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Community and Q&A

Using Water for Thermal Mass Heat Gain

LGMUwEJGTk | Posted in Green Building Techniques on

I have a room in the basement that has a large south-facing window. We don’t really use this room but it would be nice to use the solar heat that collects in there to warm the basement. What is the cheapest way to use water as thermal mass? If tubes are the answer, what kind of tubes? If a barrel, what color? Can I use black plastic 55-gallon drums? Thanks for you help!

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  1. GBA Editor
    Martin Holladay | | #1

    The solar heat that comes into the south-facing window in your basement is ALREADY heating your basement.

    It's unclear whether adding a water-filled drum will make much of a difference, especially since it sounds like you are talking about one large window, not an overheated greenhouse.

    But if you want to experiment, you can paint a 55-gallon drum black and fill it full of water. Locate the drum so that the sunlight strikes the side of the drum.

    This will take time and cost some money for black paint and for the drum, unless you already have a free drum. I can think of many ways to spend your time and money that will save more energy than this experiment, but go ahead. Lots of people did the same thing in the 1970s.

  2. Riversong | | #2


    What makes you think that the solar heat that collects in the basement doesn't already warm it? Or, perhaps, you're asking about how to store more of it for later use? If the basement is not overheating when the sun shines, then the solar gain is being properly absorbed and distributed. If it is overheating, then there's excess solar heat which could be stored for night time distribution.

    If the basement has a concrete slab floor, particularly if that slab is insulated below, then you already have the best passive solar storage device. Painting or staining it darker will improve its solar absorption.

    While water has a more than double the volumetric heat capacity of concrete, its thermal diffusivity (or ability to move heat internally) is only 1/7 that of concrete, and it's thermal mass index (or usefulness for diurnal thermal storage) is only 67% that of concrete.

    Because of water's relatively low thermal diffusivity, which is closer to wood and plastic than to earthen materials, it works best as solar thermal storage in very thin containers with lots of southern exposure. Water barrels are next to useless because of the low surface area to volume ratio - there won't be enough temperature rise to help warm the space later. And a bank or big black barrels will simply prevent the sun from warming the floor and walls.

    A 4" thick concrete slab, somewhat darkened on the top (earth tones work well) and insulated below and at the edges, is the ideal diurnal solar mass.

  3. LGMUwEJGTk | | #3

    Yes, of course the solar heat warms the room during the day, I know that. I just thought it might be possible to keep the heat in there longer with water. I already have 2 black plastic 55-gallon drums, but it sounds like those wouldn't help much. The floor is hardwood, not concrete, but I also have some dark slate tiles I could spread out on the floor. Thanks for your suggestions.

  4. Riversong | | #4


    Is the basement floor hardwood over a wooden frame or hardwood over concrete? If the latter, then it can still serve as a thermal mass floor if it's not too shiny. If it is highly reflective, then it will deflect solar energy to the walls.

    Dark slates are good solar absorbers, though they are not massive enough to store much heat.

  5. Mr. Greenguy | | #5

    If you have a basement undoubtedly it has concrete walls with a concrete floor. Using solar heat is a great idea and a perfect application, but you don't need to install complicated water tanks and pumps. Water has two wonderful characteristics. It has an incredibly high specific heat and can hold lots of energy. It's also a liquid which means it's easy to move. Unless you want to move the heat to isolated rooms far from the window, your best bet is to maximize absorption at the source and minimize night time heat loss. If it were my basement, assuming no finished floors, I'd paint the concrete dark to maximize absorption of solar energy (dark brown/green absorbs 80% of the energy while shiny white may only absorb 15%-20%) into the thermal mass (concrete). Shades on the windows in the evenings can do a lot to make the heat you do have stick around longer.

    But if you're sure you want to use water to store the heat, it will cost you about $1/gallon and that's just for the tank. You probably won't want to put it in front of the window given most basements predisposition to being dark, which means collecting the heat and then moving it to the storage tank (now it needs to be insulated too) and then using the hot water to feed in floor heat or little radiators. At this point, the best efficiency (at least in the north west where I'm from) is vacuum tubes on a drain back system. But they're expensive so to maximize performance you should probably put them up higher (more pipe). At this point, there's no reason not to supply all your domestic hot water with solar heat because you now have the infrastructure to do it!

    However, for the vacuum tube domestic hot water systems I've run numbers for (cost optimization), the pay off is usually in 5 years and a good system should last at least 20. That's 15 years of free hot water!!

  6. Riversong | | #6


    Are you answering someone else's question? Beth is not interested in active solar, but a simple passive storage option, she already stated that the floor was hardwood and you're repeating advice already given here.

    If you read first before answering, you might have something more useful to contribute.

  7. LGMUwEJGTk | | #7

    THanks for all your comments. I should have been more specific in the question. What is now the basement used to be the living area, until we built a second floor, that is now the main living area. The lower level is concrete block walls with wood floors. I think I got the answers, that it isn't worth trying to heat water to extend the solar heat that enters during the day. again, thanks for all your time and ideas!

  8. user-885167 | | #8

    If the window is only single pane, you might want to consider replacing it with something that lets in solar energy but then retains infrared. Have you studied up on "solar gain" and all that? You could also add an insulating shade of some sort, for some discussion see:

  9. samofvt | | #9

    I know a couple that had a sun room with lots of glass, wood framed with good insulation above and below. Seemed they didn't use it much because it was always either too hot or too cold in there, so it turned into a storage room. Then one fall they parked their Harley in there to keep for the winter. All of a sudden, the whole first floor stayed warm all night even though they turned the thermostat all the way down. Couldn't figure out what was "wrong".

  10. Jonnie101 | | #10

    Quote: Riversong - "While water has a more than double the volumetric heat capacity of concrete, its thermal diffusivity (or ability to move heat internally) is only 1/7 that of concrete, and it's thermal mass index (or usefulness for diurnal thermal storage) is only 67% that of concrete."

    It's an 0ld post but I don't like to see inaccurate values left unchallenged as people quote them.

    Concrete is generally given as around 1.0 W/mK but varies between 0.7 and 1.7 dependent on mix, aggregates and so on.

    Water is given as 0.6 W/mK

    So taking the average for concrete, concrete and water have similar values (ish) for thermal conductivity rather than the 1:7 ratio given in the quote above.

    In addition thermal conductivity values do not include the contribution of mass heat transport, convection which in many fluids will dominate or be significant - a black barrel heated by the sun is going to develop internal convection currents unless the heating is confined to the top part only.

    1. Jon_R | | #11

      Agreed, water, having about 2x the volumetric heat capacity and allowing convection, is typically the better thermal storage medium. But don't confuse thermal diffusivity with thermal conductivity.

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