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Community and Q&A

Is anyone using radiant wall heat?

user-7631340 | Posted in General Questions on

I’m building an 1,800 SF, 3-bedroom, well-insulated home in Massachusetts (5A) and we can’t decide on a heating system. We’re building with 12″ double-stud walls, 18″ roof with thermal breaks, and dense-pack cellulose throughout. We don’t expect the heating loads to be very high, and aren’t that interested in AC. We’re planning on some solar HW panels and some PV. We’ll have a wood stove (Aspen C3) that we will probably use often, but not all the time.

We started off planning on radiant floors, then switched to mini-splits after a round of “value engineering”. However, we don’t love mini-splits, mostly because of the noise and because we’re trying to avoid using refrigerant (you know, for the environment). In researching options, I’ve come across some examples of radiant wall heat and it makes a lot of sense to me. It seems comfortable, easier to install/maintain/repair, and brings up other interesting ideas. However, in 14 years of building I’ve never encountered it.

The first question is: is radiant wall heating a good option for us? In that are all the expected questions: Does it work well? How much does it cost to install/operate? Which systems work best?

The second question is: can I use it for thermal storage? We’ll have the solar HW, so I love the idea of optimizing it by “banking” heat while the sun shines. I’m considering making ~2″ concrete radiant wall panels with tubing cast-in to act as the radiant wall heat. The panels would be visible, decorative, and (hopefully) functional. I have ready access to a professional concrete form shop (mostly used for countertops), so I’m not concerned about the logistics of making the panels; I just don’t know if they would work as intended.

Oh, and we’re pouring the foundation this week, so we’re down to the wire on deciding what system to go with. Any and all input is greatly anticipated!

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  1. walta100 | | #1

    Depending on the equipment and current outdoor temp it will cost 2 to 6 time more to use resistance electric heat than a heat pump.

    Solar thermal has proven to be mostly a pipe dream. It turns out most of the time the house is too hot or too cold and rarely just right. Lots of people have been claiming they have the secret recipe since the 1970s if that were true it would have dominated the market long before now.

    You may want to read this article given the current pricing on solar panels it makes more sense to install electric solar panels and electric water heater.


    1. user-7631340 | | #4

      Thanks for the input Walter. I am consistently getting the feedback that solar thermal for space heating is indeed, a pipe dream. It does sound great, but I'm increasingly realizing the reasons it doesn't make sense.

      That said, I do still see the benefits of solar thermal over PV for DHW, especially in our case where roof space is at a premium. The roof plane that we have with good exposure is not large enough for all of our electricity needs, no matter how we cut it. With solar thermal being more space efficient, it seems like a clear decision for us. If we had more roof space, it might be a different story...

  2. Expert Member
    NICK KEENAN | | #2

    There's no theoretical reason that your radiant surface couldn't be in the wall. Floors have a practical problem that you can't avoid touching them, so you're limited in how warm they can be, which limits their heat output. I think the practical problem you'll run into is that you'll want to put your heating on the exterior walls, and typically all of the available thickness of an exterior wall is used for insulation. It's easier to hide that extra inch in the floor or even ceiling.

    You don't say what kind of heat source you want to use for your radiant heat.

    Thermal storage is a mirage. Your house isn't big enough to store enough heat to make a dent in your annual energy bill. Plus, it makes your house less responsive to the thermostat and therefore less comfortable. If you look at the high-end radiant systems like WarmBoard, they now boast about their quick responsiveness. When you turn the heat on you want heat now, not in a few hours.

    Concrete is a material whose use should be minimized in construction. Its production has a devastating environmental impact. If you insist on trying to create thermal storage, it's not even particularly good at that, having a lower specific heat than most common building materials.

    1. user-7631340 | | #5

      Thanks for the input DCContrarian. For new construction, the space consideration with the insulation seems easy to navigate (we just make the walls a little bigger where needed). For retrofits, I definitely see that being a constraint though.

      I also appreciate you bringing up the point that concrete should be avoided where possible. It is spot on, and a habit I still struggle to break sometimes, having developed an appreciation for the beauty and performance of installed concrete.

      I'm still curious about thermal storage, mostly because I haven't found the resources to do the math and see how it works out. I'd be very interested in any hard data on the issue. I've looked a few times and haven't come up with anything.

      As far as heat source, I had the notion of an air-to-water heat pump. (Much less refrigerant going 5' to the water heater, rather than all over the house.) However, my HVAC installer is disabusing me of the notion. According to him, the technology is coming, but it's not reliable yet. I'll definitely be watching that develop, though it doesn't seem like it'll go in our house...

      1. Expert Member
        NICK KEENAN | | #7

        This recent thread had a discussion about thermal storage:

        I'm a thermal storage skeptic and I made the case that it's not effective. If you see a flaw in my logic please let me know.

        1. user-7631340 | | #8

          Thanks for that share. Definitely some interesting conversation. I see what you mean about thermal mass failing to be comfortable, even if it was designed to have the theoretical capacity to store that heat.

          I definitely see the failing of solar thermal for space heating, in pretty much any residential application. What do you think about solar thermal for DHW and, in particular, when solar real estate is limited, given the higher output/SF as compared to PV?

          1. Expert Member
            NICK KEENAN | | #9

            If you live in a place with net metering PV will always win. The big problem with solar is the variability, both from day to day and season to season. Where I am, I get over three times as much insolation in June as I do in December. But the efficiency of solar thermal depends on the outside temperature, so I actually net out four times as much energy per square foot in July as in December. So if I'm doing DHW, do I size the array for July and have it only meet a quarter of my needs in December, or do I size it for December and have it be four times as big as it needs to be in July?

            With net metering, 100% of what I produce is used, every day of the year. It more than makes up for the efficiency difference.

            PV systems get cheaper every year. It's getting really hard to buy solar thermal systems any more. Although I do see people giving them away on Craigslist...

  3. Expert Member
    NICK KEENAN | | #3

    I found this article very influential, it has detailed instructions for building ceiling radiant panels under drywall. He says the same construction can be used on walls.

    1. user-7631340 | | #6

      Thanks for this too. I appreciate the calculations provided. I find myself feeling skeptical about ceiling radiant, though I'm still digesting. It just seems counterintuitive to heat from above. For instance, we will have a "walk-out basement" as we are building into a steep hill. It will be completely unfinished/unconditioned initially, though we likely will finish it down the line. The ceiling height will be 14', and there's a concrete slab for a floor. Of course, this is not a typical situation, but I can't imagine heating the ceiling in a room like that.

  4. Expert Member
    BILL WICHERS | | #10

    If you want to try thermal storage, the easiest way to do it is with a water tank — typically a big inebriate (250+ gallons). You can calculate how much energy it can store by working out an allowable “delta T”, which is the difference between the highest temperature your heat source can raise the tank temperature to, and the lowest temperature your heating system can acceptably heat your house with (the temperature required for the minimum required BTU output). You then take that delta T and use both specific heat of water to calculate how many BTUs (or any other unit of energy you want to use) you can store in the size tank you have. You can also flip things around and calculate how big of a tank you need to store whatever amount of energy you require.

    There is a lot of info on thermal storage in hydronic systems in the outdoor wood boiler forums on People with wood boilers can greatly increase their fuel efficiency by adding thermal storage to their systems, when they also reduce the amount of work they have to do throughout the day to keep their system operating.


    1. Expert Member
      NICK KEENAN | | #11

      The difference is that the efficiency of a wood boiler doesn't really depend on the water temperature. (And even if it did, with wood you often are generating more heat than you can use anyway.) The efficiency of a solar collector depends greatly on the delta between the water temperature and the ambient temperature. It's another reason solar thermal never really caught on.

  5. BirchwoodBill | | #12

    You can use warmboard r in ceiling, the walls or floor. It can be connected to a heat pump or other hydronic source. I run my hydronic system off my DHW tank through a heat exchanger. Cooling is also an option if you add in the dew point controls. The Messina system looks like drywall panels.

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