Of all the uses for solar energy, direct thermal is the simplest and most efficient. No photovoltaics, inverters, and fancy wiring. There is always use for hot water, and there are a number of designs out there. The goal of this project was to design a solar-powered water heater so simple that it could be built anywhere with only hand tools. The materials are simple and inexpensive, and it requires no power or plumbing.

To start out with the basic design, you will need a new (un-used) oil catch basin, available at auto parts stores. This will serve as the water reservoir. For more water, it is easy to stack more oil basins, and you can modify the design to be tall enough for the top one to gravity feed for a shower, depending on your needs. The dimensions for this build are based on the FlowTool 15-quart container. The spout is convenient, and it has a carrying handle. Completely full, it weighs 30 pounds, but it works just as well partially filled—or you can drain it into a smaller container, if you wish.

Build The Frame

Frame Materials:

(2) 2-inch by 4-inch by 10-foot framing lumber

(3) Strips of 1/2-inch CDX (exterior grade) plywood 4-inch by 30-inch

(1) 1-inch by 6-inch by 8-foot board

(1) 2-foot by 2-foot clear corrugated greenhouse glazing

(2) 4-inch strap hinges

(1) Pound 1 1/2-inch deck screws

(1) Pound 2 1/2-inch deck screws

Frame Tools:

  • Drill or driver with bit for screws and 2-inch hole saw
  • Skill saw or handsaw
  • Framing square
  • Tape measure

Note: Sawhorses will make construction much easier.

Step 1: Cut The Frame Pieces:

Cut the 2 by 4s for the frame. Cut two pieces at a 45-degree angle 54.5 inches from end to tip. If you use a skill saw, you will need to finish the cut with a handsaw. Cut two pieces at a 45-degree angle 39.5 inches from end to tip. Then cut one piece each: 20 inches, 16 inches, and 7 inches long.

  • Cut the strips of plywood (1/2-inch CDX) as follows:
  • One strip 12 inches wide by 19 inches long
  • Two strips 4 inches wide by 30 inches long (end to tip) at a 45-degree angle
  • One strip 4 inches wide by 20 inches long
  • Cut two pieces 4 inches wide by 6 inches (tip to tip) at a 45-degree angle

Note: It is easiest to cut 30 inches off the end of a sheet of plywood, then cut the pieces listed below from it.

Step 2: Assemble Legs

Use 6-inch plywood piece to make a gusset at the top of the leg.  Then use two 2.5-inch deck screws to attach the 54.5-inch and 53.5-inch 2 by 4s at a 45-degree angle.

Use two 1.5-inch deck screws to attach a 4-inch wide plywood strip 10 inches up and 90 degrees to the shorter leg. Use two 1.5-inch deck screws to attach the plywood strip to the longer leg. Repeat the process to make a second pair of legs.

Step 3: Assemble Frame

Set the legs 12 inches apart (inside to inside measurement). Attach the two lower cross braces. The 20-inch wide plywood strips go between the shorter legs, 10 inches up from the bottom (use 1.5-inch screws). The 20-inch long 2-inch by 4-inch braces go between the longer legs, 16 inches up from the bottom (use 2.5-inch screws).] Attach the hinged pieces. These will support the heater at different angles.

The frame is complete.  You can set the frame in a sunny place, fill up the reservoir, and the water will heat up to about 20 degrees F. above the outdoor temperature. Building an insulated box for the tank will dramatically improve the efficiency of the water heater.

Build The Insulated Box:

The solar water heater works fine in warm weather, but needs some help on cooler days. Here are plans to add an insulated box to improve the performance of the unit.

Box Materials:

(4) 1/2-inch CDX (exterior grade) plywood 10-inch by 23.5-inch

(1) 1/2-inch CDX (exterior grade) plywood 24-inch by 24-inch

(2) 3/4-inch by 3/4-inch by 24-inch strips

(8) 3/4-inch by 3/4-inch by 22.5-inch strips

(4) 3/4-inch by 3/4-inch by 8.5-inch strips

(2) 2-inch by 4-inch by 12-inch strips

(2) 2-inch by 4-inch by 15-inch strips

(1) sheet 3/4-inch rigid Styrofoam 2 feet by 3 feet

(1) 2-foot by 2-foot clear corrugated polycarbonate greenhouse glazing

Note: Clear plastic window glazing may be substituted

(1) Foam strip for corrugated greenhouse glazing

Note: Not necessary if plastic window glazing is used

(1) Self-adhesive flat weather strip

(2) 4-inch strap hinges

1.5-inch deck screws

2.5-inch deck screws

Box Tools:

  • Drill or driver with bit
  • Skill saw or handsaw
  • Framing square
  • Tape measure

Step 1: Construct the Basic Box

Using the 24-inch by 24-inch as a base, attach the four sides.  Note: at this point, it is easiest to use finish nails to hold it together. Attach four 22.25-inch strips in the bottom of the box, using 1.5-inch screws on the sides and bottom. Then attach four 8.5-inch strips to the four corners of the box. Attach the remaining four 22.25-inch strips around the top of the box.

Install two layers of 3/4-inch insulation in the bottom and sides of the box. The first layer should fit snugly inside the wood strips, and the second layer should lock them in place. It may help to use screws to hold the insulation in place—just bring the screw head flush with the foam to hold it.

Drill a 2-inch diameter hole in the center of the front of the box (any face can be the front), 7-inch from the bottom of the box to the center of the hole.  Then drill a second hole about 1 inch below the first one. This will make it easier to remove the water container.

Step 2: Construct the Top

Attach the two 24-inch strips to two opposite sides of the box, 3 inches below the top. The top of the collector box will rest on these strips.  Use the four 2 by 4 strips to build a frame around the outside of the top of the box. It should fit loosely enough to be easily lifted off. I found it helped to temporarily screw the top frame to the sides of the box while assembling it.

Lift the top frame off the box. If it fits too tightly, you may need to adjust the pieces to loosen it. Make sure it lifts off easily before proceeding.

Cut strips of corrugated foam sealant for two sides of the box. Cut the corrugated greenhouse glazing 26 inches long.

Note: The clear plastic will cut with a skill saw, though it is best to use a fine-tooth plywood blade, if you have one. It will throw plastic chips, so wear eye protection if you cut the glazing with a skill saw.

Add the Glazing

Square the glazing and foam strips to the top and nail it down using panel nails. There is no need to pre-drill the glazing, but it does help to use a nail set to drive the nail. The nail head should make a slight dimple.  Work the insulating foam strips under the sides of the glazing. The self-adhesive type works best. I used Frost KingD-Profile Self-Stick Weatherseal 5/16-inch wide by 1/4-inch thick.

If the greenhouse glazing is not locally available, or the $22 is a bit steep, you can stretch clear plastic window glazing over the frame and tighten it with a hair drier. It does not hold the heat quite as well, and you will probably need to replace it a couple of times a year, but it will serve well as a cover.

Step 3: Attach Box To Frame

Rest the box on the 2- by 4-inch cross member on the frame. Attach the two strap hinges [Box_11]

Make A Water Nozzle

Nozzle Materials:

  • 4-inch section of garden hose with the male fitting attached.
  • Hose shut off-valve
  • Fusible electrical tape (A rubbery material that sticks to itself and forms a permanent water-tight seal.
  • Wrap the tape around the end of the hose to build it up so that it fits snuggly into the outlet of the tank. Wrap the tape around the outlet of the tank and the hose to make a watertight seal.   Screw in the hose shut-off valve.

How To Use The Solar-Powered Water Heater

Set the frame in a sunny area, facing south. Use the 16-inch hinged support to hold the frame horizontal.

Set the water tank in the box, and fill with water. Try not to splash water into the box, as it will cause the glazing to fog up. The one I used holds 15 quarts, so I heat three and a half gallons at a time.

Place the cover on the box. Find the best angle for the angle for the box to face the sun. You generally want the box to face the sun as directly as possible.

Allow Water To Heat

That’s all there is to it! In a few hours, you will have warm water. After a bit more time in the sun it will reach 130 degrees F. or more, so be careful, and supervise any children who use it. You can either use the hose to fill a bucket with the hot water, or remove the tank and carry it where you need it. You can even hang the solar-powered water heater from a branch and use it for a gravity-fed shower, but you may need to mix in some cooler water to get it to a comfortable temperature.

The portability and independence of the heater from utilities mean that it is a perfect companion for a solar cooker. I personally would not use it to heat drinking water, since the container was never intended for contact with food, but you could easily adapt the unit to hold a suitable water tank if you do want to heat drinking water.

As a final note, there are many ways to heat water with solar energy. Hopefully, after you build this solar-powered water heater, you will try other ideas. Photovoltaics that run a small circulating pump, for example, would improve the performance. Or you may want to build a pressurized system to incorporate into your home plumbing. Please drop a line with a photo of your project, and let everyone know how it works!

This article is from the summer 2017 issue of The New Pioneer Magazine. Grab your copy at


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