A Double-Duty Solar Solution: How to Build a Solar Water Heater

You can build your own space and solar water heater for just a fraction of what you would pay for a commercial solar water heating system.

| February/March 2012

Gary Reysas House

The author’s solar water and space heater at his home in Montana.


This simple solar water heater provides both domestic hot water and space heating. You can adjust the size and design to meet the needs of your home. You’ll find nearly all the materials at your local hardware or lumber store, and to build it, you need only basic carpentry skills and a little plumbing know-how. Amazingly, the cost of this DIY system is only about one-eighth of what you would pay for an equivalent commercial system!

How It Works

The system takes water from near the bottom of a solar heat storage tank and pumps it through a collector — where it’s heated by the sun — and then back to the tank. This continues as long as there’s sun on the collector. An off-the-shelf controller monitors the temperatures of the collector and the tank, and it turns the pump on only if the collector is hotter than the tank. When the pump is off, water drains from the collector back to the tank. This type of “drainback” system is especially useful in cold climates because it keeps the water from freezing inside the collectors.

Water is preheated in a single pass through a large coil of PEX pipe immersed in the solar storage tank. The preheated water then goes to your regular hot water tank. This simple one-pass system works well because the PEX pipe coil is large enough to hold quite a bit of preheated water right in the coil, and it has so much surface area that it acts as a good heat exchanger after the initial hot water from the coil has been exhausted. The water in the tank is used strictly to store heat — it is not part of the potable water system.

The floor heating system pumps water from near the top of the tank through the radiant floor loops, and then back to the bottom of the tank. The control system monitors the room temperature and the tank temperature, and it turns the pump on only if the room is cold and the tank is hot. The control system is made from two standard thermostats.

A key feature of this design is that the storage tank is non-pressurized. This gives you a lot of storage volume at a low cost and also eliminates the need for a separate drainback tank and heat exchanger.

My aim with this solar water and space heater was to create a design that would be simple, low-cost, long-lived, reliable, low-maintenance, and as easy to build as possible. Over the past five years, the design has progressed through several versions with a lot of feedback from early builders, and I think that together, we’ve made good progress toward these goals. I hope you’ll find it a fun and rewarding project.

9/13/2015 5:21:34 AM

I think the price of $2000-$3000 is too high. I guess some ready devices might cost less and have higher efficiency. www.alloma.ca

10/15/2014 9:21:45 AM

Hi Emanuensis, The tank is more like 4 by 4 by a little less than 4 ft high. Full details here: http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Main.htm Gary

1/27/2014 3:28:16 PM

Looks great, am building a house in Seattle:) http://i.tgu.ca/bright_plan_f Any reference on the Cistern design? By the pic it seems ? capacity, 2x4x4 feet, with 2x4 ribs, and ?" Plywood sides...

1/12/2014 9:55:42 AM

Hi Brian, Basically you need a pump that has a static (no flow) head equal to about 20% more than the vertical distance from the tank water level to the top of the collector -- this is for getting the flow started. Once the flow is started, the pump has to be able to push about 0.04 gpm per sqft of collector area through the collector and plumbing. There is an easy to follow step by step on this page: http://www.builditsolar.com/Projects/SpaceHeating/PipeSizing/PipeSizing.htm Its pretty easy to figure out, and for the most part the pump size does not have to be exact. Gary

Brian Powers
12/26/2013 9:13:50 AM

How do you determine the size of pump needed. I know it will depend on the size of collector used/vertical lift, and the length of coil in the storage tank, but is there a recommended gpm or hp for the pump? Should a valve be used to regulate the flow rate for maximum heating efficiency of the collector?

Landon Smith
9/20/2012 3:07:28 PM

Renewable energy really is the future. I like the idea of a solar water heater and I hope many people use this. I bet it could make a great deal of difference. I will start making plans and hopefully I can contribute more to lower energy demands and help keep the world green. http://independencealliance.com

3/4/2012 2:58:04 PM

Hi -- I guess you found the flow distribution test: http://www.builditsolar.com/Experimental/FlowDistWideCol/FlowDistWideCol.htm On the riser spacing, it depends on the thickness and material of the fin. The fins I use are thicker than the fins on commercial collectors, so they are able to transfer heat into the riser just as efficiently as narrower fins that are thinner -- a calculator on this here: http://www.builditsolar.com/References/Calculators/FinEficCalc/FinEficCalc.htm Gary

Angela Hammond
2/28/2012 9:08:33 PM

Found the answer about flow on your other site. Still interested in your thoughts about a 3" or 4" spacing vs the 6" spacing. Again, this is awesome information - my plan is to build it for the heat exchange system for the hot water tank right now and then add in a floor heating system later (will size accordingly)

Angela Hammond
2/28/2012 2:16:33 PM

Excellent article, thanks for the info. Questions about the copper riser tubes: Does the T manifold design ensure all the 1/2" lines are being used? How can we be sure the pump is not just forcing water through the first 3 or 4 risers? Would it be better to snake a single line throughout the whole collector or would this provide too much resistance and not enough volume? Also, would it be more efficient to have 3" spacing instead of 6"? Thanks again for the info, this is definitely something I can see myself building this spring/summer.

Gary R
2/4/2012 2:36:35 PM

Some people do use sand bed heat storage for space heating -- this appears to work pretty well if you have the right space for it: http://www.builditsolar.com/Projects/SolarHomes/SandBed/SandBed.htm Gary

Gary R
2/4/2012 4:47:21 AM

Hi Cindy -- I guess you could use sand, but water holds a lot more heat per cubic foot than sand does, and its easier to transfer the heat into and out of the water. Gary

Gary R
2/4/2012 4:46:03 AM

Hi Wayne -- If you think the trees might be a problem, you might try the Solar Survey on my site: http://www.builditsolar.com/Projects/Hydro/hydro.htm It does not take long to do, and it tells you how many hours of sun you will get on any day of the year. Gary

2/3/2012 8:20:03 PM

Can you use sand in the holding tank instead of water?

2/3/2012 7:17:33 PM

This is the BEST article EVER on this topic! I've seen / collected several how-to articles, but this puts it all together---great photos are essential. This is very doable--esp. for me since I have hot water heat. I can certainly do this---if I can find the right location. Having a southern exposure on your house makes all the difference. Mine is covered with trees, which is great in the summer, but there is room for experimentation. Thanks MEN!

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