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Boiler sizing for small house and DHW

Gene Davis_2
Gene Davis_2 Member Posts: 71
New construction, small house with only 36K Btu/h calculated heat loss. Two full baths, no whirlpools, one DW, hot water needs to be handled with an HTP SuperStor Ultra 45g.

We can easily have a Munchkin T80 wall-hung mod-con do the heating, and in fact their 50 would probably suffice, but will such a small mod-con go into long cycles to do DHW, and affect recovery time?

And, if we size up for the DHW picture, won't it mean short cycling in the heating mode?


  • jp_2
    jp_2 Member Posts: 1,935
    my house

    had a 30 gallon 29,000 btu standard hot water heater, with 3 students in the house, never had a complaint about hot water.

    at roughly 70% eff. that was a 21,000 btu DHW heater, not sure why so many people seem to think 50,000btu's are too small?
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    The Munchkin has

    an advantage, at least in the 80 size (and maybe in the 50 size?) in that it will boost output to 110 MBH (in the 80 size at least) for DHW production.

    As to how this affects heating during the DHW production time goes to mass of the building and radiation. Typically this is not a problem with most radiant floors, especially concrete slabs but also thin set systems. Cast iron radiators also do not tend to show any droop.

    Only on prolonged DHW production periods during the coldest weather and with say, copper-aluminum fin emitters might you notice some droop.

    The key principle is the large input (100% of boiler goes to DHW) which keeps the time away from heating pretty low, 20 minutes or so depending on sizing of the heater and draw.
  • zeke
    zeke Member Posts: 223

    I doubt that 29K BTUH will handle 2 showers at once or even one long shower.

    Assuming a delta T of 80 deg F and a 2gpm rate of usage, I get
    7.5*2*80*60= 72000 BTUH for 1 shower.With the 30 gal tank the capacity will be depleted After about 15 minutes of usage and the makeup of 29K BTUH will not be sufficient to handle the load of 72000 BTUH.

    Not to mention the loss of heating during this interum.
  • Mad Dog_2
    Mad Dog_2 Member Posts: 6,884
    My experimental trend has been leaning to smaller and smaller

    boilers and larger storage capacity for domestic with a mixing valve on the tank (encouraged in part by Mark & Darin when I was a little nervous about it). The only time I ever had a problem in boiler down-sizing was on Steam...that unforgiving Art. As Brad said, may have a temporary lull during the coldest day of the year during a long dom hot water draw. Most folks could live with that as long as they understand the upside of fuel savings the rest of the year. Mad Dog

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