Venting oil furnace and wood boiler into one chimney

Home Maintenance Questions and AnswersCategory: House ChimneyVenting oil furnace and wood boiler into one chimney
Question by AlanS

I’m glad I stumbled upon this website. I have an older home with a fairly large chimney. The chimney has two clay-tile-lined flues. One flue was for the fireplace in the kitchen and the other is ventilating my fuel oil furnace. I am considering installing a wood gasification boiler in the basement. It would be very easy for me to connect the flue from my wood boiler into the flue that the oil furnace uses.

I’ve read, and been reminded on this site that the same flue shouldn’t be used for dissimilar appliances. This flue however is big enough that I could drop a flexible duct down through the top and connect directly to my boiler. Is it “legal” then to still allow my fuel oil furnace to use the flue that my new flue is encased by, or do I need to have an insert installed for both appliances in the same flue? Thanks so much!!

3 Answers
Dariusz Rudnicki Staff answered 1 year ago

Thanks for stopping by Alan,
A couple of things I would consider: by inserting a flexible duct into the oil furnace flue you’re downsizing it. If it was designed for venting your oil furnace only, eliminating several inches of its cross-section will compromise proper draft. It would definitely make sense to properly calculate your chimney flue dimensions requirement for both appliances, try to get a professional.
If it is too small for 2 appliances, you’ll need an external chimney for one of them. If it’s big enough to vent both appliances – install two separate stainless steel flues (unless otherwise specified by the manufacturer) in that one large flue. With new boiler flue only, you’re exposing its exterior surface to sulpheric acids contained in oil furnace exhaust.
Also… your local code enforcement division may have some different opinions :-). Good luck!

Alan answered 1 year ago

Thanks for your comments,
It would seem to me that the size of the duct leaving the fuel oil furnace technically would be the minimum flue size required going up the chimney. Is that poor logic? If my logic is correct, then I have plenty of capacity for two flues in my existing.
Good point on the sulfuric acids from the fuel oil furnace. My goal is to virtually eliminate the use of my fuel oil furnace….only using that for an emergency (or long-weekend-away) back-up. If that is the case, and I’m only using the fuel-oil furnace the equivalence of a couple days per year, is that going to be enough to deteriorate the new flex flue?
Again, I thank you for your help!!

Dariusz Rudnicki Staff answered 1 year ago

I don’t think it would, but it might become and issue if such installation is not permitted in your area (flue inside the flue used by other appliance), and you offer your house for sale. Also, does your chimney flue has an offset? Single flexible liner could partially block the flue in such design, and two liners will provide dedicated path for the exhaust gases. Just a thought.

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