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Community and Q&A

Best Scheme for Efficient Electric Radiant for Comfort in Vermont

J M | Posted in Mechanicals on

In Spring 2018 I am breaking ground on a high performance home in Vermont with polished concrete floors in the basement (conditioned and finished) and on the first floor. The home has a net heated floor area of just over 3000 sq ft with a design max winter heating load of 15000 btu/hr. It will have at least 10 kW PV and likely eventually 15kW total for electric car charging. Energy storage will not be installed until it’s cost effective (doesn’t seem like it’s anytime soon).

Everything in the home will be electric other than a woodstove for aesthetic (mostly) and emergency heat reasons – we’re considering the Blaze King Sirocco 30.1 cat stove which burns up to 30 hrs on low which is 12000 btu/hr.

It became clear that we would have to put radiant hydronic heat in the floors for the first floor and basement for comfort reasons as I’m not the only one living or designing the home (yes I understand it’s costly and pointless for anything but perceived comfort). The rest of the heat will be supplemented by Fuji XLHT(or Mitsubishi H2i) for making up the difference and upper level heating/summer cooling (for the entire home). The second floor is all wood floors other than the tiled bathrooms which may or may not have electrical mat for on-demand comfort.

I am trying to figure out the best way to accomplish this while remaining all electric.

I’m not sure the uncommon Chilltrix CX34 or Spacepak Solstice Extreme are systems I want to gamble on given how cold it gets in Vermont and the limited support I’d like get for either system. Unless you can give me a good reason otherwise.

Alternative designs for the first floor and basement hydronic heating are:

1-The new Sanden 3rd gen hot water heater in a combi application – was told that’s a Taco X-Pump Block and the new 3rd generation can supply more for spacing heating than the old one – closer to 10K BTU/hr although as a whole it’s 15.4K BTU/hr and this is four bedroom home design for use primarily by two adults and two kids (the fourth bedroom is a guest room). Water tanks range from 43-116 gallons.

I was told the COP of the Sanden CO2 system is 1.2 at 0 F. That seems unimpressive and it does get colder than that here so in those situations the heat pump would be about the same as simple and reliable electric boilers.

2-Instead could we buy two indoor-only heat pump water heaters, one for each use (DHW and radiant) in combination with an appropriately larger Fuji (or Mitsubishi) cold climate heat pump for heating and cooling. The water heaters can be ducted to the outdoors during heating season and ducted to the indoors during warm weather. Rheem Platinum Hybrid 3.5 EF 80 gallon sell for $1900 each and there’s $500 rebate on them from Vermont and the heat pumps are rated at 4200 BTU/hr

This has detailed specs for the Fuji which are helpful for efficiency:


It’s 2.43 EF at 5F with a maximum capacity of 21,600 BTU/hr (but it can be as high as 3.91 if it’s not running at maximum capacity at 5F) vs the Sanden which is about 1.5 EF at 5F.

At -15F the Fuji maintains 73% of max capacity which is about 16,000 BTU/hr.

A plus with the indoor heat pump water heaters is they have backup resistance coils for hybrid or high demand use so for very cold weather for days at a time they can be switched to hybrid or electric mode at the expense of efficiency.

A negative is the required input air but the shared mech/storage room they are going is nearly 3000 cu ft – 1000 minimum cu ft is recommended for one heat pump. The question is that suitable for two heat pump heater and/or will HRV (core will be changed to ERV once all the building materials and moisture dry up for comfortable year round humidity) help keep that mitigated with the constant circulation (we can also pull air from elsewhere in the house for at least one of the units)? Louvered doors can be used on the mech/storage room if needed but nearly 150% of minimum recommended for each unit seems ok, but it’s atypical for two to be operating in the same room sometimes at the same time.

Thoughts or recommendations?


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  1. J M | | #1

    BTW, the Rheem is 3.5 EF at DOE standard test conditions which is 58F for ambient temperature and water input. This will be installed in conditioned warm basement but the water temperature and delta will probably be from around 58F from the well. To me it seems for the expense and more wear and tear plus expense of having an outdoor Sanden wouldn't be more efficient even if we're moving heating around the house or am I missing something?

  2. GBA Editor
    Martin Holladay | | #2

    J M,
    You have a political problem, not a building science problem.

    The building science answer is clear: you should heat your house with cold-climate minisplit heat pumps manufactured by Mitsubishi or Fujitsu.

    The political challenge is to convince the other members of your family of the wisdom of this conclusion. Good luck.

  3. J M | | #3

    Not really political, some people have cold feet or poor circulation, I'm usually warm though. The Efficiency Vermont person we're working with said her feet always feel cold on concrete floors even in high efficiency/passivhaus buildings so that certainly doesn't help sell it to the family and some people are in fact not as comfortable. It's certainly a costly problem.

  4. J M | | #4

    I did try to sell the idea of tiled or wood floors with electric mats in select areas on the first floor for on-demand comfort use, no sale...

  5. GBA Editor
    Martin Holladay | | #5

    J M,
    If a slab has continuous horizontal insulation under the slab, and the house has a good thermal envelope, and the member of the family who is most likely to complain of being cold is allowed to set the thermostat, that person will not be uncomfortable.

  6. J M | | #6

    There's also this third option but it probably ranks only just above SpacePak or Chilltrix. Daikin Altherma 3 which isn't yet announced in the US:

  7. Ethan ; Climate Zone 5A ; ~6000HDD | | #7

    J M - I lived with radiant floor heat for a while, and the only spots where the floor actually "felt" warm, was at spots where coils had been placed too close and at the point where water was just leaving the boiler. On cold nights, tiles were still "cool" on the feet. I agree with Martin that you may want to do a bit more diplomacy before installing an unnecessary system...

  8. J M | | #8

    Hi John,

    Do you know of any experts to consult with?

    I found John Siegenthaler in NY but haven’t contacted him yet.

    Do you think the Rheem indoor solution or Sanden outdoor/indoor solution is best or do you have another efficient electric solution we should be considering?

    Would controls to prioritize use of the hydronic before using the ASHP be pretty straight forward to implement?


  9. Jon R | | #9

    Normally a low load house wouldn't be able to run full coverage radiant floors at a very high temperature (perhaps only 1 degree above room temp). But using a HPWH to move heat from the interior air to the floor means that you can run the floor much warmer (ie, more comfortable) before the air overheats. Such a plan will decrease, not increase the size of the ASHP needed to heat the building (because of waste heat from the HPWH). But DHW from a HPWH will require more ASHP.

    Evenly distributing cool air from the HPWH and the HPWH duty cycle are issues to consider.

    Hydronic systems allow for low cost heat storage.

  10. Stephen Sheehy | | #10

    My concrete floors seem comfortable enough for me to go barefoot much of the time, but a nice pair of socks warms my feet considerably more than heating the floor would.
    Radiant heat in the floor seems like old tech to me, from a time when the heat was always on in winter. It's 20° F and sunny here in Maine. My minisplits are hardly running. I'd be concerned that J.M. would spend a bundle on heated floors but not see much benefit.

  11. GBA Editor
    Martin Holladay | | #11

    I agree with Stephen Sheehy.

    You have listed a lot of complicated and expensive equipment, all of it unnecessary, and all of it intended to achieve a goal that will remain elusive: Floors that feel warmer than your feet.

    If you build a good house, the hydronic tubing will almost never have hot fluid flowing through them -- certainly not hot enough to give that "warm toes" feeling.

    Invest in a well-insulated thermal envelope. Then install some electric-resistance heating cables in the bathroom -- at a much lower cost than all of the air-to-water heat pumps you are thinking of purchasing -- where the warm toes feeling will occasionally be appreciated.

    For more information on this issue, see All About Radiant Floors.

  12. Stephen Sheehy | | #12

    When someone new walks into my house in the winter, the first comment always is: "Wow, it's nice and warm in here." The next comment, after they see the concrete floor, is: "I guess you have radiant heat." Then I explain the whole efficient building envelope concept and show the minisplits.

  13. J M | | #13

    I agree that radiant isn’t necessary for most people.

    I think the best strategy is to only use radiant in select comfort required areas of the first floor only in the 2” slab over the basement area so we can use higher temperatures for the same btus over a significant smaller area so there’s a noticeable temperature delta.

    This is obviously already has a good thermal envelope given only 15000 btu/hr max for 3000 sq ft heated floor space during the coldest time of the year in the mountains of Vermont.

    At our current location outside the mountains in Vermont this past week it was -11 one morning and -8F another.

  14. Jon R | | #14

    I don't know of a consultant to recommend but others might. I see no problems with controls. You could use a water-to-water heat pump and fan coils to move heat from various areas of the home to the floor. And such HPs would be rated for a high duty cycle. But it does seem like a lot of expense just to get warm floors (perhaps 5F above room temp - which is about optimal with shoes on).

  15. Jon R | | #15

    Select areas will help, but do the numbers. It may still only hit optimal floor warmth at design day conditions. And it may overheat closed off rooms (ie, rooms like a bathroom where you want high coverage and a warm floor). Rugs are a simple solution for bare feet and an electric foot warmer mat can cover a fixed position.

  16. J M | | #16

    With no effort on my part the radiant was eliminated. Makes things much more straightforward now.

  17. Steve Wolfe | | #17

    I don't want to create problems for you, but I have to say that I love the radiant heat in our concrete floors. I agree with others you can't necessarily say the floors are warm in most rooms. But in the bathroom, ours are definitely warm. Consider electric radiant there, but I"m not sure that you can do that under a shower floor and that is where we most love it. I have a natural gas boiler to supply the warm water so my situation is different.

  18. J M | | #18

    We’re still doing the mats under the bathroom tile but not for primary space heating purposes. Thanks

  19. Stephen Sheehy | | #19

    Our tile guy used Ditra heat under the tile in our bathroom and it works well.

  20. Walter Ahlgrim | | #20

    The luxury vinyl tile is most foot friendly floors in my home. This change cost less to build and does not run up the power bill. I say buy a box and dry lay them around where you are now I think you will be sold. The LVT is much warmer than the concrete / ceramic floors and noticeably warmer than the hardwood.


  21. J M | | #21

    Polished concrete is a required floor covering for the first floor and PVC is an undesirable material for many good reasons.

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