through a single flue. (ie; gas and oil). If an accident occurred and your insurance company was made aware of your prior configuration your claim would be denied. Also it is a code violation to exhaust multiple heat sources from different floors (ie: basement furnace & bedroom insert). Hope this helps

Chris Bell says:

I have a gas furnace space heater (Warm Morning Gas Heater). It vents out the back with a metal vent pipe which connects to a masonry chimney. I want to add another gas furnace to a second story addition to the same chimney. Is this possible? On my first floor there are two openings in the chimney like I could attach two appliances. One of those openings are sealed. So, it seems like the flue was probably built to accommodate more than one heater. Also, I’m thinking since the metal vent pipe has two 90 degree bends before it connects to chimney – I’m thinking a gas furnace on the second floor with the same double-90degree vent pipe would prevent exhaust from entering from the first story. Any thoughts???

Les pohle says:

Can the top of a crumbling chimney be taken off roofed over and a liner used in the rest of the chimney for a water heater and 80% gas furnace. I would add a cap on the roof and extension to proper height

Dale Howard says:


Any fuel burning appliance must be vented to the outdoors. Your chimney top may not be roofed over if you intend to vent the furnace. You need to either repair the top masonry or you can remove bricks to a point below the roof-line and continue up with factory built chimney.

If you just want to be all-done with the chimney, you can take the top down and roof over the hole, but then will need to replace the appliances below with ones vented through the wall. If your furnace is in reasonable condition it’s probably best to put in a stainless steel liner and then deal with the chimney top as you like best (I’d choose to rebuild the bricks personally; I like that appearance best, though it probably cost a bit less to finish with factory built chimney) .

John Deschaine says:

Could one flue be utilized to vent an oil fired hot water tank, oil fired hot air furnace and a pellet stove if:

1. the pellet stove is the top pipe
2. the hot water tank is the second pipe
3. the oil furnance is the lowest
4. all pipes are offset on the chimney
5. all three pipes have motorized dampers (spring closed)
6. If one appliance is calling – it will open its damper first, when fully open it will activate a switch and allow the appliance to start. while inhibiting the other two appliances from being able to start.
7. When this appliance is done its thing – it will remove the inhibit contacts from the other two appliance circuits thus allowing either to start — when any one appliance starts it also inhibits the other two from starting until it is done its task.
8. Therefore, time sharing the one chimney flue via mechanical and electrical interlocks thus preventing any cross drafts between the appliances with the normally closed dampers.



Dale Howard says:

Sorry we’re just getting back to you now John – your comment slipped past the radar.
The answer is “maybe.” The quick answer is, and I’d bet it probably will be, “no” because of the dissimilar fuel types. But because you intend to hard wire the systems to not overload the vent AND because pellet stoves have a high efficiency the answer is maybe.

I’ll say this. The NFPA recommendations on which codes are built were made assuming we were talking about cord-wood appliances (wood stoves or fireplace inserts) but the pellet stove is a different heater. It is true that it puts out higher volume of flue wall deposits, but IMO if wired as you describe it is probably a safe installation. As to whether you can get an inspector to pass it is quite a different question.

My suggestion would be to submit your permit application ahead of time, have copies of the code references that you know will raise questions- especially the part about dissimilar fuels. Then have some documentation on the efficiency of modern pellet stoves and, if you can get it, something authoritative that addressed how the codes were written when pellet stoves were not a consideration i.e. they had cord-wood burners in mind when writing. Not sure what that might be but perhaps something from the appliance manufacturer.

BTW, as long as you have the capacity you can run the oil HW heater and the gas furnace in the same flue. It’s only a problem when the second fuel is wood. Hope this helps some.

Joe McGuinness says:

I own a 2 family house in MA with two stories and an unfinished attic and basement. There is at least 30″ from the basement to the top of the chimney. There are currently two 105,000 BTU gas steam boilers and two stand alone 40 gallon gas water heaters in the basement (one for each unit). These appliances are vented thru two 8″ x 8″ clay tile flues in a chimney, one for each boiler/WH combo. There are no fireplaces or other heat sources. I had the chimney inspected by three different chimney sweeps and they all agreed the clay flues are badly deteriorated and should be relined with SS liners. Two of them said this could only be done by installing two SS liners due to the number of appliances, but the third one said they all could be vented with one 7″ SS liner. Obviously, one liner is cheaper than two, but I was concerned about the safety of venting a total of about 290,000 BTU’s thru one 7″ liner. Do I really need to be worried?

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mike says:

can a kitchen hood vent tie into a boiler vent?

Vance Simons says:

I want to add a natural gas heater in my garage. I already have a natural gas hot water heater in the same corner I want to add the heater so there is a gas line and a outlet in that corner. Can I run the vent for the heater upline from the hot water heater. I don’t believe either the hot water heater or the heater have fan assisted venting. The heater does have a fan to move air through the coils. Can I do this safely?

Thank you

Dale Howard says:

Hi Vance,

If I understand the question properly you are asking if you can vent the new heater into the same vent and would like to do that with the heater connector closer to the wall than the hot water heater. You can vent both gas appliances into the same vent if the BTU capacity is OK; I assume it is but you don’t know for sure until you check the charts. As to the order or the connectors, probably not, but it depends on connector sizes.

If this is a small heater maybe so; assuming it’s not bigger than the hot water heater you’re OK. The rule is to have the big connector primary, with the small connector joined into it.

If however the heater is a whole house heater (central heat) the BTU input is undoubtedly higher than the hot water heater so that connector has to be the primary one.

As to the rest, the gas lines etc. that’s HVAC work and I’m not a gas-guy. This is something for which to bring in professionals who actually know their stuff. Gas hook-ups are “no foolin’ around propositions” and not good candidates for do it yourself projects. For example, whether you should have those heaters in a garage in the first place is questionable to me: if it were wood the answer would be no. Just a good time to bring in an HVAC professional I think.

Good luck and stay warm!

Judy Doyle says:

We bought a ranch built in 1962 with a fireplace in the living room. Below, in the basement, is a wood stove which vents into the flue of the oil-fired furnace. We are told this is illegal. Can the woodstove remain in the basement and be vented instead into the fireplace flue?
Thank you for your advice.

Dale Howard says:


You have been advised correctly; wood and oil cannot be vented into the same flue.

And yes, a stove can be vented through a fireplace, with these caveats.

There should be a liner directly to the top of the stove and is to be the same size as the flue outlet. In other words, don’t dump the exhaust of a 6″ woodstove into a big fireplace flue.
Another thing to know is that only one appliance is vented per flue. That is to say, after you put that 6″ liner into the fireplace flue you won’t use the fireplace anymore, only the woodstove.
Since the stove is in the basement and the fireplace is on the first floor, that means you’ll be coming through the floor of the fireplace, which ruins the fireplace. Be sure to close off the front of the fireplace safely (no official way to do that; bricking it up is common though) and be sure you aren’t running your liner through any combustible materials- sometimes there’s plywood on the bottom of where the fireplace was built. Not supposed to be, but we see it all the time anyway.

So yes, if you observe these notes you’ll be OK. Sounds like you might be working with a competent professional already; show him these notes and he can probably propose a way to do the job that will work for you.

Stacy says:

I have a house that was built in the 60’s. My chimney has 3 flues. There was a woodburning stove in the main fireplace, an oil furnance and the fireplace in the basement was sealed up with cinderblock. I am having a lp gas log set put in the main fireplace. The old oil furnace was removed from the house and has not been functional in 5 years. Can I use the flue from the old furnace for the wood burning stove?


Dale Howard says:

From what you describe, I don’t see why not. All the usual caveats apply of course. Maintain your clearances to combustibles in placement of the woodstove, and make sure you have a properly sized liner to the stove. That probably means installing a chimney liner inside an existing tile liner. But yes, go for it.

BTW, if your woodstove was “a slammer” i.e. just slammed into a fireplace without a proper liner attached, you may have a creosote nightmare up inside the smoke chamber and flue. While your gas logs won’t catch it on fire (because of the low temperatures involved,) the condensation may help create a drippy mess, not to mention possible creosote order. Be sure everything is nice and clean up inside the fireplace flue.

Stay warm! Enjoy the flame!

Sarah says:

I hope you can help me. I have a chimney which on the front side has a wood stove insert. Previously it had been a fireplace but was very inefficient. I’m assuming the piping that was put in for the wood stove is correct for that stove as I know the installer did charge for new piping. On the opposite side of that fireplace, I have a gas cook top. That gas cooktop is currently not vented. Can I run additional piping and install a direct vent hood utilizing the same chimney?

Dale Howard says:

Hi Sara,

Maybe. Each appliance needs to be on its liner so it depends on whether both liners will fit inside the masonry chase. And there needs to be room for insulation as well of course.

If you find that two liners and insulation won’t fit inside the masonry structure, here’s a suggestion for you. Apparently you have an interior chimney, which is in your favor. You can vent the gas appliance through B Vent. Although you’d still have to install a pipe running through your house, at least B Vent is the least expensive way to do that.

Hope it’ll all fit in the existing masonry structure though; that would be best of all. Good luck.

Sarah says:

Thank you so very much. I just want to be knowledgeable enough to oversee what the installer is doing as I’ve had conflicting reports on this one and was getting very nervous about safety. I can’t thank you enough for your input.

Kent says:

I have an old victorian house (3 floors) with an 8″ chimney. When I bought the house it had a 160k btu Natural Gas furnace for the whole house. I’m now changing it to baseboard heating and I was wondering if I could put 2 Natural gas boilers (each 70k btu) into the 1 chimney (of course the flues offset from each other)?? It would be very difficult for me to add a 2nd chimney due to how the foundation is constructed and space constraints. Thank you for your advice!

Kent says:

I don’t know if you need additional information but the chimney height is approx. 30′ and the 2 boilers would be only 2′ away from the chimney. The chimney is a masonary type.

Dale Howard says:

Yes, two appliances of the same fuel type can be on one flue. From what you have written it sounds as if you probably do have the BTU capacity in your flue, but taking advice for sizing from “some guy on the internet” is no substitute for having the HVAC company give you the thumbs up on this. As a general statement, sounds like you will be OK, but keep in mind: wood flues that don’t work smoke up the house; oil flues that don’t work stink up the house; gas flues that don’t work just silently kill the whole family. Get the HVAC guys to make sure; you never know what variables there are and the mechanics on the job will actually see all that.

Kent says:

Thanks for your advice! I do have CO detectors all around the house including near the boilers.
I will go ask an HVAC person. I just want to be informed so that when I speak to them I don’t want to be taken for a ride.

Donna Strand says:

We have a 3 story home. It has the original chimney in it. We currently have a wood burning stove in the basement that we have hooked up to the chimney. Can we install a wood burning stove on the main floor and use the same chimney? Our furnace is vented out the side of our house, so not connected to chimney. We love the heat from the wood burner and it saves so much on our heat bill. We also are cleaning the dead wood out of our shelter belt. Any help would be appreciated.

Meg Lamont says:

Greetings, we are in the process of retiring our oil spaceheater stove in our garage. Can we install a pellet stove and use the same chimney we’ve been using for the oil spaceheater?

Many thanks,


Dale Howard says:


The fast answer is yes.

The trick is to make sure the chimney is properly sized for the appliance. When using a chimney for a pellet appliance, that usually means a 4″ vent (even though almost all the pellet stoves are vented 3″ on the appliance themselves.) So said, if it’s a short chimney, 3″ may still be appropriate. So yes, plan to use the existing chimney but with the sized liner recommended by the hearth appliance’s venting instructions. The people at the stove store should be able to help you with the specifics.

A side note: I think it would be smart to have a sweep make sure your chimney is “dry” i.e. that you don’t have oil residue in the flue. If oil is present, I would suggest removing the tiles before putting in the new liners (a messy job you leave to the pros.) Check it out at least.

Good luck, and stay warm!

Jesse says:

Hi, I currently have a wood burning insert in my fireplace and am not sure but believe that it is a “slammer”. My place is an old school house converted into condos in the 80’s. I was told that these were popular at the time. Would I be able to remove this unit and use the original fireplace again? Thanks a bunch!!

Dale Howard says:


Fast answer is “probably” as long as it’s a masonry fireplace. Not being there and seeing up the chimney, condition etc. you don’t want some guy on the internet telling you it’s safe, but assuming nothing out of the ordinary I think you can reasonably expect to use a masonry fireplace for its original intended use. Some caveats might be: if it’s actually a factory-built fireplace (converted to condos makes me wondering if it is) there should never have been a wood stove there in the first place. Or if the fireplace was damaged in order to install the stove, that might change things as well. These are things you want a professional to assess for you; get a Certified Chimney Sweep out there to check it out for you.

Good luck and stay warm!

Jesse says:

Thanks for the useful info!! One last question, if there is a liner that goes from the top of the chimney to the insert, do I have to remove this liner and use the original brick or clay liner?? Basically, would I be able to use that same liner from the insert??

Dale Howard says:


Well, “slammer” just means it was slammed into the fireplace without a liner- a popular installation in the 1980s but never a good one. If however there’s a liner attached to the stove, that’s a good thing- but it’ll be sized for the stove, not the fireplace. If your fireplace chimney is in good condition you may well be able to use it with its original clay tiles. That’s a big IF of course, so be sure to have a proper inspection on your chimney. As you call various sweeps, ask them if they video scan chimneys. Lots of sweeps do, and this is a service you want before you make a change. They can look at the walls and joints of the tiles and determine if things are fit for continued use.

And if you had a liner on your stove all along (in which case it wasn’t really a slammer after all) there’s a fair chance things will be fine. You just want to know for sure before you put smoke into it. Don’t want to learn differently the hard way!

If you learn that for some reason the tile fireplace liner isn’t fit for use, you’ll want to have the flue relined with a liner sized for open fireplace use. Get a video scan inspection and know for a certainty.

Good luck, stay warm!

Jesse says:

Thank you soo much, keep up the great work, hopefully I discover that the clay liner is in great shape!!

Jesse says:

I have a three family home, each unit has a fireplace. I am adding a rooftop deck and in order to do so, I need to raise the height of the chimney. I am going to install chimney pots from superior clay that will raise the flue’s to the proper height requirements. They are very expensive so I want to know if I can get away with using only two pots. We have closed our second unit fireplace completely and it now resides behind the drywall. I’d like to know if I can close off that flue from the chimney cap and only place the new chimney pots on the two functional flue’s. I hope that makes sense.
Any thoughts and recommendations are greatly appreciated,
Thanks again!

Dale Howard says:


The fast answer is yes. Two appliances on two separate flues- nothing wrong there at all.

Something to check into. Fact is I don’t know a lot about chimney pots. They look cool of course, but that’s mostly all I know. However, I think I understand that the codes say don’t use them. You might want to check into that. I can imagine a reason for not using them: if the flow of the exhaust is impeded the temperature of the structure below gets hotter than it would otherwise.

I know clay pots are very popular nowadays; I’m just not sure they’re legal. I might be wrong, and I can’t cite a code reference for you, but check into it because I think I might be right.

But the direct answer to your question- yes. Good luck and stay warm!

Holly says:

My husband and I are wanting to install a wood stove on our main floor of our home and tap into a chimney that is currently being used to vent our gas water heater from our basement. We are going to install a pre-insulated chimney liner from the stove up. Am I correct in thinking that we will have to vent our water heater some other way once we install the wood stove or is it considered a separate venting because the wood stove will vent completely out the liner rather than the chimney? Is there even a way to do this considering the chimney cap for the liner will cover the rest of the chimney?Then, does the rule about appliances on separate floors come into play? Lastly, is this specific to where you live or is this nation-wide? Looking forward to your input!

Dale Howard says:

Fast answer- no you can’t use that chimney for the hot water heater anymore. You can’t mix fuel types in the same flue, and you can’t put obstructions in a chimney (which describes the chimney liner from the gas hot water heater’s point of view). These alone make the conversation about separate floors moot. So yes, figure out something different for the hot water heater.

You may have lots of options, I can’t know what they all might be from here, but I can tell you what people often do in your circumstance. The fastest, easiest and least expensive option is probably just to switch to an electric hot water heater. May or may not cost a couple extra dollars for hot water, but any increase in ongoing costs are greatly offset by the additional expense of almost any other option. Think about it anyway.

Good luck and stay warm!

Alan Ohms says:

I recently upgraded the hot water tank to a tankless water tank. Therefore my old water tank exhaust pipe is no longer being used. Can I install a wood burner and use the exhaust vent that the old hot water tank used?

Dale Howard says:

Fast answer: probably/maybe. You should certainly find out because you very well may be able to.

Assuming the flue is sized right, of structural integrity, etc. – all the usual caveats – you should be able to use it, yes. There are good reasons for all those caveats, including safety, so be sure to bring in a professional to pass judgment on your particular situation. But assuming the chimney itself it serviceable, you probably will just need to install a round insulated chimney liner and be good to go. Hope so!

Stay Warm.

Chris Bell says:

I have a gas furnace space heater (Warm Morning Gas Heater). It vents out the back with a metal vent pipe which connects to a masonry chimney. I want to add another gas furnace to a second story addition to the same chimney. Is this possible? On my first floor there are two openings in the chimney like I could attach two appliances. One of those openings are sealed. So, it seems like the flue was probably built to accommodate more than one heater. Also, I’m thinking since the metal vent pipe has two 90 degree bends before it connects to chimney – I’m thinking a gas furnace on the second floor with the same double-90degree vent pipe would prevent exhaust from entering from the first story. Any thoughts???

Tammy says:

We have a oil furnace and oil hot water heater in our basement, two flues in the chimney both of these are hooked to this chimney. We want to put a pellet stove down in the basement, is there a way we can use the same chimney for the pellet stove? maybe putting a pipe up through the chimney from the pellet stove to the top of the chimney? Thanks Tammy

Tammy says:

Also wanted to add this chimney is in the middle of the house, not on the outside wall.

High's Chimney says:


The answer is: maybe. The two heaters you have now can be on the same flue, but the new solid fuel appliance (pellet stove) has to have its own liner. So the question becomes, can you get two liners down the existing chimney? (and no, you can’t just line for one and let the gases of the other rise around it)

If you find there’s not enough space for the two liners you’ll need, see if they’ll fit if the tiles are removed. It’s done all the time.

So do get someone out to see what your possibilities are; you may be able to do it!

Stay warm.

Joe Meyer says:

Hi Dale, I have a red brick chimney in my home that currently services one wood burning fireplace on the 1st floor of our 3-story home. I can’t tell / don’t know yet if the chimney has one flue or two but assuming the chimney only has one flue, can a second flue be installed within the existing chimney so as to allow the chimney (which is on the exterior of the house) to service two different wood burning fireplaces (one on the main floor of the house within the house, and the other on the main floor of the house outside the house on the outdoor porch)?

High's Chimney says:

Quirky question, but maybe. As a masonry project it’d might be less possible than installing two stainless liners inside. The trick is that a fireplace liner is pretty big usually and might use up all the available space. It really does depend on how much room you have how work with. My hipshot guess without seeing anything is that it’s going to be a problem.

My suggestion: step one is to find out what the inside dimensions of the flue opening are. As a general statement you need to maintain a 1:12 ratio between the fireplace opening and a round flue. If the two flues needed to be 10″ diameter you’d need about 24″x14″ space to work in (less might be enough theoretically, but starts to get tight for installing.) In real life you can do with 1:14 or even 1:16 on round liners as long as you have sufficient height on the chimney. Assuming you learn that you just plain don’t have the room to do this, here’s your second chance: have the fireplaces built (or rebuilt) as Rumfords. This design works on a thinner ratio- like 1:20 and I have heard reports of 1:24 even. This work should be done by somebody who has done it before. Get an specialist mason who knows Rumfords and has done before. Also, do a web search for Belfire. Many chimney sweeps who do lining and masonry work will also install a Belfire (or competing brand.)

One more thought for you: you can make the fireplaces direct vent gas units. I’d imagine your opening is sufficient to handle the venting for that type of fireplace. Gas isn’t the same as gas, and that’s both good or bad depending on what you want. It’s only 90% as romantic, but it’s 90 times easier to deal with. A thought anyway.

Lots of decisions ahead. Good luck.

Seana says:

Hi! We are in New Hampshire and currently have an oil furnace. We have an old house with no heat upstairs and were considering adding an oil boiler to the already existing hot air system, as getting smaller pipes from a boiler system would allow us to get heat upstairs, where as we cannot get forced air up there. Can we run both an oil boiler and furnace on the same flue? the next part of this question is, we had originally been considering upgrading our existing forced hot air furnace(oil) with a oil/wood system. If I understand the code exception, this system could run on the same flue, even though typically wood and oil are not supposed to run on the same flue, is this correct? If so, we have been looking at Maine wood/oil furnace and boiler systems, and noted they have an option of a wood add-on for an already existing system. If we did this, would it still meet the exception? Basically, to make this simple, we want to purchase a new oil/wood boiler system to heat areas of the house that we cannot use the hot air system, and we want to keep our current oil hot air furnace, but purchase the add-on wood furnace to make our current system a wood/oil furnace to save money, rather than buying a whole new oil/wood furnace. If we did this could both appliances run off the same flue? Thanks for your help!

High's Chimney says:


Fast answer: … I guess so. (very useful, ‘eh?)

As you already know, the general answer is that no- you don’t mix fuels. I am aware of wood/oil units. I’ve never been clear on their relationship with the codes (which can vary around the country, but most places are on IRC) But if the unit has been listed to some UL standard, which I can’t imagine it wouldn’t be- I doubt they could get insurance if it weren’t- apparently it’s OK.

As you can tell, I’m not an authority on this particular question. My best advice is to call the manufacturer and ask these question of them and listen to see if you get good solid answers or sort of vague or slippery answers. I imagine you’ll get good answers, but the exercise will be worth the while. Be satisfied there really is an exception and be satisfied it’s as safe as they say it is. Be real clear on why an exception is OK.

Good luck!

Zack says:

I am drawing a floor plan for a house I would like to build. I am wanting to build a three-sided basement with a wood burning fireplace on the exterior wall that leads out to a patio. I am wanting this to be a double-sided fireplace, accessible from both outside and inside. I am also wanting to build the great room of the house directly above the basement with another double-sided fireplace accessible from both the great room and the outside deck. How can I make this work? From what it sounds like here that would be impossible. Thanks.

High's Chimney says:


I have to admit that as described I can’t fully picture what you want to do here, but generally speaking, I don’t know why you couldn’t have one fireplace above the other (probably slightly offset.) Specialty hearth shops design stuff like this all the time. I think you would do well to engage the design services of a high-end hearth shop.

I wouldn’t toss in the towel yet. See what folks who install these things for a living have to say.

Good luck with it!

Scott says:

I just had a 98% high efficiency furnace and water heater installed that no longer vents to the roof via a 5 inch double-walled metal pipe that would get really hot; they now vent separately via PVC piping to the side of the house. Am I able to use this unused 5 inch pipe that goes straight to the roof for a very small fireplace, or would that get too hot? I’m no expert but to me I’d say a small wood-burning fireplace would produce the same amount of heat the old furnace/hot water heater did. I’ve tried googling this for ever and I can’t find any information.

Thanks in advance,


High's Chimney says:


Fast answer: I am pretty sure you can NOT.

If that vent got hot before, it would surely get seriously hot (burn-down-the-house-hot) using wood. I’m guessing the vent is a B vent, which is made for gas venting. That alone preclude using this vent as a chimney. It’s a case of different venting materials for different uses, and if the old one got hot on just a water heater, sounds line there may have been a problem with it anyway.

You could call in a local professional to see if there are mitigation facts I don’t get from the info provided. But I’m pretty sure the answer is NO.

Best of luck to you!

susan says:


I need to line an old chimney and plan to install a pellet boiler. I want to leave the old oil burner as a backup.Can I just exchange the oil exhaust into the liner and take out the wood pellet exhaust, if I need to use oil for any reason. If I am only using one exhaust at a time is it necessary to have two chimneys?

High's Chimney says:

Assuming the flue outlets on both appliances are the same you could use either appliance. That said, when you change from one fuel to another an inspection (and often a cleaning) is indicated. The big message here is to make sure you don’t have an undersized or oversized flue for an appliance. If one were vented 7″ and the other vented 4″ for example you would have a problem with at least one of them.

If in doubt, call in a professional to look over your particular situation.

Sweep’s luck to you!

Elsa says:

I have a two-family house. There is an oil furnace for each apartment in the basement. One oil furnace is quite new. The other oil furnace is more than 35 years old. They both vent to the same flue.
I would like to replace the old oil furnace with a new naturally aspirated natural gas furnace. Natural gas is piped to the building. Would it be permissible for the remaining oil furnace and the new gas furnace to be vented through the same flue?

High's Chimney says:

Fast answer: yes.

If you can, it would be nice to put each appliance on its own flue. If there’s room to accommodate two liners in your chimney, please consider it. the idea being to have the appliances attached to properly sized liners.

But in fact you can put both a gas and an oil appliance on the same flue. There is nothing to describe how much difference between the two breaches into the clue, (I’d say maybe 9″-12″) but be sure the gas appliance connector enters the flue higher than the oil connector.

GAS + OIL is OK but GASorOIL + SOLID FUEL is NOT OK. You’re OK.

Good luck!

Eric says:

New homebuyer here. Just had inspection done and the report came back as follows: “Exposed flue pipe from the heating system was found in the flue opening. Consult with a contractor to create a proper wall between all flue pipes.” When I asked for more details, he said that we needed a brick wall separating the fireplace from the heating unit flue pipe. How much does a job like this typically run? Thanks!

Esther Chen says:

We live in a 2 story condo where the downstairs unit’s hot water heater shares the same exhaust pipe as ours.
And we’d been experiencing back draft issues. Sometimes it would be so bad our entire condo would smell like exhaust/gas.
Our landlord doesn’t want to fix this and simply tape up our AC box to prevent exhaust from being suck in to the AC.

We are trying to decide if it’s safe for us to stay.
Any suggestions would help!

Nan says:

An elderly friend on very small budget who can no longer split wood due to health issues, is having to hook up his older model hot air oil furnace after many years of not using… He doesn’t know what kind of pipe (6″) must be used used: single, black, or double, stainless, what gauge (24?), etc. How much acid & heat do oil furnaces create in venting? He has no choice but to pipe it out a back basement window (and hope no one sees it). How is it secured when done out through a window pane? Is it badly unsafe? I know it’s not to code…
So next question is: How HIGH does it have to be once outside?? It’s a 2-story cape…
Do any other issues other than what I’ve asked come to mind for advice on?
Many thanks for some knowledgeable info on this that I can pass along..!
Hope this site is still active for a response… Have been searching online for over an hour for info, but haven’t gotten anywhere… Thanks again! Nan

High's Chimney Service Inc. says:

A 24g galvanized pipe is a good connector for oil. It doesn’t need to be double-wall unless you are passing less than 6″ from combustibles (18″ for the wood stove though) Connectors are just that, connectors. They are meant to be inside the house in the same area as the appliance. As soon as you pass through a wall or a ceiling you are supposed to be using chimney or L Vent for this. (do not use B Vent at all; it’s only for gas)

As for venting through the window, I don’t like to think about that but again, as long as you keep your clearances… but I don’t see how you can. Glass isn’t combustible but the pane separators are I’ll bet. Then going up, you really should have L Vent or chimney. L Vent is meant to be inside the house or in a chase. I know people have run it up the side of a house as if it were chimney, but you’ll have way to much condensation in the pipe and it’ll rot out prematurely.

And you really mustn’t go up with single wall pipe. It’s not safe for several reasons, an important one of which is that the gases will condense and the condensation and will freeze blocking up the chimney with ice. This is sure way to asphyxiate oneself, assuming you breathe the fumes long enough even though they smell terrible.

I urge your friend to see if there is help from the State if he can’t afford to do this safely. Between the opportunities to burn down a house or kill it’s inhabitants with CO, venting heating appliances is very serious business.

Good luck with this. I know that he can’t choose to freeze, but he shouldn’t choose to fill the house with oil fumes either.

John says:

I live in an old farmhouse in Italy, stone walls, wooden beams, etc. The previous owner had a diesel boiler in the basement venting to a chimney that runs stright up to the roof (single flue, 8 inch by 8 inch). He then quit using the diesel boiler and cut a hole in the chimney in the first floor and stuck a small wood burning stove in the hallway and ran a 4 inch pipe into the chimney.

I want to remove the diesel boiler and install a pellet boiler. Local code says I have to run a 4 inch pipe to the roof through the old flue. No problem with that. However, the wood fireplace is really nice to have in the winter both because radiant heat is nice but also because we usually have enough scrap woodthat it is nice to have a way to burn it.

In a square 8×8 flue, there is physically enough room for 2 round 4 inch pipes. Is this safe however?

High's Chimney Service Inc. says:


This is the first question from Italy for me. I can comment of this only in the context of US standards, so the info isn’t necessarily “code” in Italy. But I will say that smoke goes up the chimney the same world-arround, so this general response should be useful re considering safety even if not the local law.

If the two 4″ liners will pass through the 8×8 then you should be fine. In the US an 8×8 is actually more like 6.75×6.75 and two liners would NOT go through- particularly considering the outside diameter of an 4″ stainless liner is probably more like 4.4″

Assuming the two liners do fit however, and assuming the masonry structure is 4″ thick plus the tile liner, you should be OK.

Arrivederci e bouna fortuna!

Nick says:

I have a oil furnace in my basement that goes into a single 8×12 flue. This same flue has a 4″ stainless liner that has a pellet stove attached in the basement. The 4″ stainless liner runs all the way up to the top of the chimney with a cap on it that still allows the oil furnace to vent properly.

The chimney has a second flu that was being used by a wood burner insert previously before that ran into a 6″ stainless liner. From what I am reading, having the pellet stove in the same flu as the oil furnace is a “No-no.” If I took the 4″ stainless liner for the pellet stove and ran it up the other flue into the 6″ liner, would I have any issues.

Also, I burned 7 pallets of pellets last winter, which is double what I was told Anyone else burns. I need to figure out why its so inefficient. Is this because I had the two appliances running in the same flue and the draft was pulling my pellet stove heat out with the oil furnace. Or just because I have the pellet stove in the basement and it needs to be on the first floor?

High's Chimney Service Inc. says:

I’m glad you’re getting the 4″ liner out of that oil flue. That really is a bad situation. Keeping the pellet stove on a 4″ vent is important so just take the liner out and run it down the 6″ flue that used to have the wood stove on it. Be sure the 6″ flue is nice and clean first of course.

As to why you’re burning so many pellets: I don’t know and won’t hazard a guess, but 7 tons sure is a lot of pellets. Something is off there. Hope you find somebody who can figure that out.

Good luck! Stay warm!

Jennifer says:

I am going to reline my chimney with a flexible liner, and attach it directly to the existing insert, but my wood burning insert does not have a collar on top, the opening on the top of the insert is 8 inches, How do I go about attaching the liner to the insert?

Teresa says:

We had a new boiler installed (replaced oil boiler for new oil boiler) there seems to be smoke coming from the flue when the heating is not on. Is this ok or something to worry about ?

High's Chimney Service Inc. says:

That doesn’t sound right at all. I won’t try to diagnose that with the given info, but no- doesn’t sound right at all. There may be a blocked flue, perhaps crossover from other appliances involved, or if you live in a row perhaps crossover from the neighbor’s… lots of possibilities.

Do have a professional in to get to the bottom of this. Good luck,


Martin says:

I have a house built in 1878, it has an 18″ unlined chimney that currently has one aluminum liner venting a boiler and a water heater in the basement, the liner is properly dimensioned.

I was hoping to add a wood burning stove into the same chimney but use a separate flexible steel liner for its venting. There is sufficient space in the chimney. Is this ok to do?

High's Chimney Service Inc. says:

Without actually seeing it, from what you describe it sure seems like you have the room.

One extra thought for you: I’d suggest also switching the aluminum out for stainless steel while you’re lining for the woodstove. Aluminum liners have a relatively short life to start with, but there’s also the issue of electrolysis. When the steel (which would presumably be from the mesh around the insulation on your woodstove liner) touches the aluminum there’s a little electrical reaction (dissimilar metals) and it ultimately makes holes in the liner.

It’s smart to insulate ALL liners, but you don’t have a safety issue with the boiler so could forgo if you want to. So said, you’ll get better performance from the liner if you insulate. 1/4″ cerwool is appropriate for heater flues (but 1/2″ for the woodstove flue)

Stay warm!

Stephen Spinella says:

I live in a condo in Manhattan with gas boilers and a flue tiled lined flue. The construction is recent: 2005. We do not have a stainless steel liner. I’ve read several things saying this is okay if the flue is checked periodically for cracks and other things saying it is unsafe and that we must get a UL liner. Some further details: the total BTUs is 1.4 million: the only liner that would fit in our flue is 12″ requiring a fan at the top.

Dwight says:

I live in an old farm house built around the turn the century. The house originally had an old oil furnace and chimney. My great uncle replaced this set up about 20 years ago with all electric baseboard heaters. Me and my wife moved in about five years ago and have struggled each winter since then with extremely high electric bills. We have begun to consider putting in a wood burning stove and utilizing the existing chimney. It is an old brick chimney with some cracks in it especially at its base in the basement where water has damaged it. We want to put the wood burner on the first floor so I was hoping we could scrap the original base anyway. What I want to know is if it is possible to use an old oil furnace chimney for a wood burner? I know we will need to repair it and put a liner in it but what else would you recommend?

Marcello Rodriguez says:

I have a high efficiency gas furnace. I was told i can not put the PVC flue pipe in the chimney and that it has to be routed off the side of the house and 6ft away? why wouldnt i be able to use the chimney for a high efficiency? currently i have a gas water heater flue going to the chimney (it is right next to the new high efficiency furnace). Also, if this is indeed the case, can i route the PVC pipe straight up along the chimney? the chimney is in the ideal location.

High's Chimney Service Inc. says:

It is correct that you can’t put it into a chimney. Not only that, you really don’t want to either. Here’s why.

High-efficiency gas units put a lot of water into the vent. This is because the heat exchangers in the unit take away so much heat (therefor high-efficiency) that the gases going into the vent are very low temperature. The moisture condenses without warm flue gases (which would be “lost heat”) to carry it up the chimney. Not to mention the carbon monoxide (of which there will be enough that you have to get it out of the house)

What happens if you run the PVC to the chimney is that water soaks the bricks. Very acidic water I might add. And the carbon monoxide being produced would not go outside, it would stay in the house and make everyone sick.

Not what you hoped to hear I know, but the furnace guys are telling you right. I understand that by not using the chimney it creates some inconvenience etc for you, but it’s important. Vent it outside.

Brenna says:

Hi I am buying a home and the chimney from the basement which is used by the boiler is not lined. It is brick. What damage could be caused by it staying unlined? What would you recommend?


Cindy says:

We recently renovated a house and had gas lines installed in our main floor and basement fireplaces. We can’t afford the gas inserts right now so there’s nothing covering the fireplace entrances. When it’s cold outside there is a HUGE draft. We can’t close the flue. Should we be able to still close the flue when a gas line is installed? We aren’t sure if it was just done by someone who doesn’t know what they were doing or if that is just how gas lines are. Do we have to live with that draft?

High's Chimney Service Inc. says:

I presume you are talking about an updraft that you did not have before? That suggests to me the dampers were removed.

Removing the damper is not bad practice; it’s the installer making sure you don’t do something to asphyxiate your family, namely operate the gas units with the damper shut.

The first choice of course is to get the inserts in there as soon as you can. If cash doesn’t allow for that just yet, some temporary solution is in order. I’d say this: DO NOT install a top sealing damper. Those are for open fireplaces and you’d be asking for trouble knowing the things are going to have gas units. If aesthetics are important, glass doors will work, but that could be an expensive temporary solution. If you can live with plastic covering the fireplace opening, it’ll give you the results for zero cost, but it’s not going to look so great.

You could also go to the roof and just block off the chimney; again, I don’t care for solutions that might backfire. If you install the gas units but don’t open the top back up… you asphyxiate your family. Plastic across the front is a good solution as long as you can live with the look of it for awhile.

Good luck! Hope it works out well for you.

Margo says:

Our home was built in the early 30’s in South Dakota. It was built with a large wood furnace in the middle of the basement with the chimney through 3 floors. My father removed the wood furnace and replaced with an oil furnace. We added a wood stove in the basement when we moved in 30 years ago. The chimney has ruined one of the upstairs bedroom ceiling due to water damaged (we think) and am thinking of adding a chimney liner to fix. The chimney is about 30 -35 feet high. What would suggest for the type of liner–could we use a flexible one ????

High's Chimney Service Inc. says:

Each appliance should have its own liner; hopefully you can put two
through the existing chimney. I’m glad you are doing this.

Yes! to putting a flexible liner. They cost less and are easier to
install: especially where there are bends. Costing less may seem
counter-intuitive I know, but it’s so. As to which alloy, while 304
liners are certainly available and they are suited for wood, 316 is more
common these days and has more corrosion resistance so may as well have
the state of the art. For the oil flue you definitely want 316.

While there is a chance that your upstairs water damage is from the oil
flue condensing, more often that sort of damage is seen in gas flues.
I’d suggest checking other things as well; maybe especially the
flashing. I presume you already have a good cap- that sort of goes
without saying. It’s never bad advice to tell anyone with a water
problem to just do it all at once; waterpoofing as well.

Good luck with it.

Whitney-richards says:

Thank you for having this website. This is a very informative, but I am a bit confused on the terminology regarding flue vs. chimney liner, and the recommendations for a single flue with wood and oil fired appliances seem to be mixed. There’s at least one reply that states that two separate steel liners in a single flue chimney for an oil furnace and a steel stove are acceptable. Everywhere else you state that a single flue is unacceptable. Does this mean that a stainless steel liner for echo appliance qualifies as a separate flue?
Again thank you.

High's Chimney Service Inc. says:

A flue is just a passageway for gases. A chimney is generally a
structure that would house one or more flues. For example, think a
brick structure with both a tile flue liner for the gas furnace and
another one for the fireplace running through it. That would be one
chimney but two flues.

It can be simpler than that; for example in the case of “Class A”
chimney, more properly called factory-built chimney. It’s insulated
pipe and is the chimney and flue all in one. This doesn’t apply in your
situation, but I mention this so you get the full picture about chimneys
vs flues.

A stainless steel flue liner is a SS tube that goes through a chimney
(often through an existing tile liner in fact) Generally speaking, each
appliance (i.e. a woodstove, furnace or boiler, hot-water heater etc.)
should have it’s own flue. However, if you have a gas furnace and a gas
hot-water heater, both can be vented into the same flue as long as there
is sufficient capacity to do so.

But there are combination that you can’t mix. To the point, you can’t
mix gas or liquid fuel exhaust in with solid fuel exhaust. (tiny
possible exception: there may still be manufacturers of oil/wood units
that are listed for use with a single vent, but I haven’t heard of them
in years and if even if they are still sold, you might not be able to
get a permit to install it because the codes are clear about this being
bad practice)

I think that may be more answer than you are looking for. Suffice to
say that lots of appliances can be on one chimney to be sure, but not
all in the same flue. Does this make sense?

Stay warm!

Jay C says:

I have a gas furnace and a gas hot water heater that are both in my basement but on opposite sides of the chimney. The furnace goes into the front of the chimney via a standard 4″ vent. Inside the chimney is a 4″ 90 and then above that is a 4″x3″x5″ Wye. The 5″ top of the Wye hooks to a 5″ flexible liner that goes all the way to the top of the chimney. The water heater comes into the back of the chimney and hooks up to the 3″ branch of the Wye. A plumber told me I needed a separate 3″ vent for the water heater because there’s not supposed to be a Wye inside a chimney. I have enough room inside the chimney flue to run both the 3″ and 5″ liners but I don’t have enough room on top of the chimney for both rain caps to sit side-by-side. Can I tie the 5″ and 3″ liners together inside the chimney just below the flashing at the very top so there is only one rain cap? I.e. both liners would run parallel to each other for approx. 30′ and tie together 6″ below the top of the chimney. Assuming “no,” can I extend the base of the rain cap for the 3″ liner after it exits the chimney flashing so it sits next to but above the cap for the 5″ liner?

High's Chimney Service Inc. says:

This is actually fairly easy. You can certainly bring both liners to
the top and there are chimney covers that will cover both flues. I’m not
there seeing the details, but it’ll probably go one of two ways: you may
be able to mount a short section of tile up there (say a 13×13) and put
on a standard chimney cover. If not with the tile, the two liners just
come up and terminate a couple inches above the top. On this you
install a crown mounted cap. Any chimney sweeps should be able to help
you with it.

Good luck!

Jay C says:

Thanks very much for your help and such a speedy reply! Great site!

rachel says:

Typically how large a chimney does one need for 2 flues, one for a furnace and one for a small gas fireplace? That is if you can have this setup with 2 fuel sources.

We currently have an oil furnace, which has a hot water coil for our hot water, in the basement of our home that vents through a brick chimney. On the first floor there is a boarded up hearth, connected to the same chimney, that we would like to put a gas stove/fireplace. There is no aluminum/steel flue for the furnace, just the brick chimney. If we put the oil furnace vent into a steel flue within the chimney, would we be able to put the gas appliance on a separate steel flue that is within the same chimney?


High's Chimney Service Inc. says:


“Typical” is a wide enough range that I can’t really give you a straight
answer to it. Let’s just say that >50% of the time typical is probably
large enough to house a couple liners. What’s more, if your existing
tile lined chimney did not allow for that, tiles can be removed. It’s
done all the time. That would make the chimney substantially larger if

As for if you can run two flues though there, yes of course (assuming
they fit, as you mention) I will suggest this: if you put a gas
appliance in the fireplace, don’t let them put aluminum in for that one-
go with stainless steel. The reason is that you’ll have a stainless
steel liner for the basement appliance and you don’t want dissimilar
metals in contact with each other if you can avoid it. Also, aluminum,
while legal etc, just doesn’t hold up in wet conditions. If you get
condensation in the AL liner, and it’s very likely you will because the
appliances are really quite efficient, those acids eat aluminum right
up. Run stainless steel for both appliances.

Any chimney sweep who does chimney lining should be able to help you.

Good luck!

Joel Stonebraker says:

I have a stand alone nat. gas water heater. I have a 5 inch chimney in a one story house. the appliance is in the basement, we seem to get a back draft as we get sick and the plastic rings around the water inlet and outlet are melting, what can we do to fix?

High's Chimney Service Inc. says:


There’s more to this than I can diagnose from afar, but I can say it’s
serious. CO poisoning is serious business indeed. You may in fact have
a back draft, though life’s not usually that simple. And the melting of
plastic is really alarming; it makes me wonder if the thing is venting
at all.

Joel, I’m not kidding here and I’m not given to over-cautiousness or
excitability- you need to turn off the hot water heater and get somebody
in there. Might be the vent or it might be the appliance- regardless
you’re better off without hot water for a few days than to fill the
house with CO. ESPECIALLY when you already know you are sick from it.
A quick solution is an electric hot water heater. Electric may be more
expensive, but balance it against the expense you’ll go through to
preserve the present circumstance.

If it were mine, I think I’d call the HVAC company first, and make them
confirm the vent is open while they’re there.

Good luck!

Jonathan says:

I have a natural gas separate hot water heater that is 15 yrs old and a year old natural gas high efficiency furnace that was flued together to a common flu that enter the chimney
I am getting copious water stains on the foundation wall below and around where the flue enters the chimney which is clay lined the house was built in 1959
What should be done to correct this problem

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