Gas Boiler/Gas Water Heater Exhaust Size - HVAC - DIY Chatroom Home Improvement Forum

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Old 02-03-2007, 12:28 AM   #1
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Gas boiler/Gas water heater exhaust size

I have a 35+ year old house with a baseboard boiler furnace. I recently had a home inspection done and the furnace exhaust and water heater exhaust are tied into the same exhaust pipe. The inspector said that he thought the pipe had to be upsized on the out side of the junction when the two are tied together. Mine is not. DIY what is the standard?


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Old 02-03-2007, 08:35 PM   #2
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Hi Tim Lucas

The general mechanical code calls for the pipe size to be increased 1" above the primary heating devise. When the increase is made it is supposed to be done where the secondary device is connected. Example: Boiler has a 5" flue, the flue should be 5" until it gets to the water heater. The water heater will be 3", where the two flues intercect or tee however you want to say it. From that point to the end of the flue should be 6". However this does not apply in all city codes. Check your local mechanical codes to verify what is required.

Good luck


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Old 02-03-2007, 08:46 PM   #3
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Exhaust pipe sizing

Thank you. Repair already done and this is what I thought and installed the proper size.
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