Oil and Gas venting

I have been inspecting several oil fired furnaces and or boiler systems venting into the same flue and chimney as a gas fired hot water system. I have been told that this is NG. Does anybody know the answer to this problem. Why is this always called out? What is the reason?

What kind of draft system is coming out of the furnace?

Got any photos?



I would think the same rules apply in the U.S. seeing as how we get most of our rules from U.S.

Hope this helps



I didn’t answer part 2 of your question

Natural gas is a clean burning fuel.The fumes they produce are cooler and contain high levels of water vapor,which are more likely to cause condensation,these vapors also contain chlorides thus the flues are subjected to more corrosive conditions.This combonation of water laden gas vapors and old oil soot speeds up process.


P.S. this is the short answer!!!

I don’t think I’ve ever seen an oil-burning appliance here in CA, but my interpretation of the codes suggest that oil and gas CAN share the same venting system.

I’ll research this further, but I think I have come across information that describes harmful chemical reactions (in addition to those described by Mario) caused by mixing exhausts from different sources.

Are the appliances “listed” as “combination gas and oil burning appliances?”

Yep. The UMC allows it.

You can have a Combination Oil wood burning furnace Regulation3.2.1`of the CSA B365.
They used to allow oil furnaces and wood furnaces to use the same flue. I believe it is only allowed in Combination units now .
I have seen a couple of older oil and wood separate furnaces still operation .
There is a standard 3.2.2 wood gas combination boiler that was certified ( I know of no one who has ever seen one ) they do require a complete separation of combustion and venting systems for each fuel.
The test standard ( Can-CGA 2.27).
You can run two gas appliances into the same flue but they must be on the same level .
You Can not put Gas and oil into the same flue.

Roy Cooke

It appears that the IRC allows it as well, according to G2427.5.6.2 (503.5.7.2) Liquid fuel-burning appliances.

Commentary: Gas-fired and oil-fired appliances are allowed to share a chimney or flue.


In the U.S. But not in Ontario.Not a natural draft chimney anyways.

The only prohibition I can find is with regard to SOLID fuel, which must have it’s own, independent chimney/vent.

This has to do with the extreme burning temps and deposits of byproduct (creosote), which are not produced by gas or oil.

I’ll admit my ignorance when it comes to Canadian standards/requirements.

Same here for U.S. requirements!


This same rule applies here as well.

I have an oil fired boiler and gas water heater sharing the same chimney. My understanding is that as long as the larger (in BTU) unit is lower than the smaller unit where it enters the chimney, it is not an issue. I don’t have a problem with draft.


Thanks to all that replied. It seems that every body has a different oppinion. No body knows for sure. I was told several times that gas burns hotter then oil. Oil leaves an after product that the higher gas temperatures can ignite with. No one knows for sure. Further investigation needed!
Thanks again


For sure? The model codes say it’s allowed. Is that not definitive enough for you?

Thats what I was told. But not to many people know tis. I just wanted to be sure. If I find any more information, I will let all know.


Read this section of the IRC. It IS allowed. There’s no need for “further investigation” unless you feel there’s a problem with the venting system itself.

IRC G2427.5.6.2 (503.5.7.2) Liquid fuel-burning appliances.