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

Let’s start with a few definitions.

“Things” are properly called “appliances.” This means fireplaces, woodstoves, furnaces, boilers, pellet stoves or hot water heaters etc. They’re all individual appliances. 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?” chimney flue ventingA 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.)

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.

Let’s start with 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 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 and 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.

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

  1. C. August

    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?

    Reply
    1. Paul Bancroft

      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

      Reply
      1. Chris Bell

        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???

        Reply
  2. Les pohle

    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

    Reply
    1. Dale Howard Post author

      Les,

      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) .

      Reply
  3. John Deschaine

    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.

    thanks,

    John

    Reply
    1. Dale Howard Post author

      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.

      Reply
  4. Joe McGuinness

    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?

    Reply
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    Reply
  6. Vance Simons

    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
    Vance

    Reply
    1. Dale Howard Post author

      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!

      Reply
  7. Judy Doyle

    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.

    Reply
    1. Dale Howard Post author

      Judy,

      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.

      Reply
  8. Stacy

    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?

    Stacy

    Reply
    1. Dale Howard Post author

      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!

      Reply
  9. Sarah

    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?

    Reply
    1. Dale Howard Post author

      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.

      Reply
  10. Sarah

    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.

    Reply
  11. Kent

    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!

    Reply
    1. Kent

      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.

      Reply
      1. Dale Howard Post author

        Kent,
        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.

        Reply
        1. Kent

          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.

          Reply
  12. Donna Strand

    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.

    Reply
  13. Meg Lamont

    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,

    Meg

    Reply
    1. Dale Howard Post author

      Meg,

      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!

      Reply
  14. Jesse

    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!!
    Jesse

    Reply
    1. Dale Howard Post author

      Jesse,

      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!

      Reply
  15. Jesse

    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??

    Reply
    1. Dale Howard Post author

      Jesse,

      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!

      Reply
  16. Jesse

    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!

    Reply
    1. Dale Howard Post author

      Jesse,

      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!

      Reply
  17. Holly

    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!

    Reply
    1. Dale Howard Post author

      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!

      Reply
  18. Alan Ohms

    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?

    Reply
    1. Dale Howard Post author

      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.

      Reply
  19. Chris Bell

    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???

    Reply
  20. Tammy

    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

    Reply
      1. High's Chimney Post author

        Tammy,

        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.

        Reply
  21. Joe Meyer

    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)?

    Reply
    1. High's Chimney Post author

      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.

      Reply
  22. Seana

    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!

    Reply
    1. High's Chimney

      Seana,

      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!

      Reply
  23. Zack

    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.

    Reply
    1. High's Chimney

      Zack,

      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!

      Reply
  24. Scott

    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,

    Scott

    Reply
    1. High's Chimney

      Scott,

      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!

      Reply
  25. susan

    Hi,

    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?

    Reply
  26. High's Chimney

    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!

    Reply
  27. Elsa

    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?

    Reply
    1. High's Chimney

      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!

      Reply
  28. Eric

    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!

    Reply

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