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Condensing boiler with duct coil

bob eck
bob eck Member Posts: 930
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 Hall
    Jamie Hall Member Posts: 22,937
    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.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • njtommy
    njtommy Member Posts: 1,105
    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-Ed Member Posts: 15,335
    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
  • njtommy
    njtommy Member Posts: 1,105
    @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.
  • TAG
    TAG Member Posts: 755
    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
  • Deepfish
    Deepfish Member Posts: 2
    Anyone know much about the gf200 feel free to comment. We just had one I stalled in our new home, was just fired up a day before moving in so not alot of time to. Play with it. The heating works fine but the dhw is so far weak. It's set to 140 but you have to have shower or faucet on full hot to get water just hot enough for a shower. It's like it needs to be cranked up some. The heat just isn't there. The umbers had a mixing valve on it and it was full open to hot and water was even cooler so they took it right off. Water got a bit warmer but still have to simply turn on full hot to get even a warm shower. I think there is a calibration issue or something is amiss in the set up. It should be able to make hot water. I'm the home owner not the plumber or heating contractor.
  • bob eck
    bob eck Member Posts: 930
    Call the installing contractor and have them call NTI and ask NTI what they need to do to get you the correct amount of hot water.
    You could have a cross connection where cold water is mixing in with the hot water through another faucet.
    What is the actual temp coming out of the unit.
    If it is coming out at 120F you might have a cross connection.
    Like I said call the installing contractor get them to look at your system.