Hot Water Storage

  • Active since 1995, is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.
Not open for further replies.

Eric Johnson

Mod Emeritus
Nov 18, 2005
Central NYS
I'm thinking about turning a poured concrete cistern in my basement into a hot water storage reservoir. Does anybody have any links to (or information about) building a non-pressurized hot water storage tank?
Funny, but this info was around 20 years ago and may have been buried on the back burner. Maybe some old time solar junkies have some of the info around - I remember a system that you can build out of plywood - the company sold large rubber liners for your plywood box! Worked like a champ for building storage - heat was transfered to and from using standard 100 foot copper tubing which comes coiled. Of course, you have to do all the calcs as far as storage, etc.

Here's a possible lead - see what the folks at Real Goods Trading Co. are up to - they used to have a lot of solar experts in house and maybe they can at least head you in the right direction.

Here is a link to a similar tank type:

Try some google on EPDM site built solar storage, etc.
I’m thinking about turning a poured concrete cistern in my basement into a hot water storage reservoir. Does anybody have any links to (or information about) building a non-pressurized hot water storage tank?

I'll share some observations(from personal experience). I too have a wood fired boiler (not a true OWB but might as well be with all the 'grief I'm getting'). It has a gross output of 85,000/hr and can easily and very comfortable heat my homestead. When I fire it...I fire it "Full bore"(hot fire=no smoke)...which is tricky to do unless you don't mind 80+ storing/shedding heat is something I have dealt with. I use a 120 gallon water heater as an accumulator/buffer tank to balance the system. "Storing" hot water for heating is tricky...and not all it's cracked up to be. In theory great idea..real world? Unless you have a "super efficient" home (which I doubt either of us claim) leaves something to be desired. In milder weather it works best. I can store the heat and get a "couple of showers" worth of hot water out of it, with the "fires out" (both wood and oil). If you plan on running wood fire in "cycles" might not be so great..."continous" good idea. I would suggest you use your (what was it 4,500 gallon cistern?) more as a "radiant heater" than a storage tank. 4,500 gallons at 80-100 degrees is more attainable than 180-200 (to get it that hot I think you might 'go chernoblyl' with any As an "old timer' told me once "If your basement is warm...your house is warm". I would therefore lean towards using it as "radiant heater" to keep the basement toasty warm.
You could always add a smaller "tank upstream" at a later date.
If I had a 4,500 gallon set-up in my basement...I would use it as a "Hot tub"
I just looked up the post you quoted "10x15x6":
10 x 15 = 150sq/ft x 6' Depth = 900 Cubic Feet x 7.48(#gals/cu ft) = 6,732 Gallons

Good sized tank!
I've been successfully heating various houses with various different boilers for about 15 years without the benefit of hot water storage, but they say it's a good idea with gasifiers, which is what I'm going to be putting in over the summer, so I think it's worth a shot. My approach has always been to fire the boiler up in the fall and not let it go out until the following spring.

Anyway, I think I'll block off a section of the cistern so that I have about 1,000 gallons of capacity. That would be roughly 4x8x5. With a rubber pond liner and some foamboard insulation it's not very expensive and since I'm dealing with concrete walls, integrity of the structure isn't a problem like it would be with a similar tank made out of studs and plywood. My supply and return lines from the wood boiler conveniently run right above the cistern, so I think a three-way valve on the supply line controlled by an aquastat on the gas boiler pressure vessel is about all I need to pipe it in. When the gas boiler (zone distribution) vessel hits the set point, the valve diverts the flow from the wood gasification boiler through the heat exchangers in the tank. When the gas boiler is below the set point, the valve goes the other way and I get direct flow between the two boilers. This way (the way I've got it figured) the tank serves as a place to store excess heat. Ideally, the whole system would sit at around 180 and the feed from the gasifier would always flow through the tank. The new boiler has a rated output of 205K btu.

You all are going to tell me I'm crazy, but for a heat exchanger I'm considering using a couple of cast iron radiators immersed in the tank. It would be a lot cheaper than copper coils, and I doubt there would be enough oxygen in the water to cause much of a corrosion problem, especially if I give them a few coats of decent paint.

Maybe this is an oversimplified design, but it's what I've come up with so far just doodling on the back of a cocktail napkin, so to speak.

I'd really prefer to deal with a pressurized tank, but no way am I going to get 1000 gallons for anywhere close to what modifying the cistern will cost, which I estimate to be about $500, not counting the piping and controls, but including the ci rads, which I already have.
To add to the discussion:
1. Obviously you have to calculate the min. temp that your house radiation works properly at - my guess is 140.
2. Then you have to take the highest temp you can get the water to - which is prob 190

So the 50 degree difference is your heat storage. Do the calcs as far as 1 BTU = 1 pound of water 1 degree and you will find your heat storage....then work backwards to determine what it can do for you and how long it will take to charge.

As far as heat exchangers, do the math and make certain the exchanger can transfer the total output of the boilers. Copper coils are relatively inexpensive. You can also use finned coils - like a typical DHW coil from a boiler.
Well, "relatively" is a relative term. One design I saw uses 540 feet of 3/4-inch soft copper for an 820-gallon tank (SSTS/Tarm). At today's prices, I'm guessing that soft copper goes for around $2 a foot, if the price for rigid copper is any indication. I thought about using finned 3/4-inch copper baseboard, too.

Thanks for the math guidance. Not my strong suit, but I'll work the numbers and see what it looks like. BTW, I'd say you're about right on with your estimates. 140 is about the minimum in my system for effective heating on most winter days.

Alright, just fooling around with these numbers:

1 gallon of water = 8.34 pounds
So it would take 8,340 btus to raise 1,000 gallons of water by 1 degree F.
And it would take 417,000 btus to raise that same 1,000 gallons of water by 50 degrees F.
So a tank full of 190-degree water represents 2 hours of output by the gasification boiler.
Assuming no other load on the boiler, would it then follow that it would take 2 hours to raise the tank temp to 190 from 140? (I'm guessing longer).

I'm lost. What does this tell me?
I am looking at the Tarm or greenwood GWB's too,
wondering if the whole "storage" is worth the effort, I have no idea, it makes sense, kinda like a "Liquid Masonry heater"
What adout the controls, do they add more complexity with storage ?

Greenwood does not sugest storage ?

Eric, that starts letting you know how much storage you want to do X job. Makes sure there are no surprises as to what you want to get out of it, etc.

BTU in and out are the same, so the time to charge....from 140 to 190, would be about two hours minus whatever losses in the piping system. With 100 then, you could count on 8 hours of heat at about 50,000 BTU per hour to draw down the tank.

Nick, not much complexity here. Some relatively simple controls can do the job - either solar type electronic controls or boiler aquastats.
Have you thought about automobile radiators?
Seems they would be more efficient and easier to work with
as far as the install.
As for the cistern, why not think of it as a swimming pool.
I have a wooden pool that is lined with foam to keep the splinter
and such off the liner. I have the heat exchanger from a glass sterilizer
that I want to install to extend the swimming pool use into Oct/Nov.

The concrete and earth will help with heat storage after they get up to temp.
Try your local use hospital / restaurant supply for the heat exchangers
from steam cabinets or sterilizers.

good luck
That's what I love about plumbing & heating: there's a bunch of different ways to do just about everything.

Thanks Craig--that helps.

Nick: From what I've been able to learn, gasification is quite a bit different from conventional wood burning in the way you tend and view the boiler. One guy describes it as just focusing on the tank temp rather than what the temp is outside or how warm you want the house. If you maintain the right tank temp, everything else should take care of itself. The tank gives you flexibility to fire the boiler less often and at your convenience. That compares to a conventional boiler setup (like I have now) where if you want to heat the house up, you have to crank up the boiler to get hotter water into the system. Another big difference between gasifiers and conventional boilers is that the gasifier doesn't have a big bed of coals sitting in the firebox when it's idle. When the blower shuts off on a gasifier, the process basically halts--a lot more like a oil- or gas-fired appliance. So instead of having your heat "storage" in hot coals in the firebox, you have it in stored hot water.

My whole understanding of heating with a wood-fired boiler is centered on the conventional approach and equipment, so it's a bit of a challenge to figure out the gasification alternative, but I think the benefits of gasification (cleaner burning, greater efficiency) make up for the added expense and work involved in setting up a tank.

In short, I don't think you need a tank with a gasifier, but it works a lot better if you do. As to the Greenwood, I'm not sure about water storage. If it's like the Garn, (and I think it is), then the water storage is built into the unit. The Garn comes with something like 1,500 gallons of capacity in the boiler jacket, I believe. I'm not sure if it's pressurized.
Just finished talking to a friend at my work about his old solar system that had the tank. Here's the words straight out of the horses mouth.

I had a 2,000 gallon tank in my basement, vinyl coated cement covered with insulation. The 2000 gallons wasn't pressurized, but I had a 30 gallon stainless bung tank with stainless tubes in the middle of it that was pressurized from the well. That bung tank transferred the heat from the surrounding unpressurized water to the pressurized and gave me 30 gallons. For 9 months of the year the solar panels would heat the 2000 gallons to 140F, the other 3 months it was more like 100F, still pretty good. But, stainless is extremely difficult to weld and near the bung tank there was the smallest pinhole leak in one of the welds. That leak over time overfilled the 2k tank and the water went into the vinyl, the cement, the insulation, it was a mess and was going on for probably a month or more before I noticed. It was drained, rewelded, and happened again in another place. So, I replaced the stainless bung tank with coils of copper which worked better, they didn't develop any leaks but there wasn't 30 gallons so it didn't work as well. Then, I had problems with the solar panels and took the whole system out after 10-12 years.

His solar & tank system was installed by a fly-by-night operation of the 80's so take it for what it's worth. Does make an interesting point about leaks developing inside the storage tank, and that the coil route seams to be the better route to go. If you do stainless, better use threaded connections.
Webmaster said:
Eric, that starts letting you know how much storage you want to do X job. Makes sure there are no surprises as to what you want to get out of it, etc.

BTU in and out are the same, so the time to charge....from 140 to 190, would be about two hours minus whatever losses in the piping system. With 100 then, you could count on 8 hours of heat at about 50,000 BTU per hour to draw down the tank.

Nick, not much complexity here. Some relatively simple controls can do the job - either solar type electronic controls or boiler aquastats.

I'll catch on eventually.

If the goal is to match the heat exchanger capacity in the storage tank with the boiler output, then there's not enough room in a 1,000 gallon tank for enough cast iron radiators to do the job. Maybe I'm looking at the wrong information, but cast iron rads are rated at between 5,000 and 10,000 btus each, which isn't much when you're trying to get to 200,000 btus per hour. (I must be missing something because the heat exchanger in my late '50s vintage gas-fired cast iron boiler is no bigger than a couple of average-sized ci rads). A comparable amount of 3/4-inch soft copper coil, according to the mfg website, would be about 900 feet. At $3/foot, I don't think so.

But if btus/hour is the goal, why not use a 200K btu/hour flat plate heat exchanger or a similarly-sized shell-and-tube arrangement? I don't know if the heat exchanger needs to be immersed in the tank, but it seems that you could circulate water from the tank through the heat exchanger and back in again, just like any other heat exchange situation. Or maybe you could even immerse the flat plate exchanger in the tank. I don't know. A 30-plate, 1-inch 200K btu flat plate heat exchanger is probably less than $500 these days.

So my question is, would a flat plate heat exchanger be a reasonable alternative to an immersed coil? I mean, that's how most OWBs get their heat transfer into people's houses, mainly because the OWBs aren't pressurized and are usually full of glycol.
OK, here's a diagram of what I'm talking about. Important details like pressure relief valves, low-water cut-off, expansion tanks, DHW are included but not shown.

This may or may not make sense. Any and all comments and (polite) suggestions are welcome.


  • pipingscheme.jpg
    56.3 KB · Views: 4,230
I'll have to study the diagram later, but you are right in thinking that a flat plate may be the way to go! These have come a long way - I remember Tarm made some big advances in them over 12 years ago and have to assume that the entire industry followed along.

One warning - if any side of the heat exchanger ends up using water exposed to the air (open tank, etc.) make certain that water is protected against corrosion, etc. - it might be that some plates have one side that is designed for this....for instance, pool heating (usually cupro-nickel).

Once again I am behind the times. We used tube-in-shell heat exchangers because copper was cheap back then and the plate ones had not really been perfected.

Oh, and when it comes to heat exchange area, consider that your oil boiler throws a 2000 flame against the cast, while your unit is going to be exchanging only 200 degree water - big difference!
I updated the diagram a little so that the most efficient transfer is now from the tank back into the system. This way, the tank charges up more slowly, but that may not be an issue. I assume there are some inefficiencies in this design (constant circulation), but I'm trying to keep it simple so that I can understand it, at least at the beginning. The bells and whistles can always be added later.

The flat plate heat exchangers available today all seem to be made of stainless steel brazed together. OWBs use them in a big way to get heat from the unpressurized vessels into pressurized systems. I'm using one right now to transfer heat from my wood boiler into the gas boiler vessel and it works very well. I use it because I have glycol on the wood side. The big ones--400K btu/hour and up, are going for $200 to $300, if Ebay buy-it-now offers are any indication. I think I paid around $350 for my 150K btu 3/4 inch unit about four years ago.

Anyway Craig, as always I appreciate your help and expertise.
I am about to buy a Tarm Solo for a new house I am building. After some research I don't think I'm going to install a storage tank. The concept is good but it seems like a lot of work and money to set it up. The tank takes up a lot of space also. If I change my mind I can add the tank later.

The Tarm tanks are ridicuously expensive, if I was going to do it I'd try to find a cheaper alternative. I think any stainless tank in good condition would work, and it could be insulated with Icynene-type spray foam.
All other things being equal, I would go with a pressurized tank and forget about all the hassles involved with heat exchangers. Plus, there's no chance of anything growing in a pressurized tank. One good idea is an old propane tank, like the ones you see sitting in people's yards. My understanding is if you can find them, they go for about $1 a gallon, so a 500 gallon tank wouldn't break the bank. Getting it into your basement might be another matter entirely.
Getting it into your basement might be another matter entirely.

If you go this route Eric, let me know...I'll give you a hand! (always interested in 'what the other guy is doing' when it comes to wood boilers')
Keep us informed on what you end up doing...
Thanks keyman. I'm going to see if I can find any scrap propane tanks before doing anything on the cistern. I've got a Bilco door into the basement, but a 32-inch door going through a concrete wall at the bottom of the stairs. So I don't know. A couple of 500-gallon tanks would be nice.

Is your system gravity feed or pumped, keyman? Presurized? Are you connected with another boiler with a heat exchanger or is it piped direct? DHW?
Is your system gravity feed or pumped, keyman? Presurized? Are you connected with another boiler with a heat exchanger or is it piped direct?

...My setup is piped direct (started the whole thing 'primitive' trying to keep it simple...and economical). The add on boiler is an actual 'fire tube' boiler with turbulators...not a 'water jacket', pressurized, ASTM, H rated, the whole nine yards...Very safe set-up...self extinguishing..."overfiring" is next to impossible...a T&P is installed such that if she gets too hot...the T&P relief dumps onto the fire to 'quench' it. Eff. I figure 50% or better (conservative estimate)

The 'whole set-up' is well balanced. I could heat three house with it...If I didn't mind filling it every hour(lol) If I had the time, money and resources...I would probably build the same boiler five times bigger...and give the OWB manufacturers a run for the money.

I am at a "cross roads" at this point...and may actually move it indoors. "I have a wood (or coal)fired is operated outdoors...Does that make it an OWB?...NOT!"

Probably the smallest fire-tube boiler ever built...but it works like a charm!
I've got a 1979-vintage Royall 150,000 btu/hour pressurized boiler set up in a cinderblock boiler room in my barn. My system is full of glycol. How you you protect against freezing?
Here's how I plan to do the tank. I got most of the rubble cleaned out of the old cistern last weekend. Fortunately, no unpleasant surprises (like a pile of asbestos). Just construction debris from the past half century of renovations. I found some old radiator steam vents, so I guess this place had a steamer before getting hot water. Interesting. Also the remnants of some big old rat traps. The list goes on.

Anyway, here's the tank schematic. About $500 altogether, not counting the piping, pumps, htexch, etc. I'm not exactly sure how to configure the input and output lines in the tank. If you think about it, the flat plate heat exchanger works both ways, so sometimes it's recovering heat from the tank, while at other times it's charging it up. And sometimes it's in a state of equilibrium. The way I have it envisioned now, it's most efficient at heat recovery, which I think makes the most sense.

Please share any thoughts or (polite) suggestions on this.


  • tank.jpg
    18.7 KB · Views: 2,209
Here's what I'm working with for a tank. I have to build a cinderblock wall in the foreground to create the 4x5x7-foot space, but that's about it. I'll probably fill the blocks with vermiculite.


  • cistern1.jpg
    30.2 KB · Views: 1,766
Not open for further replies.