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On demand combi boiler or an 80 gal HWT?

We're about to install radiant heat in a 5-bed, 3 bath, 3 story house in California ( mild climate). We have gotten a couple of different suggestions on how to approach the equipment selection.
One is - go with 199BTuh combi boiler, it will be fine
The other - only an 80 gal water tank system will be able to supply enough hot water for the radiant heat and DHW.
What do you all think?


  • gennady
    gennady Member Posts: 835
    Correct me if I’m wrong, does someone offer you water heater to heat the house?
  • Jamie Hall
    Jamie Hall Member Posts: 18,927
    gennady said:

    Correct me if I’m wrong, does someone offer you water heater to heat the house?

    That's the second option @divarani laid out. 80 gallon water heater for the DHW and radiant.

    This is California, and I guess they do things differently out there, but that is one option -- and the contractor who suggested it -- that I'd run away from, fast. They are almost certainly suggesting an open system -- combining the DHW and radiant -- which is a recipe for lamentable water-borne illness, never mind other problems.

    On the other hand, the combi will be so exceedingly overkill for the radiant heating that it won't be happy either, though it will work if properly installed, mixed and controlled.

    I'd much rather see and independent very small boiler for the radiant, and an on-demand (or maybe two or three smaller ones) for the DHW.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Ironman
    Ironman Member Posts: 6,868
    Was a load calculation or any design work done? If not, you're headed for trouble. Radiant floors and Hydronics is not something that yields to opinions; it requires facts that come from proper calculations. A scientific load calc and proper design is your first step and the foundation for everything.

    That being said, a water heater that provides domestic should never be shared with space heating unless the floor is isolated by a heat exchanger (think Legionella). Even then, it's a poor and inefficient choice.

    A 199k btu combi is probably gonna be drastically over-sized for space heating and will short-cycle itself to death unless there's a lot off buffer in the system,

    Without knowing more facts about the job, a properly sized mod/con boiler + an indirect water heater is the general recommendation.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • divarani
    divarani Member Posts: 2
    Just to clarify - there is no suggestion of using an open system -- combining the DHW and radiant. The water from the radiant pipes will never be delivered back to the hot water tank in either scenario.
    The combi boiler suggested is EFTC -199 Btu/h:
    HVACNUT Member Posts: 4,592
    Can you lay it out a little better? Maybe a diagram of the near boiler piping.

    Are you saying that both choices get the combi, with an option of adding an 80 gallon buffer tank for the radiant?

    The contractor should have plans drawn for the whole job. With a heat loss calculation, radiant layout, remote manifold stations, etc. If not, then...