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Dual hot water heater vent connection

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From Sunny Lee:

My chimney flue size / diameter is 7" and has Y adapter(4" and 3"). My hot water heater exhaust vent is (3") and my furnace exhaust vent is(4").

If I install another same BTU (or Gallon) hot water heater, can i connect exhaust vent to 3" vent(first one) by using 3" T-adapter?

PLEASE, let me know.
asked in Gas Water Heater by darekrudy (21,730 points)
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1 Answer

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Hi Sunny,

Total square inch area of 3”+3”+4” vent pipes is 7.06 + 7.06 + 12.56 = 26.68 square inches. The closest vent pipe diameter corresponding to that area is 6” (28.27 sq. inches) which means that your 3 appliances must discharge into a minimum 6” diameter pipe.

You didn’t specify how big is your “Y” diameter at the chimney entrance, but for this extra water heater it should be either 6” or 7”. Two water heaters and the furnace can be connected to a 6” or 7” manifold discharging into the chimney or you can do a tapered manifold, combination of a 5” manifold pipe (with 2 water heaters connected to it), changing into a 6”/7” diameter manifold with a 4” furnace vent pipe connection discharging into the 7” chimney flue.

Examples of 3 natural draft gas applinaces venting














Using a 3”x3”x3” Tee connector could compromise proper venting of your water heaters and create hazardous situation.

More about chimney flue size calculations

answered by darekrudy (21,730 points)