Water as heat storage

I was just thinking about water, which is such an excellent heat storage device. Heat it up, cool it down. BTU’s in and out, almost no loss if things are well insulated. It’s a very efficient way to store energy. If you could solar-heat up enough water during the summer, you could “coast” all winter. You can also capture waste engine heat if you’re generating power. Then, when the water is too cool to heat the house, the system still works as a massive low temperature boiler, which can be wood fired etc.

Apparently the name for this is Seasonal Thermal Energy Storage (STES).

But I haven’t seen anyone do the math. How much water do you need? How hot, how cold?

So, here’s a few numbers, in Metric and SAE. Note, BTUs and kJs are roughly the same size, so I’ll use the same number for both.

  • The safe maximum temperature is around 90 C (194 F). Similar to your hot water heater.

  • The minimum effective temperature is around 24 C (75 F), a little warmer than you keep the house.

  • So, your thermal range is 66 C (119 F).

  • Metric folks, 1 L of water can store 262 kJ (or BTUs)

  • SAE folks, 1 gal of water can store 995 BTUs (or kJ)

Now, the million dollar question - how much energy do you need to store? This varies a LOT by geography and house construction. I’ll base it on a round number, and you can adapt it to your own situation.

Let’s imagine a big house that uses 100 million kJ (BTUs) for the heating season, 30,000 kWh, about 5 cords of wood.

  • 100 million / 995 BTUs = 100,000 gallons (379,000 L) = 13,368 cu ft (378.5 cu m)

Now, to make this more realistic, I’m going to cut that by 2/3. Why? Because the system that created the heat will continue to run, and produce more heat that we can use. A solar water system sized to heat the house on a sunny day will still output heat on a cloudy cold day, perhaps only 1/3 as much. If half the days are sunny, then on average we’re getting 2/3 of our heat direct from the solar. We then supply the difference from our storage system.

  • 33 million / 995 BTUs = 33,000 gallons (125,000 L) = 4,411 cu ft (125 cu m)

  • This much water needs a cube shaped tank measuring 16’ 5" each side (5 m)

  • Or a cylindrical tank 18’ across, 17’ 4" high. Weighing 138 tons.

EDIT: It looks like this might be a slight overestimation. I’m not sure exactly why, but real world tank sizes are even smaller. Perhaps they’re better insulated homes.

[quote=“Wikipedia says”]
An eight-unit apartment building in Oberburg, Switzerland was built in 1989, with three tanks storing a total of 118 m3 (4,167 cubic feet) that store more heat than the building requires.[/quote]


Who’s going to be first to turn your basement into an indoor pool?
No , seriously this is something I have been interested in for a long time. .I have gathered many of the things needed to build a drain back hydronic water heater. I plan to play with this on a smaller scale to start with but the inputs are endless. I know of a guy who has heated his shop using a giant compost pile to heat water. Not sure how the solar will work here as we go months with no sunshine and sometimes weeks at a time never getting above 0* F. The nice thing about water is it is so portable. When the sun don’t shine the generator will run and the fire will burn. The car must warm up and it will cool down .This water can warm the car and be warmed by the car.
I have read somewhere I don’t remember where , but they just buried some coils in a sand bed 15’ below a slab house on grade. they just circulated the water through the coils deep in the bed all summer. It was not till late summer /early fall before the heat made it to the surface and started heating just in time.
Very interesting topic. Thanks for doing the math and posting the numbers Chris

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I think the better insulated home part is the key. My theory is if someone is looking at solar hot water they are the type of person who would also invest in a high performance house. All the standard heating models are a little wonky I’ve found. When we tried to have a hvac guy size a furnace for us he politely listened to my speech about passive solar, window placement, extra insulation and cube construction and then proceeded to recommend a standard furnace with a 50000 btu burner. 10 years later we have gotten along just fine with the water heater running the back up heat through the radiant floor. We only use it when away but I know it meets all the heating needs in midwinter. Curious what others bring up here. I’m ready to incorporate some solar thermal next year. Stored in slab maybe a small tank not ambitious enough for something big enough for a year.
Best regards, David Baillie


A little bit about solar collectors, because the size of them is related to your storage needs. Also a refinement on the storage sizing.

Assuming our goal is to supply exactly our winter heating requirements with no more collector than needed, but also not run out before winter’s end. Clearly this is impossible to figure out, but we can get a ballpark estimate.

Dropping to a typical for our area, 50 million BTU house, 2-3 cords. This way I can use local solar numbers.

So, start by finding your solar insolation numbers. This tells you how much sun you get in summer, and in winter, on average, in “peak hours per day”. So 8 hours of cloudy weather might add up to 2 hours of “peak sunlight”. You will get more in summer and less in winter, of course. This matters too.

Taking an average summer here in Kentucky, we get nearly 6 hours of peak sun. In winter that drops to 3.5 hours. On average, we get 5 hours. Heating season is 5 out of 12 months, Nov-Mar.

You could draw our year like this:

|--------------------- Summer 6 hrs -------------------- | ------------- Winter 3.5 hrs ---------------- |

  • All solar collectors receive 300 BTUs per square foot per hour, at peak sun.
  • Most are 50% efficient, leaving 150 BTUs/hr per sq ft usable heat.
  • We have 7 months x 6 hrs/day = 1280 hrs in summer.
  • We have 5 months x 3.5 hrs/day = 534 hrs in winter
  • Total = 1,814 hours peak sun per year
  • 50 million BTUs / 1,814 hrs = 27,600 BTU/hr = 183 sq ft of collectors
  • 70%, or 35.3 million of those BTUs will be collected during summer non-heating months, and stored
  • 35,000 gallon tank = 4678 cu ft (132 cu m) = 16’ 9" cube

Now, given the cost of such a large tank it might make more sense to increase the collector size and store less water.

  • Increase collector area to 500 sq ft = 75,000 BTU/hr
  • Winter solar = 534 hrs = roughly 40 million BTUs
  • Storage tank to hold the remaining 10 million BTUs
  • 10,000 gallon tank = 1336 cu ft (38 cu m) = 11’ cube
  • Tank will heat from 75-195 in 22 summer days

Last exercise. Setting aside solar for the moment, let’s take up the case of a rugged north woodsman who has a water cooled diesel generator. In the summer, he gets enough power from his solar panels, but he uses around 15 kWh per day, and burns 2 gallons of fuel per day in winter.

  • If the above is correct, the generator is 20% efficient.
  • Assume 70% of the waste heat is captured, from both exhaust and coolant.
  • He captures 155,700 BTUs / day.
  • This heats 160 gallons from 75 to 195 F.

His house is small and well insulated, but it’s a very cold climate. We’ll stick with the previous 50 million BTU (2-3 cord) usage, but over 8 months.

  • 50,000,000 BTUs / 8 months = 205,000 BTUs / day (25 lbs of wood)
  • Reclaimed heat from the generator meets 75% of that need (19 lbs wood)
  • Entire winter wood supply is reduced to 1/2 cord.

Great post Chris,

Depending what you want to achieve: where does the heat comes from, vs where you are going to use it;
the water/liquid temperature should be in a 'safe" range, meaning forming of bacteria should be avoided ( once in a timeframe water should be hotter then 72 degree celcius )

Then, using the hot water as buffer is an excellent idea to keep the above house in a constant state/temperature by convection.

Using multiple tanks, lets say PE 1500 liters each, makes it possible to use it as partial heated or partial cooled for summer. ( heating 1 tank is more rapid then one huge tank )

Normal basement with agrex around the tanks as insulator would be a good idea.

Make your calculations based on the energy/heating need for the house you would like to install it ( europe average 30 kwh for modest house )

Buffer size… ? how much place available :grin:


House/water heating - my favorite topic for decades (until I stumbeled on DOW).

When we moved to this house 20+ years ago I started off doing the roof. New lath, tarpaper? and tiles. While at it I built integrated solar panels. Only 10 m2 (100 sqft ?) but enough for 80-90 % of our hot water needs from mid April to mid September and a little bit of spring/fall space heating.
I have a 4 m3 (1000+ gallon) storage tank. The solar anifreeze feeds the top of that tank via a heatex. That way the relativly large size of the tank doesn’t matter since water temps will stay in layers.

When building the solar panels I bought alu stripes with pressed in copper pipes. I soldered them together and put it into my homemade insulated roof boxes. Now days I see only complete panels offered. Much more expensive. If I were to do it again I would do it from scratch with tin and tubing - just a bit more surface area compensating for less efficiency.

A German guy across the river started up his solar buissness a couple of years ago. When he built his new shop he dug out what I thought was going to be a basement, only he put all the material back in the hole. It turned out he insulated towards the ground and snaked 1000 m of pex into the material when filling up. His over sized solar panels heats up the masses during summer. He claims he usually manage on stored solar alone until Jan/Feb.


You brought this up on a friday hangout a while back and it has been stewing in the back of my head since.
If one were to scale this down to a 8’x8’ building, let’s say someone has a generator shed in northern Minnesota and is currently feeling the effects of cold temperatures on his battery bank, could this work?
I’m not talking about maintaining 70 degree temps but extending a reasonable above freezing temperature for even a few hours after the generator stops. I am thinking like running the exhaust through a 55 gallon barrel of water. I haven’t been able to hook up my in floor heat yet. I have some more thinking to do with that. I think I can find room for a 55 gallon drum in front of the generator. The 2"x6" walls will be insulated and then lined with poly.
I have a 2" exhaust that the wasted heat is going directly outside. How much is enough surface area to absorb the exhaust heat, how much is too much? I’m going to guess this water may freeze if we have a day of sun (which isn’t common in the winter). I’m going to guess the exhaust heat Is are 160-180 degrees? I can touch the exhaust pipe with my hands right after the muffler. The generator will run about 2 hours at a time twice a day.


Hi Bill, the exhaust heat is out of reach until you build a nice gas to water heat exchanger, which means having your shop set up. However the coolant heat is available now.

If I remember right, there’s a lot of distance between the generator and the batteries, correct? You will have to run insulated pipe all the way if you go that route. Alternatively, an insulated exhaust pipe flowing directly to the batteries would keep them warmer. The batteries themselves are good heat sinks.

If you’re just using the water loop for heating, some antifreeze will help. Or what they call a “drainback” system, ie drain it out when it’s not in use.

Carefully insulate the battery house, or all this will be rather pointless.


Last year there was quite a distance. The generator shed serves two purposes. A reasonable climate for the generator to run in and hopefully keep the batteries out of the extreme temps as well. The wall is going up today and hopefully most of the insulation. I have the batteries about 12" off the floor on a wooden stand.

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Oh, well that’s much easier. Yeah, you could make a water storage tank, insulated on all sides except one, exposed to the batteries on the uninsulated side, with a coolant coil (and exhaust if you like) running through it. Insulate around the batteries and tank together.


Keep some ventilation available in the battery area because of the H2 gases given off during charging. If you have ever had a battery blow up in your face, you know the problem. TomC


Bill I keep thinking about a oil fired hot water heater to be used as a heat exchanger for the exhaust. Fill with water or antifreeze and pipe the exhaust right into the burn chamber exhaust through the top of the tank… someday


Not familiar with oil fired water heaters but my plan was to use a propane or natural gas water heater.

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Same idea a lower section burner and a tube through the middle. I just mentioned oil because they are available very cheaply as people switch out their oil fired equipment for propane.


Here propane is what is available at the dump store as natural gas came to town last summer.


We call our dump " the local walmart"


There was a experimental house built about 100km far from me last year. If l understood the princip, it uses 2 big water tanks in the ground. One contains cool water, the other hot water. So, in the summer, cold water gets pumped trugh heat exchangers and heatpumps, storeing all the internal heat of the house, cooling the house and heating the water. When the cold days come, the situation turns around and water starts to fliw in the other direction.
I dont know the measurments or how it works but its a interasting principle.

Althugh lf you ask me, there is nothing like the heat that comes from a wood fired stove/boiler. If you allso have a glass sight like JO does on his boiler, this can eaven substitute TV :smile:


Hey Bill, as long as you’re running the gen. anyway, why not put a couple of elec. heating elements,( 12v 120v,240v) and pop off valve in the drum, unless you are using close to max out put of gen. add a little anti-freeze , and you could pump that through your floor. Don’t know if that is feasible in your situation. Al

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We had a few people try those up here. They are called “heat pumps”. They work like a refrigerator. You pump water out of well which I believe is in the neighborhood of 50 degrees F. You cool the water. The heat from cooling is blown through the house and the colder 40 degree F water is pumped back down the well. In the summer it is the opposite. The heat from the rooms are pulled in and the water gains heat and is sent back down the well. So far they have been expensive to operate like an electric refrigerator in you house only much bigger. Plus a lot of mechanical issues. My friend fought with his for years and then replaced it. TomC