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Community and Q&A

Hydronic Radiant Floor With Heat Pump

JesterBlackDog | Posted in Green Building Techniques on

Hello, I’m designing a new home and want hydronic radiant floor heat but am trying to be carbon-free so no gas-fired water heater.  Can it be done with an air heat pump in a zone 5 (central NY) climate?  Or would it have to be a hybrid heat pump+ electric supplement?  Thank  your for any guidance.

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  1. charlie_sullivan | | #1

    Can definitely be done!

    Here are some articles on air-to-water heat pumps in general:

    The good news for you is that of all the different types discussed, a radiant floor is the easiest way to get sufficient heat output with very low water temperatures, which will run the heat pump very efficiently. And Zone 5 is no problem--a lot of that discussion is Vermont, largely Zone 6.

    Just be aware that a luxurious feel of warm tile on bare feet is not the typical result in a well insulated house--you simply don't need as much heat as an 80 F floor will deliver. And a 72 F tile floor can feel cold with bare feet. A wood floor will be more comfortable when it is running at low temperatures.

    But regardless of tile or wood or some other material, you'll get the benefits of the lack of clutter and lack of constraints on furniture layout that you get from floor heat, and you'll get the potential for very low water temperatures and thus high efficiency if the design is done right.

    Consider adding slim fan coil units, e.g. from Chiltrix, to add air conditioning capability.

    1. JesterBlackDog | | #2

      Thank you for that comprehensive reply... very helpful to me. I will check those links. I have read in other blogs about super insulated homes getting cooked by radiant floor never made sense to me... isn't that what thermostats control? I could see a problem with thermal mass in a fast-changing temperature situation like in a desert environment with 30+ degF swings everyday but not in central NY winter time...

      1. charlie_sullivan | | #4

        It is true that control is sometimes tricky, particularly if the heat is in a thick slab. There's a long delay between the heat being delivered to the slab, the slab warming up, and then finally the air heating up. You might have the slab up to 90 before the thermostat turns off, and then all that heat comes out of the slab into the room even after the thermostat kicks off. But there are controls that can avoid that problem, through a combination of a slab temperature sensor, some intelligence, and an "outdoor reset" that sets the water temperature according to the outdoor temperature. When you get it right, it can maintain a very constant, stable, well controlled temperature. People think that the thermal mass of the slab is important for that, but it's really not, and it some ways it's easier without it.

        Generally the designer of a hydronic system has a lot of flexibility. That means there are a lot of ways it can go wrong, but it also means you can get it to do exactly what you want if you have a good designer. It can work best to hire an expert designer, likely not local, and a local installer.

        1. JesterBlackDog | | #5

          Thank you Charlie for the additional info. I plan to use something like a Warmboard subfloor with engineered wood on top and basement below. So no slab, no massive thermal mass. Good controls will still be key and you informed me on some. I'm wondering if there are any that actual monitor the forecasted weather (via WiFi) to be 12 hours ahead of temperature changes instead of monitoring current outdoor temperature and reacting to only that. That would be great!
          Warm floors are not my biggest attraction but rather the eveness across the area and the stability across time. I'm in a forced hot air standard house now and can't wait to get away from the cycling "too hot" to "too cold" and very little "just right"!

          1. charlie_sullivan | | #7

            I haven't heard of anything that uses forecasted weather. But there's some time delay for the interior of a house to react to the outside temperature, so that's really not needed.

          2. Expert Member
            NICK KEENAN | | #9

            The WarmBoard website boasts of "faster response times." When radiant floor heat first got big around 30 years ago, a lot of the early literature claimed that "thermal mass" was necessary for it to work. This was exactly wrong. The biggest issue with radiant floors is uneven temperatures, what you need is something with high heat conductivity. Concrete worked well because it conducts heat well. But aluminum works even better because it holds little heat and responds quickly. The WarmBoard system is a layer of aluminum over MDF.

  2. Jon_R | | #3

    While it isn't ideal for efficiency, consider only installing the floor heating in areas where you will walk on it. With too much area, the temperature will almost always be low and probably won't supply the "warm floor" comfort that you are expecting.

    Don't be afraid of a little bit of supplemental electrical heat that only runs when it's super cold - it won't run enough to be a cost problem.

    1. JesterBlackDog | | #6

      Hi Jon R, that's good advice. For cooling, I may use min-split wall mounts and those could double as my supplemental or "fast acting" heat when needed.

      1. charlie_sullivan | | #8

        I think it would be cheaper and more efficient to use the air-to-water heat pump to supply chilled water to mini fan coil units in the summer.

        1. Expert Member
          NICK KEENAN | | #11

          Fan coil units also have the advantages that they come in much smaller capacities than mini-split heads, and you can throttle them down without issues. The biggest drawback of minisplits is that the smallest head is way too big for a room in a well-insulated house. Running them at part capacity gives a real efficiency hit and may not result in proper dehumidification.

          1. JesterBlackDog | | #12

            Thank you DCContrarian. Do you have some suggestions for me to get educated on fan coil units? I'm not familiar and the concern with min-splits you cited is exactly my concern. My first floor is fairly open but the bedroom area is always a problem

          2. Expert Member
            NICK KEENAN | | #14

            Chiltrix has a pretty good roundup:

  3. BirchwoodBill | | #10

    We remodeled with Warmboard 2 years ago and replace our boiler with a heat exchanger and a hot water tank. In zone 6, i have the heat exchanger on OTDR and it runs at 93F. Next house will be warmboard with a hi velocity air handler and air to water heat pump.

    1. JesterBlackDog | | #13

      HI William, what is "OTDR"?
      I am heading toward warmboard with air to water heat pump for the new house I'm designing.
      ERV will take care of humidity.
      A ductless mini-split will take care of cooling on the first floor which is quite open. However I'm concerned how to cool the bedrooms effectively... maybe a ducted mini split upstairs.
      I am on the border of zone 5/6
      I just hope the cost is not out of control for all this.

      1. BirchwoodBill | | #15

        Outdoor temperature reset. The temperature of water going through the hydronic units will vary based on the temperature outside. For cooling, the chiller water from the heat pump is controlled by dew point. Make sure your pex tubing for chiller water going to the mini splits is insulated otherwise you can get water damage. But an air to water heat pump should provide heating, cooling, dehumidification and DHW.

        Read Allison Bailles article on humidity control with a ERV. (Energy Vanguard is the company).

        Warmboard is a premium product, but it is very comfortable. Much cleaner look to the house and uniform temperature. No regrets and the energy bill is lower on a monthly basis.

        1. JesterBlackDog | | #16

          William, how can the air-to-water heat pump provide cooling? From what you are saying a single air-to-water HP would take care of all my needs which is exactly what I'm trying to confirm. Other articles I ready here tend to steer people away from floor radiant because for cooling, you end up with a mini-split air-to-air solution anyway which means double HVAC systems, double $$. I would like to do everything with an air-to-water HP plus an ERV or CERV and no mini-splits or ducted splits etc.

          1. charlie_sullivan | | #18

            The air-to-water heat pump chills the water. You then run that chilled water to slim wall-mounted fan-coil units like the chiltrix ones. They are sort of like minisplit heads, but they are slimmer, and the small ones are much smaller. And they are cheaper. So it becomes practical to put one in each bedroom. In some ways, they are a lot like panel radiators, but they do more convection, so they are sometimes called "emitters" rather than radiators.

            If you are in climate where heating and cooling loads are similar, you can skip the floor heat and just heat with the fan coil emitters. But you can also just shut them off for the winter and heat with floor heat. Or you can do a combination for heating, with or without using the fans.

          2. AlexPoi | | #19

            You can do radiant cooling if you add a whole house dehumidifier and a controller that can take in account the humidity level like the tekmar 406. An ERV will help your dehumidifier but it's sure is not a replacement for it.

            Otherwise, you can add air fan coils to to handle the cooling like it was already mentionned.

      2. Jon_R | | #17

        > ERV will take care of humidity

        You will find that an ERV helps, but isn't enough.

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