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Forced / Gravity Vent Through Common Flue


Inspector Will
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I came across a venting system I have not seen before. The water heater and furnace are installed in a basement. The original furnace and water heater were equipped with gravity vents and connected to a common flue (8") that terminated at the roof top. A new furnace was installed just prior to my inspection. The new furnace is a direct vent system with 4" PVC. The PVC vent pipe is run through the original flue (8") and terminates at the roof top. Is there any concern that the PVC run through the original flue will prevent proper water heater venting.

Thanks,

Will

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Hi,

I think I'd be concerned with that fact that by putting that plastic vent up through the center of the B-vent vent they've just reduced the cross-section of the B-vent by the cross section of the plastic vent. Now you have to calculate whether what's left meets the flue size rule of equal to the largest connector plus 50% of all others for those original two appliances. Bet it doesn't.

The high efficiency furnace extracts most of the heat from the flame and discharges air that's basically just a little warmer than we are. You want the exhaust gases from those two gravity devices to be as warm as possible as they rise up the vent so that they'll maintain their buoyancy and won't condense inside the B-vent and damage the vent or furnace. By running that plastic up through the middle of the B-vent and then running colder exhaust from the 90+ through it, you are cooling the exhaust gases that leave those gravity appliances as they surround that cooler pipe. Cool them too much and they won't draft properly. That means you're facing damage to the appliances from condensation and those gravity appliances will be prone to backdrafting, which raises the question of CO poisoning.

Then there's the unanswered question of what the long-term affects of hot acidic exhaust gases surrounding PVC are going to be. Will it eventually break down the PVC and allow that forced vent to push the exhaust from the gravity appliances back down the flue and out?

The person that did this didn't have a clue so I'd suspect it was a homeowner. If it was an HVAC "tech," the owner of the HVAC company needs to be notified that there's a dullard that has to go to the proctologist to get a tooth cleaning working for the company .

ONE TEAM - ONE FIGHT!!!

Mike

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I Is there any concern that the PVC run through the original flue will prevent proper water heater venting.

Thanks,

Will

Yes. It's a bogus installation.

If you sent those pictures to any manufacture you would have all the backing you need. Or.... ask the installing contractor to provide proof that this type of installation is approved.

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That is so wrong it's frightening. The WH exhaust vent needs 1" min to anything combustible. Building materials are considered combustible unless they're rated as being non-combustible. PVC is not rated as non-combustible.

BTW unless the furnace air intake also originates above the roof, that's a high efficiency, condensing furnace but it's not direct vent. A direct vent appliance draws its combustion air from the same place where the exhaust goes.

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