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Condensing boilers and domestic hot watet

Kevin_10 Member Posts: 20
i am convinced that condensing boilers are the way to go with radiant heat in a slab. However, I have heard conflicting stories about these boilers and getting enough domestic hot water production. Any thoughts ???


  • Steve Ebels
    Steve Ebels Member Posts: 904
    Good question

    I'll answer it this way.

    We fired up an 8-32 Vitodens Monday. The domestic water is handled by a SuperStore SS-45 which is of course a 45 gallon tank. The Vitodens (on LP gas) fires at a max rate of around 112,000 or so. The water temp in the tank at start up was 49*, fresh fill right from 150' down. The Vito took 21 minutes from light off to bring the tank to 122* initial setpoint. I don't think that's too shabby.

    The Vito is programmed to maintain a 30 degree differential between tank temp and entering boiler water temp. Cool huh? As the tank temp comes up the Vito brings up the boiler temp to match. Doesn't expend any more energy than neccessary to do the job that way. You could hear (barely) the burner modulating up and down to hold the 30* diff.

    It's as much a function of the size of heat exchanger in the tank as it is the boiler. Pick a tank with as much surface area as possible on the HX and you'll be OK.
  • Mark Hunt
    Mark Hunt Member Posts: 4,909
    Pretty much

    the same results with the Munchkin. We program a 30* delta T as well and it ramps up and down as needed.

    Besides, show me a 92% AFUE water heater.

    So many really good options out there today, but hey, don't take my word for it. Read this.

    Mark H

    To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"
  • Matt Connolly
    Matt Connolly Member Posts: 67
    Condensing units are always better...

    because they have no low return temp limits. That means that if you have an indirect with a lot of heat transfer area, the boiler will run constantly and in condensing mode for a longer period of time during the recovery phase. A conventional boiler is limited usually to a 20F delta T and the pump will cycle if too much heat goes to the DHW, to protect the boiler.
    A nice feature in a condensing boiler to have is multi-temp control with priority. The Monitor MZ has that on its models which allows any temp for radiant and an auto priority 180F for DHW recovery as a standard component. The included boiler pump handles both jobs and is controlled internally as standard as well. Check it out at www.mzboiler.com

    Good Luck!
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