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Heating and finishing a basement in a well-insulated house

user-973600 | Posted in Green Building Techniques on

Hi Everyone,

3 years ago I built my own house using techniques from this site, and now I need to finish the basement I built for my quadripeligic brother in law who has to move in with us. The basement is a walk out under a gambrel colonial that is highly insulated. The home construction was doing using dual exterior walls and filling the space with a combination of closed cell spray foam and cellulose insulation. I heat my entire 2000sq foot home with two Mitsubishi mini splits.

My basement has a couple of options. The slab is poured over R20 of insulation and the basement walls have gotten the same treatment as the wooden walls above (2 inches of spray foam and approximately 8 inches of cellulose) The basement slab has radiant tubing run inside of it that currently gets heated by a solar water heater on the property. This has allowed my basement floor to run around 62 degrees year round, as I have run the system imperfectly and use water inside it, so I never stop the circulator when the solar hot water heater is not being effective due to time of day.

So right now its a sheetrocked, concrete floor basement with radiant tubing in the floor.

Here is the question, in order to keep it at around the 75 degrees my brother in law will need, how should I proceed? I know they make tankless water heaters that will heat radiant floor tubing. I have also though about a pellet stove down there for them. Lastly would be a third mini split system.

Has anyone run into a situation like this before? Are there better ideas that what I am coming up with?

Thanks so much in advance


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  1. charlie_sullivan | | #1

    My guess is that a pellet stove is not a good choice, because it would probably produce more heat than the space needs, and it would probably require fussing that might be difficult and frustrating for the occupant.

    Whether to go with a minisplit or combustion to heat the water in the floor depends on what fuel you have available, what your electric cost is, your interest in your carbon footrprint (and if so, whether you can install PV, or purchase low-carbon electricity, and what the carbon footprint of your grid mix is).

    Another option might be an air-to-water heat pump for the floor, or even a water-water heat pump using your solar heated water as a source.

    And of course at some point in this process you'll want to do a load calculation. It might not take much.

    Note that you'll also reduce your heating load upstairs when you keep the basement warmer.

  2. user-973600 | | #2

    Hey Charlie,

    Thanks for the response, I have propane or electric available to me right now. I currently use propane for hot water and my kitchen stove, electric for everything else. PV is an option, but down the line as cost is an issue here.

    I know its going to throw off my house environment because my basement will have to be the warmest room in my house when this is said and done.


  3. GBA Editor
    Martin Holladay | | #3

    You didn't provide many details about your solar thermal system. Some solar thermal systems have one tank, but most have two tanks (a solar tank that preheats the water, and a second backup tank that brings the water up to temperature when necessary using an electric-resistance element or gas).

    If your solar thermal system is a one-tank system, I think you could turn it into a two-tank system pretty easily. That would raise the temperature of the water circulating through the tubing embedded in the basement slab.

  4. user-973600 | | #4

    Hey Martin,

    I think its as simple as possible. Its definitely a one tank system, the water circulates through the floor and then through it. The heater itself is at ground level outside and the tubing to the home is buried in the ground before it enters the basement.


  5. charlie_sullivan | | #5

    My reading was that he has a zero tank system--just circulates water through the solar panel and the slab. At night it takes heat out of the slab to keep the collector from freezing and during the day it puts some heat back into the slab. So he would need a substantial upgrade to get 75 degree heating from that system. As a DIY project it could have a good payback; less likely but still possible with hiring someone to design the upgrades and install them.

    With electric and propane as options, a mini-split or hydronic heat pump is likely to be the lowest operating cost option. But with superinsulation below grade the load is likely to be quite small, and it's not clear that it will be worth the cost to install those, vs. just electric baseboards or electric water heating used with the hydronic tubing below grade. I don't think you've mentioned your climate, but the heat flow between a 75 F floor and 55 F soil, with R20 insulation, is less than the heat flow from the 75 F basement to a 70 F room above, if the floor is equivalent to R2 or R3. So you might actually want to include the heat flow up to the house in your load calculation....and it might even be enough that you end up heating the house primarily with the heating system in the basement, at which point it would be worthwhile to do it efficiently.

  6. user-973600 | | #6

    Charlie is right, its zero tank, excuse my mistake.

    Climate is Connecticut so I'm zone 5.

    Its looking more and more from your answers that I need to have this thing calculated by someone with some ability. I flew by the seat of my pants with the home design using this site, but figuring out the amount of heat to correctly make the basement space comfortable seems like it will require more attention than that :)

  7. user-2890856 | | #7

    Denis ,

    If one of these fine gentlemen would perform the load calcs based on information from you about your basement assemblies . I would certainly be willing to assist you in the radiant design , ODR curves , water temps and such to as efficiently as possible keep your brother in law comfortable .

    Hopefully you have the amount of tubing and number of loops and how they were originally run to make this successful ? What do you currently use to supplement the solar hot water system ? Do you prefer electric or Propane as the primary fuel for whatever is added ?

  8. Dana1 | | #8

    Getting to the right solution starts with the heat load calculation for that zone using 75F interior design temp against the 99th percentile outside temperature bin. (In coastal CT the 99% outside design temp is in low positive double digits Farenheit, in the interior is positive single digits.)

    Without the load numbers we can conjure up all sorts of possible solutions, almost all of which are likely to be a sledgehammer solution to a fly-sized load, given your high R. (A pellet stove is more like a pile-driver type solution than a sledgehammer solution.)

  9. chrisjri | | #9

    The Chiltrix air to water heatpumps look very ineresting for this application.

    I just don't have any info on how good they are. Honestly they seem too good to be true.

  10. Expert Member
    Dana Dorsett | | #10

    A 2-ton Chiltrix is probably a bigger sledgehammer solution than even a 3/4 ton mini-split for the likely fly-sized load.

    A ~1000'-2000'(?) basement with R20 under the slab and R12 foam + R28 cellulose on the walls and fully conditioned space above could have a design heat load under 1000 BTU/hr, assuming it's not a walk-out, and has typical basement window area. The quadriplegic occupant is good for at least 200 BTU/hr, the lights/TV whatever else he needs/uses might even add up to more the whole load. We simply don't know.

    Without more information it's impossible to estimate the actual load, which is an essential first step for closing in on a reasonable solution.

  11. user-973600 | | #11

    Thanks for the help all, I'm going to try and get all pertinent information together and post it up here.

    The basement is a walkout, but the walls are treated similarly and the slider / windows are triple pane like the rest of the house.

    Again, thanks for all your help, I will try and get everything up by Sunday.


  12. user-973600 | | #12

    Sorry it took so long, here is the measurements of the downstairs space. Ceiling height is 8 feet.

    Id love it for people's best recommendations.

    Thanks again so much for your time and ideas


  13. charlie_sullivan | | #13

    You've got R20 under the slab and R40 walls. We don't know how much of the wall area is above grade and how much is below. I'm not sure we know what the sizes or types of the windows are, and the door. With the highly insulated walls, the loss through the windows might be significant.

  14. user-973600 | | #14

    WIndows are

    Slider - 6ft wide by 8ft tall - U .23 SHG .19 Visible .32
    Bathroom - 25 1/2 by 40 1/2 same ratings
    Bedroom - 43 1/4 x 66 5/8 - U .35, SHG .48, Vis .52

    Basement half away from the windows is below grade, then tapers to walkout level.



  15. Dana1 | | #15

    The majority of the heat loss is through the windows & slider. Assuming an outdoor design temp of 0F and indoor design temp of 75F that's a 75F delta-T.

    Slider loss:

    U0.23 x 75F x 48 square feet= 828 BTU/hr

    Bath window loss:

    U0.23 x 75F x 7.2 square feet= 124 BTU/hr

    Bedroom window loss:

    U0.35 x 75F x 20 square feet= 525 BTU/hr

    Add those all up and you're at 1477 BTU/hr.

    Allow 200 BTU/hr for an adult human with a very low metabolic rate and you're a 1277 BTU/hr. Take off another 200-300 BTU/hr for a random collection of plug loads and it's down around ~1000 BTU/hr.

    Go ahead and DOUBLE that to cover the ventilation/infiltration load and the rest of the conducted wall & floor losses and you're at 2000 BTU/hr, whichs probably an overestimate of the real load (unless you're ventilation supply only, no HRV.)

    A 600 watt space heater covers that much load. ANY water heater would be able to put that much heat into the radiant floor, if that's the radiation you intend to use.

    A 3/4 ton mini-split can modulate down that low when it's 0F out, but would be cycling on/off most of the season- a total waste of modulating technology, and would barely beat a PTHP on as-used efficiency.

    Fixing the design inefficiencies of the solar would probably be able to deliver 100% of the basement heat, but a tiny electric boiler would do the trick for about a grand in total upfront cost, all in, including pumps & controls if you're careful. A 1.1kw EMB S-1 is good for 3800 BTU/hr ( more than 2x oversized for the likely load), and should be easy to hack in:

    For about the same money you could drop in the smallest PTHPs, which would use half the power, and is capable of air condtitioning, but the floor would run cooler (the solar might be enough to keep it comfortable enough under foot though.) The Amana PTH073G25AXXX would be extreme overkill (more than 3x oversizing) for both the heating & cooling loads, but I don't think they come any smaller than that.

  16. Chaubenee | | #16

    Who makes these tiny electric boilers that you speak of, Dana?

  17. charlie_sullivan | | #17

    Joe, in the link Dana provided, you'll find:

    2150 West River Street, PO Box 538, Monticello, MN 55362
    763.295.4138 800.922.4138 fax 763.295.4434
    [email protected]

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