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Induced draft furnace and natural draft water heater venting

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From Bill Moss:

My hot water heater exhaust vent (3") is connected to the exhaust vent of the furnance. When hot water heater is heating water, and furnance comes on everything is fine.. but if the pilot lite is only on the moment the heater fan comes on the lite goes out.

If you try reset it while the blower and heater is activated the match goes out, no draft seems like a lack of oxygen. What can I do, it was installed before I bought the house by a lic professional?
asked in House Chimney by darekrudy (21,730 points)
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1 Answer

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Hi Bill,

A picture of the vent pipe setup would be helpful. Assuming that you have an induced draft motor equipped furnace, both vent pipes (from furnace and WH) must be connected together with a WYE type connector – check this post Chimney Flue.

Otherwise, furnace induced draft motor might be forcing air / exhaust gases into the water heater vent pipe. Whenever the water heater burner starts operating first and heats up the chimney, the furnace exhaust gases have some extra draft help from already warm chimney and instead of flowing back into the WH vent pipe, they continue into the chimney.

If you have a "Y" type connector properly installed (I’ve seen them up side down), section of your chimney might be partially blocked, and before it picks up proper draft, it spills some of the exhaust gases back into the WH vent pipe.

It might be also an oversized, cold chimney that causes back-draft. One other thing that I can think of is a missing or collapsed WH draft hood… ?

Please, let me know.

answered by darekrudy (21,730 points)