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Draft induction fan and hot water heaters.

Focal Point

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Not really. Unless it's an on-demand water heater, the flue gases are kept warm and relatively buoyant by the pilot in the water heater. There is normally a momentary increase in CO around the draft inducer, until that first slug of pressurized air from the furnace hits the top of the flue, and then it levels out. If the water heater were shut off and the flue gases were cool, the duration of spillage would be greater until the flue gases had been warmed all the way to the top of the flue and then buoyancy would take over, assisted by the inducer.

It's not dangerous unless/until you have a flue blockage or you're losing buoyancy due to something else - single-walled vent in unheated areas, improperly located/sized flue suffering from stack affect, insufficient make-up air for combustion, clothes dryer or return air intake nearby, etc..



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Originally posted by Focal Point

Question, Hot water heater connected to furnace flue equipped with a with a DI fan, wouldn't the DI fan push exhaust back out the vent hood of the water heater?

No. The draft inducer fan doesn't increase pressure in the flue. It just draws a regulated amount of air over the burners. Pressure in the flue is still negative. (While the flame is burning, that is. During the purge cycle, it might cause some increase in pressure and the water heater might momentarily backdraft.)

What is the rule for WH connections into furnace flues? cant seem to find it.

IRC chapter 24.

- Jim Katen, Oregon

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David, forgive me for posting this - This question is a good illustration why every inspector must spend time with another inspector in the field for a period of time. Having the code books does not get you the experience every inspector needs. Book learning ain't enough.

David I know you realize this comment is global!

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Very good point Les. A code book would not have helped in this situation. It would not tell you how a draft inducer fan works or what it does. But every home inspector should have a copy of the IRC, the IBC is for commercial applications.

Better yet invest in a set of Code Check flip books and the JLC Field Guide.

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