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Converting existing radiant floor heat to electrical heat pump charged by PV panels

Larry Minden | Posted in GBA Pro Help on

I have a 3400 sq ft home with existing radiant floor heat (flexible water tubing in gypcrete between floor joists) powered by an old Lars Teledyne propane boiler. Winter temperature lows tend to be in 40s-50s, rarely dipping below 30 degrees F.
I plan to install a solar PV system and convert as much as possible to electrical heating & appliances.
I read lots about the new electric heat pump heaters and heat pump hybrid heaters but no one locally has any experience using them in this situation.
Please advise if using electric heating for a radiant floor is advisable and if yes, what are some good hardware/design solutions to achieve this.

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  1. Expert Member
    Dana Dorsett | | #1

    An electric boiler would be an easy retrofit, and if your heating loads are short the low efficiency of that solution might be "worth it", spending the heat pump money on a few more PV panels. Electric boilers are cheap & rugged, purpose made for the application. Water heaters are not, but some are capable of heating low load buildings.

    Heat pump water heaters located indoors doesn't improve efficiency, since it removes heat from the house that it's heating. Sanden's split system heat pump water heater might cut it, if your loads are low enough, since it's taking heat from the outdoor air. It maxes out at something like 15000 BTU/hr though, which isn't going to heat a very large code-minimum type house.

    To get to the optimal solution requires first calculating the heat heat load. Using an online Manual-J tool such as can work, if you're aggressive rather than conservative on all R-value and air leakage assumptions. With the load numbers and location (for weather and design purposes) the range of reasonable options will become clearer.

  2. Larry Minden | | #2

    Thanks Dana-
    My ignorance shows - should have been asking about electric boilers not water heaters.
    RE the heat pump option, I'd put it in the unheated crawl space under the house, so the issue of removing heat from the heated space should not be a concern.
    Will look for the original heat calcs for the house when built 25 years ago.
    The house has 10 ft ceilings so lots of volume - not going to get by with a low capacity solution.
    Location is Santa Cruz Co. California & temperature zone is 9b - annual minimums of 25-30 F.

  3. Expert Member
    Dana Dorsett | | #3

    Heat load is not a function of volume- if it's 2x6/R19 construction the relative air tightness makes a bigger difference than the higher ceilings.

    You can't heat a house by pulling heat out of the crawl space anymore than you can heat your house pulling heat out of the refrigerator. Like the refrigerator, as heat gets pulled out, the temperature in crawl space would plummet, dropping well below freezing pretty quickly under any significant heat load.

    If the heat load and seasonal energy use is big enough to rationalize the higher cost, (which may be the case for a 3400' house) there are reversible air source chillers available suitable for radiant heating & cooling at fairly high efficiency. eg:

    A well designed reversible chiller solution would use about 1/3 the amount of electricity that an electric boiler would.

    The 99% outside design temp in Santa Cruz is 38F, which is quite good for air source heat pump efficiency. Annual heating degree-day averages are in the 2500-3000 HHD/base 65F range, so it's not a heavy heating climate by any means.

    From the heat load numbers we can estimate the annual heating energy use and convert that to kwh of electricity used in an electric boiler, and how much PV panel it would take to offset that use, then estimate what it would be if using a reversible chiller instead of a boiler.

  4. Larry Minden | | #4

    Have a look at the pdf of original energy calcs I've just uploaded.
    Does this answer the questions you raised?
    Also, a question related to my original - if we use a reversible chiller, can this system also be used to circulate chilled water in the warmer summer months?

  5. Expert Member
    Dana Dorsett | | #5

    The accuracy can't be vouched for, but the typed/printed portion looks like an IBR or earlier version of Manual-J type load calculation.

    The "HEATING AND COOLING LOAD SUMMARY" section shows 15,422 BTU/hr of infiltration, which would be a pretty leaky house. The total heating load of 45,906 can probably be knocked back to 30-35,000 BTU/hr for sizing a radiant floor system, which would be about 10,000 watts of electric boiler, or 2.5 -3 tons of chiller.

    Yes, a chiller can indeed circulate chilled water in summer to provide sensible cooling. In your area that probably works 90% of the time with low risk of condensation on the floor, but you'd have to insulate all of the plumbing & manifolds to avoid dripping condensation elsewhere.

  6. Jon R | | #6

    > solar PV system and convert as much as possible to electrical heating

    If you want to do as much as possible with solar PV, then you probably want water thermal storage. Ie, a large water tank.

  7. Larry Minden | | #7

    Thanks loads Dana
    Any suggestions for a 3 ton reversible chiller?
    Also, regarding insulating the piping, it's really low humidity out here so not sure how much concern we'd have re dripping condensation. But will certainly bring it up to the contractor we work with.

  8. Larry Minden | | #8

    Please expand. If we go straight PV and no solar thermal, the large well-insulated tank would be used to store water heated by the electric heater/reversible chiller for use in the radiant floors?, or for domestic hot water? or both?
    trying to understand your precise point.

    1. Expert Member
      Dana Dorsett | | #9

      I suspect Jon R is referring to a larger tank to buffer the chiller's output, running the chiller and heating/cooling the tank whenever there was excess PV output.

      I've personally never designed a system around a chiller, but at your loads and outdoor temps the Chiltrix CX34 looks like a good fit. It depends a bit on the water temp requirements of your radiant system, which require a level of analysis not well suited to "design by web forum", but it probably makes it. You probably don't need water more than 95F to stay warm at your 38F outside design temp, temperatures at which it's good for at least 30,000 BTU/hr. (see p. 19 for capacity at different outdoor and output temperatures.)

      I understand how low the summertime humidity is in your area, but you'd be looking at water temps in the low 40s F or cooler on the warmest days to get much cooling out of a radiant slab, which is colder than your average outdoor dew points even in coastal CA. Until they repeal the laws of physics it will require insulated plumbing &/or controls that keep the output water temps above the dew point as conditions change, even in your area.

      1. Jon R | | #10

        Yes, a large water tank would allow you to create hot or cold water while the sun shines and then use it later with either a radiant slab and/or fan coils. Domestic hot water is also possible. But ignore the whole idea if you have favorable net metering (I should have said this earlier).

        You might pull it off with the help of a local with commercial hydronic experience. Plus the review of someone else (Dana? Possibly me).

        I come up with 50-60F water temp for cooling at peak load.

  9. Walter Ahlgrim | | #11

    Consider abandoning the radiant system and installing a few mini splits.

    Radiant floor cooling a delicate art form, and most HVAC contractors have no clue.

    My guess is a set of minis will cost 1/3 the price of a well engineered radiant system.

    Or spend that money on PV panels and an electric boiler.


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