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general chimney question

Good morning. In you very common situation where you have a boiler and water heater connected to a common chimney(just an example) why isn't the chimney oversized whenever only one of the appliances is running? In particular during the non heating season when the boiler never runs why aren't there issues with inability of the smaller water heater to heat the chimney and establish draft? This is an academic question. Thank you.


  • John MillsJohn Mills Member Posts: 741
    You are right

    The chimney won't warm properly with just the WH running. We have to hope that the boiler cycles enough to help out. In mild weather when the boiler isn't on, the chimney likely is warm enough that there is little condensation. But I'd guess most get pretty wet. The danger in chimney venting.

    The NFGC addresses the size of the chimney vs. the equipment. One area talking about the this is the 7 times rule. The flue area of the common flue (chimney) cannot be more than 7 times the outlet area of the smallest draft hood appliance. So it is saying that your water heater draft hood outlet determines how big your common flue or chimney can be to help reduce condensation. Your typical WH has a 3" outlet even if 4" pipe is on it. 3" is 7 sq inches. So your chimney can't be larger than 49 sq inches. Around here, most homes other than little bungalows have a liner 6.5 x 10" so most chimneys are too big for a furnace/boiler and WH without a chimney liner being dropped. And many have offsets making it murder to do.
  • Tim McElwainTim McElwain Member Posts: 3,765
    Run "B" vent up through the existing chimney

    as the chimney liners themselves are 20% less BTU capacity as the same size "B" vent.

    One of the things that has affected the water heater all by itself is the higher efficiency of the water heaters means less flue loss, they also have reduced the size of the orifices on pilots so less BTU being generated there also. The two pilots one on the heating system one on the water heater helped in the past to keep the chimney pretty warm giving you a head start on drafting when one or the other came on.
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