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Please poke holes in this scheme

Gordan Member Posts: 891
My goal was to get the most out of a high-efficiency low-temp heat source (hydronic GSHP) while providing proportional (or full, if the GSHP breaks) boiler backup. The 30kBTU/hr GSHP would be used for the bulk of the space heating (design load is around 50kBTU/hr) and DHW preheat; in summertime, the GSHP should be able to provide full DHW heating most of the time. The GSHP would maintain its buffer tank at a temperature controlled by outdoor reset during the heating season (and about 120 deg. F during the cooling season.)


The boiler is an existing 136kBTU/hr DOE output atmospheric high-mass cast iron beast. It would maintain its buffer tank at the minimal temperature required to avoid condensation, with a wide-enough differential to ensure infrequent and long runs.


The system circ is a dP-controlled variable flow circ. Zoning is via TRVs. The emitters are radiant floors and ceilings, with a design return temp in the sub-100 deg. F range. System supply is mixed by an outdoor reset controlled 3-way mixing valve between the GSHP and the boiler. All system return is to the GSHP buffer tank. The return to the boiler buffer tank is via the GSHP buffer tank, and it is piped in reverse to provide tempering. The buffer tanks are reverse indirects, and they provide DHW (pre)heating. The DHW output is piped in basically the same way that the system supply is.


Does this concept, and the layout, seem sane? Thanks in advance for a reality check!


  • Gordan
    Gordan Member Posts: 891
    Two possible enhancements...

    1. An ESBE 3-way thermostatic bypass allowing me to dial in the max system return temperature that will still go through the GSHP buffer tank. This would effectively remove the GSHP as a system heating source if the system return ever exceeds that temp (but it would still be available for DHW preheat.) Alternatively, a motorized 3-way bypass valve could be used, controlled by a diferrential setpoint control, to only return system water to the GSHP buffer tank if it is cooler than the water already there.

    2. An ESBE 3-way thermostatic mixing valve on the boiler return, as boiler return protection. This would allow a wider differential on the tank.
  • Paul Pollets
    Paul Pollets Member Posts: 3,451
    4 way vs 3 way

    A 4 way mixing valve provides boiler protection, needed for cast iron boilers; the 3 way does not.
  • Gordan
    Gordan Member Posts: 891
    Paul, thanks for your input - can you please elaborate?

    I'm not using the 3-way valve to mix system temp in this case; I'm using it on the boiler return from the buffer tank as in this updated diagram, where it's mixing in some of the boiler supply to limit the return temps. What am I missing?

    Thanks again...
This discussion has been closed.