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Indirect HW Tank v. Continuous/Flash DHW

Doug_3 Member Posts: 14
....but which is more efficient? Every time there's a call for dhw, the boiler will have to run if there's no indirect. In the case of a storage tank, maybe when just a little bit of dhw use, the boiler won't be called to heat the cold water replaced into the storage tank? In this sense, maybe the boiler would run less often and thus, less energy would be used? If a customer can live with the practical limitations of continuous/flash/instant dhw, is that all they have to live with, or must they absorb significantly larger energy use costs as well if they forego the indirect?


  • Doug_3
    Doug_3 Member Posts: 14
    I'm assuming there's also a difference between...........

    ...........the old style "coils" used in oil boilers and the newer technology used in the (mostly gas and some oil--Quietside, for one) boilers which showcase the convenience and efficiency of space-saving all-in-one design for space heating and dhw production in their continuous/flash/instant/combi units.
  • Duncan_2
    Duncan_2 Member Posts: 174
    Significantly larger energy costs.

    I went to a Paloma (on demand water heater) site, and they listed a 37,700 to 178,500 input unit as being able to produce 2.85 gallons per minute at a 100° rise. Pretty big, with a 7" vent.

    2.85 gal/min x 60 min/hr x 8.3 lbs/gal x 100°F rise = 141,930 btuh output.

    Output/input = efficiency, or 141,930/178,500 = 0.795 . About 80% efficient.

    Boilers are usually about 82% efficient. Then you have slight piping, mass, and storage losses for a boiler and indirect.

    So you're comparing 80% with low losses to 82% minus some small losses if my math and the numbers are right.

    So I'd say no, continuous/flash/instant water heater users don't incur significantly larger energy use costs, probably less. But the giant Paloma probably costs more up front and in maintenance costs than the indirect, over the long haul.

    I think replacing a standard low efficiency storage water heater with an on-demand heater might have a decent payback in fuel savings over the long haul. The on-demand would have to have a pretty good run, though. It depends on the price of fuel, too.
  • Mike Kraft
    Mike Kraft Member Posts: 406
    So Duncans answer is.................

    It Depends :)
  • Duncan_2
    Duncan_2 Member Posts: 174
    One sentence answer?

    The on demand is cheaper to operate, but cost more in maintenance and repairs.

    I couldn't get it down to two words like you, cheese. You pretty good, mistah >:-
  • Doug_3
    Doug_3 Member Posts: 14
    Thanks Duncan and Cheese.

    The models I'm comparing are basically three wall hungs available as combi units: MZ, Trinity, Quietside used alone, versus using a heat only boiler with an indirect.
  • Roy Woolley
    Roy Woolley Member Posts: 1
    wall hung combi boilers

    Hi Doug,

    Take a look at the other kid on the block!!


    The BAXI Luna boiler has been availlable in the US and Canada for 4 years, but has been operating in Europe for more than 10 years. Product and parts are availlable throughout North America.

    Give us a call if you need more info.
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