Sizing chimney liner for boiler and water heater

Home Maintenance Questions and AnswersCategory: House ChimneySizing chimney liner for boiler and water heater
Question by Zach Holmes SMWIA member

So here is the problem. I have a 140000 BTU gas boiler and a 36000 BTU hot water tank. I was given an infraction by the gas company that my chimney had to be lined. The boiler currently has an 8 inch vent opening in the top of it and reduced to 6 inch before going into the clay chimney. (It seems by reading the code book that this is already illegal). and the hot water is a 4 inch line. this is know is correct. Would i be alright to pull a 5 or 6 inch liner, insulated of course down the chimney and use a “y” type connector in the furnace room to tie them both in.? and use type c vent to pipe the boiler and hot water tank to the “y” they aren’t far away. maybe 3 feet. or am i misunderstanding the gas code that states that the size of the flue should be no smaller than the size of the vent hole of the bigger appliance + 50% of the size of the smaller appliance. There is not enough room in the chimney to pull an 8 inch liner or i would. and also could this liner be galv-alume or does it have to be stainless steel.?

1 Answers
Dariusz Rudnicki Staff answered 10 months ago

Hello Zach,
Downsizing of the exhaust pipe is not permitted and could be dangerous for a couple of reasons. It could result in the firebox temperature increase, possible cracking of its walls. It could also compromise proper venting, cause Carbon Monoxide spillage into your house.
However, I remember having a similar situation during one of my home inspections; there was a downsized boiler vent pipe, which was approved by their manufacturers (I do not recall its brand). I would suggest to confirm that with the boiler’s manufacturer before any chimney liner installation. If it has to be 8″ diameter… you’d need a new chimney or furnace / boiler that requires smaller diameter exhaust pipe (8″ seams to be a lot for 140K BTU’s). Maybe this boiler was converted from different fuel?
C vent – are you referring to a concentric vent? Those are used for exhaust and combustion air, not for discharging exhaust from two appliances.
You are correct about the common vent pipe diameter – larger pipe + 50% of the smaller = 10″ in your case (also no more than 10″ with 4″ WH draft hood) or 8″ minimum diameter if that downsized 6″ vent pipe from your boiler would be approved by the manufacturer.
Stainless steel chimney liners are required with some of the condensing type (high efficiency) appliances. Both of your appliances are most likely natural draft so no special liner would be needed (unless your local jurisdiction has different rules). Did they say anything about the liner type they’d require or just gave you a ticket?

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