How many gas/oil appliances can vent into 1 flue?

I am wondering if there is a limit to the amount of appliances you can vent into one flue. I think I remember reading somewhere that you can’t have more than 2 gas appliances venting into the same flue.
Today I did an inspection on a multi family. 2 Oil Heating systems and 2 Gas Hot Water tanks all vented into the same flue in the chimney. I can’t seem to find anything in the codes other than Oil and gas can be vented into the same flue, doesn’t list any restrictions. I just want to be sure. Thanks.

I do not think they can be intermixed.
You could at one time put oil and wood in the same but no longer .

… Cookie

Hi to all,

The gas and oil components may share a common flue (IRC 2427.5.6.2) Canadian codes differ on this I believe, the problem would be with the size of the flue itself. The codes require that a common flue be the size of the largest flue plus an additional 50% of each additional flue.

Without knowing the flue sizes of the individual components and that of the chimneys’ flue size you could not make the determination.



What type of chimney did these appliances vent into? Was it terra cotta, stainless steel liner, b-vent, brick lined, metal chimney?

With oil burning you will get carbon products mixing with water vapour from the gas hot water tanks, that creates carbonic acid if I remember correctly? What ever the liner is I would be concerned about the integrity of the chimney both interior and exterior.

Definetly something that should be checked by qualified technician prior to close of title.

This may help…

In some cases, you can still vent oil and wood appliances into the same flue. Usually it is with wood/oil combination furnaces or where the wood furnace or boiler is a certified “add-on” to an oil furnace or boiler. The appliance manufacturer’s literature will instruct on this issue.

It may have been permitted at one time, but I believe its no longer pemitted and existing installations may not be required to upgrade til replacement time.

If in doubt the heating company it may be adviseable to conduct a draft test.

How can you separate the combustion products of a wood/oil combination furnace with one flue pipe outlet?


And from the installation instructions from another manufacturer:

We recommend that the KERR SCOTTY Wood Furnace be connected to its own chimney. However, two or more fuel fired appliances, other than fireplaces or incinerators, may be connected to the same chimney.

The National Standard of Canada, Installation Code for Solid-Fuel-Burning Appliances and Equipment, CSA-B365-01 clause 5.2.9 states that:

Two or more appliances shall not be connected to the same chimney flue unless:
a) The appliances are located on the same storey;
b) A negative pressure is maintained at the flue collar of each appliance;
c) Each appliance is provided with a means to control the rate of flow of air or flue gases through the appliance combustion and heat exchanger chambers. Where limitations on flue pressure (draft) are contained in the manufacturer’s instructions, such means shall be capable of maintaining at the flue collar, the pressure specified by the appliance manufacture;

NOTE: For the purpose of this Clause, draft controls include ash pit air control dampers, dampers or draft slides wherever located, over fire and flue key pipe dampers, and check draft controls located on or in the flue pipe.
d) The flue pipes of the appliances are connected to a manifold as close to the chimney as practicable or directly to the chimney in the following order:

(i) If only solid fuel is used, the flue pipe from the smallest appliance shall be on top (downstream);
(ii) If different fuels are used as permitted in Clause 5.2.7, the oil flue pipe shall be on top (downstream); and the solid fuel flue pipe on the bottom (upstream); and
e) The chimney flue is capable of venting the flue gas by natural draft when all appliances not interlocked to prevent simultaneous firing are firing at their maximum rate at the same time.

We are not talking about wood/oil. We are talking about two oil furnaces and two gas water heaters venting into a single flue.

The manufacturers instructions likely only apply to one furnace one flue scenarios. Given this set up as in the original post, we don’t know the btu’s of the appliances nor the type of chimney whether its lined/unlined or whether the chimney is interior or exterior, its length or any other useful info.

Its a call that should be checked out by the local licenced heating contractor for flue temp, draft, et ceteras.


If you follow the thread back, I was only responding to Roy’s statement about wood and oil…on a bit of a sidetrack with current information.

The chimney is a typical brick chimney approximately 30-35 Feet tall through the middle of the house. It is unlined with 3 Flues. 2 for the 2 Fireplaces and the 1 flue for everything else.