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Using a tankless for comfort radiant heat

Mrfoamy Member Posts: 4
I am considering adding pex radiant flooring over my subfloor for comfort, and in mild weather it could be my primary heat. We are in the Los Angeles area so winter nights seldom go below 40 degrees. My thinking is that a tankless domestic hot water heater that is slightly oversized might provide radiant comfort while the existing furnace would be the thermostatically controlled primary heat source in the coldest months. This same tankless would provide my domestic hot water.

I see in previous posts that using a tankless instead of a boiler is frowned upon by many, but how about in my non-critical comfort application? I would prefer to approach an expert with any general advice that can be provided here, rather than proceed blindly. Thanks in advance for your thoughts.


  • AFred
    AFred Member Posts: 81


    If you have read up on this like you say, you wouldn't be asking about this, you would already know better.

  • Mrfoamy
    Mrfoamy Member Posts: 4
    Comfort Radiant Heat recommendations

    Thanks, but I was hoping for a more specific recommendation. I could spend a few hours reading and still wonder how much advice would apply to Southern California. Or I could approach a few contractors in great ignorance.
  • SWEI
    SWEI Member Posts: 7,356
    existing furnace

    would likely become the secondary heat source if I were doing this.  You need to begin at the beginning, which is a heat loss calculation for the house.

    Tankless water heaters are not heating boilers.  They are designed to heat cold water to a fixed high temperature on an intermittent basis.  Their efficiency plummets when the inlet water temperature increases and they do not vary their output temperature based on load.

    Hydronic boilers heat less warm water to more warm water on a 24 x 7 basis, and adjust their output water temperature to suit the outdoor air temperature.  While it is possible to make a tankless provide the heat for this task, the additional cost of a heat exchanger, buffer tank, outdoor reset control, and motorized mixing valve will easily negate whatever first costs you saved over buying a proper boiler.
  • Mrfoamy
    Mrfoamy Member Posts: 4
    More expensive than you might think

    Thanks for that excellent feedback. It makes sense to have a system properly engineered but that may make the entire system unaffordable. The location I had in mind, on a side of the house with gas and water service, would be close to building code limits but ok for a slim tankless unit. A larger boiler may not be allowed there, so running a gas line and water lines to a more remote location adds more cost. If I can fit a boiler into the ideal space that should help. Clearly I need to research this approach.
  • SWEI
    SWEI Member Posts: 7,356
    Current system

    How old is the existing furnace?  How efficient?  How comfortable?  How noisy?  What are your annual heating bills like?  How about domestic hot water?  Any plans for solar?

    It's worth your time to evaluate the whole system and see what changes you might want to make at what time(s.)  If you want to place floor tubing for a future system upgrade or square footage expansion, it may not cost all that much now but could get quite pricey later.

    The collective knowledge here runs pretty deep, so ask away...
  • Mrfoamy
    Mrfoamy Member Posts: 4
    The collective knowledge here runs pretty deep, so ask away...

    You might regret inviting me to ask away...but here goes.

    The actual situation is more complicated. My house is a 1900 sq. ft. 1965 ranch that I am remodeling / insulating so when I’m finished it will need between 2 and 3 tons of AC and perhaps 40,000 BTUs of heat (it will be energy modeled). It is potentially my retirement house so spending extra to do things properly is desired, although at some point I might skip hydronic altogether - make sense? Hydronic is purely for comfort and it can never pay for itself.

    I will hire an architect soon and when designs are finalized it makes sense to have an engineer specify a radiant design. My goal is to have a fuller understanding of my vision before consulting these professionals.

    I want to update the old and noisy A/C, replace my low-quality water heater and eliminate some unsightly HVAC duct chases in the remodel, as well as move my washer/dryer indoors from the garage. My plan is to replace the 50 gallon hot water heater located in the hall closet with a boiler/water heater hanging on an outside wall. My HVAC in that same hall closet would also disappear to be replaced with a Mitsubishi multi-split ducted system heat pump (that indoors has only ceiling cassettes and linesets). The washer/dryer will move from the garage to the resulting freed-up hall closet space.

    The setback clearances and site plan are constraints for an addition, but a very large consideration is Prop 13 in California. I want to maintain my current low property taxes so I need to be careful not to make too many "improvements". Replacing existing equipment is not an improvement - for property valuation - yet creates a “free” 20 square feet for the laundry room in this case.

    All this work (and more) would be permitted and inspected and the resulting property tax increase minimal. Sometime later, after all inspectors are finished, I might install the hydronic using the “Sandwich Method with Plates on Top of Subfloor” as a DIY project for the PEX and sleepers, and let an expert connect into the boiler/water heater previously specified by my engineer. At this point the heat pump would be purely backup heat since electricity is pretty expensive in CA.

    Originally I figured the hydronic would be for comfort only and the heat pump would provide the primary thermostatically controlled heat so a marginal tankless water heater would be ok. Alternatively, with an engineered hydronic system the Mitsubishi heat pump could be used strictly for A/C and the hydronic be the primary heat source. My understanding is that the Mitsubishi inverter will provide super quiet, zoned comfort for A/C and ditto for the heat supplied by the hydronic.

    If you feel that any of this is mistaken or have any suggestions I would be eternally grateful.

    Hopefully I did not make this explanation overly complicated.

    (Eichler) with a lot of clerestory
  • SWEI
    SWEI Member Posts: 7,356

    will benefit from envelope improvements for sure.  Depending on what SoCal location, you might be able to run the whole place on a heat pump and use radiant cooling -- or some combination of hydro-air and radiant panel.

    Don't confuse (or let your design professionals confuse) hydronic with radiant, and remember that there are numerous variations of both.
This discussion has been closed.