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Anyone have any opinions of the space heating & DHW Greenstar Combi 151 for boiler & domestic HW?

Elizabeth Croes | Posted in Mechanicals on

Need to be able to take showers consecutively for family of 4 … and fill soaking tub

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  1. GBA Editor
    Martin Holladay | | #1

    That is a wall-mounted gas-fired combi boiler. It is used for a combi system with hydronic heat distribution. I imagine that the boiler could also be used with a forced-air heat distribution system if you hooked it up to a hydonic coil in your air handler.

    Combi systems use one appliance to provide space heating and hot water. While common in Europe, they are relatively rare in the U.S. -- in part because hydronic heat distribution is much less common here. (U.S. builders prefer to install forced-air systems, which are easier to integrate with central air conditioning.)

    The main disadvantage to installing an unusual European appliance in the U.S. is that when your system develops problems, local HVAC contractors aren't able to provide troubleshooting or repair services.

  2. Expert Member
    Dana Dorsett | | #2

    The minimum input on that thing is 36MBH, which is on the high for being able to run low domestic hot water flows- expect the thing to not regulate temperatures well at low flow, or even turn the flame off in summer when the incoming water temps are higher.

    36MBH is also above the design heat load of many homes (it is about the heat load of my ~2400' 1920s antique at 0F outdoor temps.) Installing it in any home requires sufficient radiation (or radiation of sufficient thermal mass) to be able to emit over 30MBH at condensing temps. If your heating system is broken up into multiple zones, those conditions must be met for each zone separately. At 120F average water temps (a good condensing temp at which it might actually achieve it's 95% nameplate efficiency) that would take about 150' of fin-tube baseboard.

    Without a heat load calculation (preferably room-by-room) it's impossible to say if this (or any other heating device) would be a good fit to your actual heating and hot water needs.

    Taking four showers in sequence doesn't have to take a monster-sized burner, even in cold-water country. At 2gpm flows a condensing 75,000 BTU/hr burner keeps up forever, and if you have the vertical space for a decent sized drainwater heat exchanger downstream of the shower even a non-condensing 40,000 BTU/hr burner keeps up forever. Sizing hot water storage for the largest tub you have to fill, and using drainwater heat exchangers for maxing out the showering efficiency & capacity is usully the "right" solution for the domestic hot water end. If that hot water storage is an indirect tank fired off a space heating boiler or a standalone tank, or a condensing hot water heater with ports for a space heating loop for the heating end, or something entirely different depends on your actual heat loads and heat emitter type(s)/size(s).

    Combi-boilers are rarely ideal, even where they're applicable, but maybe... Only a competent hydronic heating designer willing to do the math would be able to say whether this combi heater has any merit for meeting your loads.

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