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Condensing boiler with duct coil

bob eckbob eck Posts: 826Member
Home owner is going from a oil boiler with coil to natural gas heat.
Heating contractor told me he is looking to use a 95% AFUE natural gas furnace with 50 gallon power vented water heater. Home owner wants AC
He does not have the clearance to sidewall Vent two units.
Then he asked if he could use a natural gas combi boiler ($1800 home owner rebate from the gas utility) with a duct coil on the AC air handler. If the hot water duct coil is over sized to run as low of water temp as possible will this boiler condense?
Should he use a 30 gallon buffer tank?
Will this type of system work?


  • Jamie HallJamie Hall Posts: 9,997Member
    Don't see why not -- the boiler doesn't care what heat emitters are dropping the temperature, only that the temperature is dropped low enough on the return.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • njtommynjtommy Posts: 1,102Member
    Yes a Combi boiler and hydro air will work just fine, but can cause some short cycling issues depending on TDR of the boiler. The boiler is low mass tied to a low mass coil not really a whole lot of water to keep heated. Also some Combi boilers have an interlock on them to stop the call for heat so the unit doesn’t blow cold air while their is a call for dhw.
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,219Member
    Yes absolutely use a buffer tank. You will probably have a short pipe run to the coil and no system water volume to speak of You will short cycle without a buffer.

    A condensing boiler with an indirect will be a better choice do to sizing.

    If you use a hot water coil in the AHU it will not condense. You will not get a big enough coil to run low water temp.

    Put the AHU in with the refrigeration coil in it and then size a duct mounted HW coil and make it as big as you possibly can
  • njtommynjtommy Posts: 1,102Member
    @EBEBRATT-Ed unfortunately yes I don’t think I’ve seen anything ahus for the US market designed for condensing boilers other then Lifebreath. I know in Canada there is a good amount of hydroair handlers. Designed with condensing boilers.

    At least with this set up you can add radiant floor heat to bath rooms, towel warmers and some other nice features.
  • TAGTAG Posts: 90Member
    How about that NTI GF200 in the other thread ,,,,, looks like an updated version of the idea behind the AMANA HTM of the 80's
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