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

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.

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: Gary

1/27/2014 3:28:16 PM

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

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