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?” 
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.)
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.
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?
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
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) .
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
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.
Great post. I was checking continuously this blog and I am impressed!
I care for
Very useful info particularly the last part
such info much. I was seeking this certain information
for a very long time. Thank you and good luck.