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Question DetailsAsked on 3/16/2014

Is it safe to have a Gas fired boiler and an Oil Fired burner share the same flue?

I have two burners - both Burnham (both 3 years old). One oil, one gas. They sit side by side in the basement - both share the same Flue/chimney. The building inspector's report did not point to any issues when I bought the building, but a plumber trained in Europe mentioned that it would not pass inspection there. What are the safety risks?

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2 Answers

Voted Best Answer

Yes, no, and maybe ?

If both devices are on the same floor, AND the junctions are offset into the flue (neither can run direct-through), AND the smaller BTU load comes in above the larger one, THEN the IRC allows both to be piped into one flue, assuming it is properly sized. Neither unit can be fan-driven - both have to be natural draft. Solid fuel (coal, wood, pellet, etc) furnaces, and fireplaces of any type, cannot be combined with any other type of flue.

Now the Maybe - some areas have building codes prohibiting multiple devices of any kind sharing one flue - some because those provisions hark back decades, some to be ultra-conservative.

Now the NO part - an insurance company fire inspector once showed me a section in the insurance inspector's manual put out by the joint organization that serves most insurance companies (Fire Mutual or Fire Insurance Bureau or some such) where it said this is specifically prohibited, but that may be a throwback to the 1970's or 80's era code where it was prohibited bu the UBC (Uniform Building Code). However, it is very common in my area, and I have never seen an insurance inspector (granted, their visits are rare) red flag one.

Assuming it is piped correctly and with proper sized flue, I would not consider it a significant risk - my house has a furnace and hot water heater connected to one flue. The unit cannot of course have a designation prohibiting connection to any other device 0 a warningn which should be on the tags and in the manual for all modern solid fuel devices, all fan-driven ones, and is also on some brands of natural devices - possibly to try to adhere to codes in all jurisdictions.

The only safety risks I can think of is if the lower-piped unit kicks on during very cold weather, with cold air in the flue, it takes several seconds for it to establish flow up the flue by heating the air. During that time, it could cause exhaust down the duct to the other device, which if IT then kicks on whilethere is backflow could cause backdrafting. This would be most likely where the two units are close together. A few seconds of backdrafting is not likely to put enough CO into the air to be dangerous in most cases, assuming the source air is adequate for the devices, but if the second device is improperly ducted without a draft hood (the open-bottom hood right above the device), THEN it could cause rollover (gas flame coming out under bottom of device firebox) or blow out the flame. Properly ducted however, I do not see an issue.

Answered 6 years ago by LCD


Illegal in most areas and is not recommended for lots of reasons. It's all about flue gasses and draft isues which I will not detail. Also recommend a complete flue inspection for damages as well.

Answered 6 years ago by hosey


We see it in our region.

Just depends on whether the flue is rated for the oil fired temperatures as well as the ducting is correct so as not to create a backdrafting/spillage issue.

Answered 6 years ago by WoWHomeSolutions

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