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Old 09-13-2008, 06:18 PM   #1
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Nat. Gas B-Vent sizing

What is the best size B-vent stack for water heater? Replaced my furnace with hi-efficiency sealed combustion type so no longer exhaust it out the roof. So now have only the 40,000 btu gas water heater venting out the 5" stack about 18' vertical height in MN. Conflicting info from contractors said "code don't require change, so leave it" or from another "will have high condensation in MN cold weather so better reduce size". Plan on reroofing in next few weeks so would like to change it now if needed.
Or since water heater is probably 23 yrs old do I go for a power vented out the side wall new heater?


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Old 09-13-2008, 11:29 PM   #2
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well if you gotta run a new pipe you will be fine with 4 in B-Vent. now on the otherhand if your gunna think about a direct vent for a few more dollars id look at like a rinnai tankless little costly but there sweet as hell it pushes like from 7.5 to 8.5 gallons a minute


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Old 09-14-2008, 06:16 AM   #3
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If your water heater is 23yrs old, it wont be long before you have to replace it. Might want to do it now and not have to worry about it. 4in. is the most common for residential applications.

During cold weather the furnace will cycle on and off frquently during the day, and keep the vent or chimney warm and venting properly. Once that source of heat is removed, and vented elsewhere the water heater is the only source of heat and does not cycle as frequently, or not at all when water is not being drawn from it, and the vent or chimney cools off during the off cycle.

Once it does cycle on, warm/cold creates condensation. This condensation is acidic, and will attack the mortar of the chimney, or the metal in the vent. Typicaly a stainless steel liner is installed to prevent this. Maybe you should mention this to the furnace installer, and check with your local code enforcement to see if a liner is required. You may get another few years out of that old water heater. It will also come in handy when you do purchase a new heater, and give you another option to venting through the side wall.

Last edited by 8 Ball; 09-14-2008 at 06:22 AM.
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