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Community and Q&A

Basement radiant heating

rydogg117 | Posted in Energy Efficiency and Durability on

I was considering adding a gas fireplace to heat up the basement in our tri-level home in Woodinville, as part of a larger basement remodel, but I’m starting to think maybe radiant heating (with a natural gas fueled hydronic pump) is going to be a better way to go.

Our basement is really just one large room, 20×20 feet. I’m considering an above-slab PEX and insulation system that would create a false floor. The hideous commercial ceiling tiles (drop ceiling) will also be removed and replaced with drywall and can lights, which I’m hoping will recover the overall height that would otherwise be lost to a false floor.

I’m not sure whether to go with natural gas or electric for the boiler, as electricity prices are quite low in this area, .098 per kwh and natural gas is 0.367520 per therm. We already have a natural gas furnace for the forced air throughout the house, but it really isn’t adequate to heat the basement (which only has ceiling vents at this time). If we went with natural gas, I’m also not sure about the BTU range I would need for a boiler. Keep in mind I would only be heating this basement area; the furnace is still going to heat the rest of the house.

Any recommendations for the best products to use in this scenario? And the rough cost range if I did it myself? Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    Ry Dogg,
    First, the fuel cost analysis. To create 100,000 BTU (=29.3 kWh), the cost is:
    (a) Using an electric-resistance boiler, 29.3 * 0.098 = $2.84
    (b) Using natural gas burned in a 90% efficient furnace, 0.90 * 0.367 = $0.33

    So the natural gas is about 9 times cheaper than electricity.

    Now, to your basic question: It would be highly unusual if your existing furnace were undersized. Almost every residential furnace in the U.S. is at least twice as big as necessary to handle the load. So the first order of business is to see whether your existing furnace can heat your basement.

    Probably, this is simple a ducting issue. Some duct corrections will probably make your basement toasty.

    It's possible, however, that all you have do to is some air sealing work in your attic or basement to reduce your home's total heating load.

  2. Expert Member
    Dana Dorsett | | #2

    Are the basement walls/foundation insulated? A major fraction (usually more than half) the heat load of an uninsulated basement is the above-grade exposure on the foundation, not the slab. If the walls are insulated but the slab isn't, the slab + windows are the bigger heat loss than the walls, but that loss is miniscule.

    This is probably NOT a duct issue- it's far more likely to be a zoning issue.

    The design heat loads of insulated basements are VERY modest compared to fully above grade conditioned space. But that low load also doesn't track with conditioned space loads. Basement walls don't see nearly the same swings in temperature as above grade walls, but basements also get very little solar gain. Those difference make it hard to get a satisfactory result when operated as part of a zone with the thermostat on an above-grade floor.

    A radiant floor is a bit overkill for most basements, and quite expensive if it's an above-the-slab system. European style flat panel radiators sized correctly for the load, running of the water heater (isolated with a heat exchanger) would be a fraction of the cost, and nearly as comfortable as a radiant floor.

    Radiant ceilings (also running off the water heater) as part of your new ceiling would be more expensive than panel rads, but still far cheaper than an above-the-slab radiant floor, and more efficient since there is no parasitic loss to the soil.

    Getting to the optimal heating/zoning solution starts with a careful Manual-J heat load calculations, using aggressive rather than conservative assumptions (the way it's suppose to be done, as prescribed in the Manual.)

    As lousy as rules of thumb are, a 400 square foot insulated basement is likely to have a total heat load less than 5 BTU per square foot, 2000 BTU/hr total. A load of 5 BTU per square foot is such a low load that the temperature difference with a radiant floor can barely be felt- it's very subtle even in your socks, even at the 99% design load, which is when is't 20-25F outside, in your neighborhood. (Yes, I know it gets a lot colder than that outside sometimes, but it's less than 1% of the time.) You'd barely feel the radiant ceiling too.) Even a single 2' tall x 4' wide panel rad (<$400) would likely handle the design load, at a 120F entering water temperature, and it WOULD be warm enough to sense the heat radiating at you with your eyes closed.

    But do the load calculations first. Getting enamored with any particular solution without even knowing the load is a recipe for overspending and delivering lower comfort.

  3. rydogg117 | | #3

    Dana and Martin, thank you for the responses. You bring up a good point that my current furnace should fully be able to heat the basement space. I suspect the floor does not have insulation between the carpet and slab. I will try to peel it back this weekend to confirm, but I wouldn't be surprised if it's just a minimal amount of carpet padding, maybe a moisture barrier, and then concrete. During this time of the year, the floor is cold enough to almost feel "wet" when you have socks on, though there isn't any moisture to speak of.

    The two ceiling mounted vents use very old terminations, they actually look like a floor grille. I'm sure the ducting is not properly insulated on these runs and the fact that there is a drop ceiling surely doesn't help to combat the heat loss.

    When I look under the carpet, what kind of proper insulation should I be looking for? And what kind of modern products are available today that weren't around a couple decades ago when this basement was likely last renovated? Thanks!

  4. GBA Editor
    Martin Holladay | | #4

    Ry Dogg,
    Ideally, you never want to install carpeting on a basement slab unless you have a continuous layer of horizontal rigid foam. The rigid foam can either be under the concrete slab or over the concrete slab -- but it's necessary to reduce heat loss, improve comfort, and reduce the chance of condensation and odors.

    In an older house, the only way to determine whether there is any horizontal rigid foam under the slab is to drill an inspection hole through the slab in an inconspicuous corner of your basement. In an older house, however, it's pretty safe to assume that there is no insulation under the slab.

    For information on installing rigid foam insulation above an existing basement slab, see this article: Fixing a Wet Basement. (Scroll down to the section with the subhead "Insulating an existing basement slab.")

  5. Expert Member
    Dana Dorsett | | #5

    There is very little heat loss through a 400 square foot bare basement slab, and even less heat loss through ~R1-R2 of carpet.

    Again, it's the zoning, not the furnace, not a high heat load. Run the load numbers.

    In the Puget Sound region summertime outdoor dew points are still below the deep subsoil temperatures, and the latent cooling loads are NEGATIVE. That makes it still reasonably moisture & mold safe to put carpet on an uninsulated slab, unlike the eastern half of the US. The most humid part of the year is the cusp of July into August, but even then outdoor dew points are under 55F more than 60% of the time. See:

    1. rydogg117 | | #6

      Dana, reviving this old post as I'm getting a bit closer to the basement remodel. I've looked into panel radiators as well and I think they could be a good and cost effective option. Some videos I've seen mention using it with a boiler. We have a gas hot water heater (50 gallon, 40k BTU/hr). I'm curious if we could use the existing water heater as the supply for this panel radiator rather than purchasing a standalone boiler. I understand the BTU output of these panel radiators are highly dependent on incoming water temperature; so this wouldn't be as effective as a boiler, but perhaps it would be enough for this one room.

      1. Expert Member
        Akos | | #9

        Your water heater will easily handle the heat from a basement. Since most panel rads are steel, you will need a plate HX to isolate the loop from potable water.

        If you go with hydronic baseboards that have a copper tube, you can go with an open system and directly connect to your DHW tank. If you do go with an open system keep in mind that you will need a timer to circulate for at least 15min each day outside of the heating season.

        Most tanks run at 140F, you would get about 50% to 60% the heat as you would out of a 180f boiler, so size accordingly.

        1. Expert Member
          Dana Dorsett | | #10

          One thing to be aware of is in water heater combi designs is that if the radiators are big enough and pumping rate is too high you could end up with destructive exhaust condensation on the center-flue heat exchangers of many 40K- burner standard water heaters. The turbulence in the tank from the flow needs to be brought to a minimum.

          For low-load combi-kludges like this it's useful to use a smart potable-water capatible pump with a wide range of pumping speeds such as the the stainless AquaMotion AM55-SFVL, then dial back the flow to the minimum that still delivers the heat.

          This is never a problem for condensing water heaters of course, since they are designed to take it.

          1. Expert Member
            Akos | | #11

            Just because it doesn't have a boiler, it is not a kludge. Think of it as DIY, much more positive.

            With any tank setup, you definitely have to watch the return temps and flow rate. For a plate HX feeding a couple of panel rads, even that AM55 is a bit large. Something like a Taco 003 with a balancing valve or a B&G e3-6 is about the right size.

            There are lot of basement radiant floor system around here running off DHW tanks, nothing wrong with this provided it is set up properly.

  6. rossn1 | | #7

    You'd have to look at the specs for a given panel radiator. You could look up spec sheets on a Myson or Buderus panel radiator and see how many BTUs they will put out for a given average or entering water temperature. I used the free Uponor ADS software to calculate my BTU needs, though it is not well documented.

    In my basement remodel I'm doing radiant ceiling in about 1500sf using warmboard. It's definitely a lot more technical to figure out radiant ceiling, and most do not know or understand radiant ceiling. Practically speaking you'd need at least R2.5 slab insulation for it to be effective, since it is 'shining' down. Radiant walls may be a better known solution if you don't go with the euro style panel rads, but would require new drywall. For a single room, the euro rads could be real nice.

  7. ohioandy | | #8

    Ry Dogg, your basement room is relatively small and the heat load relatively low. Instead of all the complexities and costs associated with hydronic radiant options, why not just install a single electric baseboard heater? It's drastically simpler up front and in the long term, costs under a hundred bucks, is zoned and dedicated to this room only, runs off a fuel that you said is relatively cheap in your area, takes no ceiling or floor height, and sets you up for a net-zero future. As long as you've taken care of air sealing in your basement, you won't have better comfort from any of the other options.

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