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
http://www.motherearthnews.com/diy/solar-water-heater-zm0z12fmzphe.aspx
The author’s solar water and space heater at his home in Montana.


PHOTO: GARY REYSA

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.

Building the Collector

The absorber starts with a set of vertical copper riser tubes spaced about 6 inches apart. I clipped black-painted aluminum fins onto the riser tubes to absorb solar radiation and transfer the solar heat into the riser tubes. The fins are grooved to fit tightly around the riser pipes for a good thermal connection. The absorber fins can be made from locally obtained aluminum sheet, or you can purchase them already grooved and ready to snap onto the risers. One source for these fins is Aluminum-Solar-Absorbers.

The riser tubes connect to copper manifolds along the top and bottom of the collector. The lower manifold takes water from the tank and distributes it evenly to the risers, and the upper manifold collects heated water from the risers for return to the tank.

The half-inch riser tubes are connected to the three-fourths-inch manifolds using copper T fittings. If you feel a bit intimidated by the soldering, don’t — with good cleaning and fluxing, the soldering is a piece of cake.

The collector frame is made from ordinary 2-by-6 lumber attached to the house wall with screws. The frame for the entire collector is built as a single unit right on the wall, which avoids the job of connecting multiple collectors. A layer of polyisocyanurate insulation placed against the wall separates the absorber from the house siding. Be sure to use polyiso rigid insulation board. If you use the blue, pink or white polystyrene insulation board, it will melt — trust me on this. The polyiso is a little harder to find, but most lumber yards have it.

One nice feature of building your own collector is that you can make it exactly the right size for the space you have available. In my case, this gave me about 50 percent more area than standard-sized commercial solar collectors would have allowed.

The collector glazing is twin-wall polycarbonate, which is the same material used for glazing most greenhouses. This is an attractive material that’s easy to work with and easy to find. In addition, the double glazing reduces heat loss from the absorber and results in better collector efficiency — especially in cold climates. The glazing trim and cap strips are made from PVC “wood” for low maintenance and a nice look.

Because this is a drainback system, all the plumbing from the storage tank to the collector must be sloped toward the tank so it drains when the pump shuts down.

The Water Storage Tank

A single large, non-pressurized tank stores solar-heated water for both water and space heating. The tank is a well-insulated plywood box lined with a waterproof EPDM rubber liner (usually used for lining roofing or ponds).

The tank I built for this solar water heater system holds 164 gallons of water. The plywood is framed with a carefully designed 2-by-4 frame that resists the water loads. This type of tank was developed back in the 1980s, and it has established a track record for long life and low maintenance. The EPDM liner is likely to last 15 years (some may last as long as 30 years), after which the liner can be replaced relatively easily without replacing the entire tank.

After the tank box has been built, the inside is lined with 2-inch polyisocyanurate rigid foam board insulation, and then the single piece of EPDM rubber lining is installed. This is topped off by an insulated lid also lined with EPDM. All the plumbing connections to the tank are at the top of the tank, so no lining penetrations below the waterline are required. I added a second layer of insulation to the outside of the tank to further reduce heat loss.

Domestic water is heated in a single pass through an immersed 300-foot coil of 1-inch-diameter PEX. The PEX pipe coil holds nearly 10 gallons of water that’s always fully heated to the tank temperature. After the initial 10 gallons have been exhausted, the PEX coil acts as a heat exchanger to heat the cold water passing through it. While PEX isn’t highly conductive and wouldn’t usually be the first choice for a heat exchanger, the large coil has so much surface area (90 square feet) that it actually works quite well for this purpose. The domestic water connections to the PEX coil are made outside the tank so the potable water passes through the tank in one continuous path with no fittings inside the tank, which reduces the likelihood of leaks.

Radiant Floor Heating

The radiant floor heating system consists of loops of half-inch PEX pipe that are stapled up to the bottom of the floor. A pump circulates hot water from the top of the solar tank through the floor loops and back to the bottom of the tank. There are no heat exchangers, expansion tanks or antifreeze — just pipes and a pump. I used heat spreader plates to transfer the heat from the PEX into the floor more efficiently, and I placed them under the heated floor to encourage an upward heat path. If hydronic floor heating is not possible in your house, you can also use hydronic baseboard radiators.

The radiant floor heating controls consist of two thermostats hooked together in series to switch on the circulation pump power. The first thermostat measures room temperature, and it switches on if the room is below the desired temperature. The second thermostat measures tank temperature, and it kicks on whenever the tank is above the set temperature. When both thermostats are on, it activates the pump, which will send hot water from the tank through the floor loops.

Performance and Cost

I project that for most families in most locations, this system would provide nearly 100 percent of their domestic water heating needs (although that efficiency may drop if you choose to optimize the system for space heating). Calculating how much of your heat it can produce is much more difficult, because there are so many variables. On my website, Build It Solar, I break down how to calculate this figure for individual homes. The 100-square-foot collector on my system is near the minimum size I would recommend for combined space and water heating, and while it will provide some useful space heating and good domestic water heating, it will not realistically reduce your heating bill by a large fraction. For more impact on space heating, build the collector larger and increase the tank size accordingly. In most cases, this would be a practical, cost-effective option. The only reason the collector isn’t larger on my system is that I didn’t have enough wall space.

The solar water heater as shown in the photos costs a bit more than $2,000 with the 100-square-foot solar collector. Doubling the collector size — which would greatly improve its space heating performance — would bring the cost up to about $3,000.

The payback period for the solar water heating part is about three years in most cases (see our chart for a breakdown of the costs). The payback period for the space heating is harder to estimate, but is probably a bit longer. Also, that list doesn’t include any rebates, which may further reduce the cost of your system.

You can simplify the system to do just solar water heating by downsizing the collector and tank, and taking out the radiant floor heating components. In fact, this system was derived from an earlier design intended just for water heating. It would also be good to introduce some tilt into the collector to improve its year-round performance. These changes can make a simple solar water heating system that costs about $1,000 and typically pays for itself in less than three years. Similarly, you could modify the system to provide only solar heating and not domestic hot water.

The homemade collectors can be replaced with commercial collectors. This increases the system cost, but also allows it to qualify for a federal tax credit, which offsets some of the cost increase.

You can find much more information about how to build this system at Build It Solar. If you run into difficulties or have suggestions for how to further improve it, contact the author at gary@builditsolar.com.

Read more: See how much Gary’s Montana project cost, and determine how much it could cost for you in DIY Solar Water Heating System Cost Analysis.