I'm considering a passive solar radiant heating installation in the Chicago area and was wondering if it would be feasible to store enough water/energy to support heating for most of the night. The house already has a gas central furnace, and can serve as backup, but the winter bills have been as high as $300-400 (for about 300 to 400 therms) during last year's expensive gas rates.

Roughly how much water might be needed to keep ~2000 sqft heated when it's 0 outside overnight in a below average insulated house? Orders of magnitude would be good enough of an estimate. I'm really just looking to understand if this is feasible. If this would require a basement swimming pool, obviously it's not.

Also, since the PEX would be installed on the unfinished basement ceiling, I'm a bit concerned about the basement heating up too much. I assume there are ways to shield the basement from the radiant heat via some form of heat reflectors under the PEX?

  • much easier to spitball this if you can provide Therms and a time (presumably month, preferably one that hit zero overnights) rather than $ which are useless for figuring out how many therms if we don't have your gas company...a therm is an actual unit of heat, making it more useful - we can divide by 30 for a daily figure and divide by 2-ish for a winter night. The upside of low night temps is that they often come with clear weather, so you can get some sun in the daytime. No sun, no worky.
    – Ecnerwal
    Apr 15, 2015 at 0:32
  • @Ecnerwal sorry, I didn't consider that of course it is the therms that were indicating the amount of energy used. Anyhow I updated the question (300 to 400 therms). Apr 15, 2015 at 3:22
  • @Ecnerwal also, if relevant, the actual square footage of the house is about 3,300, but I'd only be able to do radiant heat on the original 2,000 sqft footprint since the additions were apparently on slabs. I figured the zone with the additions would be primarily heated by its existing furnace. Apr 15, 2015 at 3:28
  • Feasibility is expressed here, in a picture with a thousand words: The Energy Efficiency Pyramid.
    – Mazura
    Apr 15, 2015 at 6:30

2 Answers 2


Gosh - a buck a therm is "expensive gas rates" - if I could get it (I can't and never expect to) it's more like a buck and a half...year round.

So, you're using 1 million to 1.3 million BTUs/day - if you could manage storage swing of 50 degrees F (not unreasonable with radiant floor, a low-temperature emitter) that might be 3250 gallons of water (at a round but not quite accurate 8 lbs/gallon) and perhaps as much as 2/3d's of that might be in the night when it's colder and nights are longer than days.

2000 gallons of storage is not too hard to manage. 267 cubic feet, less than a 6.5 foot cube, or slice it into a more convenient shape.

Adequate solar collectors to provide the daytime heat for the house AND to charge storage with ~700,000 BTUs in your area might be a considerably more difficult proposition - check the SRCC website and other solar resources. Call it 500,000 if you want to round off for "the other part of the house" but I still suspect it's going to be "challenging" or at least quite costly. I guess you'll pick up some direct solar gain from all those windows, at least when the sun is shining and any other collector would work, so perhaps the collectors can work more on heating storage and less on the house.

Overheating the cellar is probably not an issue - just use insulation under the radiant tubing/heat spreaders.

  • Yes, I guess I didn't realize how good we have it in Chicago with all of our utilities. I knew the electric rates were good here, at least compared to California; we pay ~7c/kWh. Seriously, $1/therm is exceptionally high here; it's typically about half that. See historical rates. Apr 15, 2015 at 13:34
  • Well, I think we can safely say that this is unfeasible from a cost/payback perspective. It looks like a single 1,000 gallon tank is ~$20,000 with a 10 year lifetime. Even if it completely zeroed gas bills, the storage tank cost alone couldn't be recouped in 10 years. Apr 15, 2015 at 13:47
  • 1
    You are shopping a bit high. I know these folks vaguely from another forum, but am not associated or even a customer (yet, anyway): americansolartechnics.com/softank.html (you have to ask to get prices on the larger tanks, but they are nowhere near what you state above, I'm 99.44% certain)
    – Ecnerwal
    Apr 15, 2015 at 15:43
  • Yes, you're right. I just called and their tanks are around $3,000 for an 1,100 gallon one, and he said they should last about thirty years because they're lined with plastic. Much more feasible. The $10k tanks must be very different. Apr 15, 2015 at 17:17

It would almost undoubtedly be a better use of your available money, time, and space to add insulation, not a complicated solar heat storage system, which after all, will do nothing for you half the year.

Start with your basement and insulate the walls and the rim joists. Then move to your attic and air seal the floor and add about 12-16 inches of cellulose or fiberglass. That should help tremendously.

Here are some articles to help you get started on the right track: (greenbuildingadvisor.com and finehomebuilding.com)

After that, keep air sealing everything, and consider replacing your windows if they're on the way out for other reasons too (rotting, impossible to open or close, etc).

Doing all of these things should cut your heating AND cooling bills by 50% or more.

  • I'm going to assume that the house is as insulated as is practical. I should have been clearer. The house was built with very high quality construction, but lots of tall ceilings ~20 feet high. Most of the space was a 2005 addition. There are walls of glass and I'm certain they're already thermally broken, and replacing them wouldn't do anything--plus would cost >$50k. There is no attic. It's a modern house with flat roof. Apr 15, 2015 at 0:56
  • "Walls of glass" and "flat roof" aren't what spring to mind when I see the words "very high quality construction". :) Never assume anything. It is highly likely that the house is NOT insulated as well as is practical, and air sealing is cheap and effective, and even in good quality construction rarely is any attention paid to air sealing.
    – iLikeDirt
    Apr 15, 2015 at 1:47
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    Flat roof is actually easy to add insulation to - it just goes on top (but it has to be foamboard, pretty much.)
    – Ecnerwal
    Apr 15, 2015 at 1:51
  • @Ecnerwal that's awesome, you're saying that you can add exterior uncovered foam insulation directly to the existing roof? Beauty of this house is that you wouldn't be able to see it. I guess I'll have to look into that. First question that pops into mind is that I wonder how it impacts roof maintenance. Any websites you could point me to with more info? Also, is this an issue: greenbuildingadvisor.com/blogs/dept/qa-spotlight/… Apr 15, 2015 at 3:32
  • It's normally covered with a roof membrane. It will at minimum want some covering to protect from sun.
    – Ecnerwal
    Apr 15, 2015 at 3:38

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