Build Your Own Solar Batch Water Heater

Public Information Office

 

INTRODUCTION

The solar batch water heater is a fun project for the do-it-yourselfer and provides an inexpensive hot water source for the back yard workshop or weekend cabin. It is easy to construct, costs only about $70 to build and produces water temperatures as high as 150 degrees F.

Remember, the batch heater is not designed to operate with pressurized water systems and should not be connected to a city water supply. If you are looking for such a system, consider manufactured solar systems, which have been designed to meet strict standards and aesthetic considerations.

The solar batch heater is made with a steel drum. Since a standard 55-gallon drum cannot withstand city water pressure, but some pressure is needed to push water through the system, gravity flow is often used. This involves a second 55-gallon drum, filled with cold water and elevated above the batch heater drum. Gravity provides enough pressure to move water from the elevated drum (cold-water source) to the batch heater drum and finally to the hot water faucet in a sink, shower or other outlet.

Follow these simple instructions to build your own solar batch water heater.

 

LIST OF MATERIALS

*Hose lengths vary depending on distances from the system to the cold water inlet and the hot water outlet.

**If you use plastic pipe instead of copper, be sure to get CPVC, not PVC. PVC cannot withstand as high temperatures as CPVC.

 

WASH AND PAINT THE DRUM

  1. Wash the inside of the 50-gallon drum. Use a bleach/water solution to get rid of any mildew; then rinse thoroughly.

    A word of caution: do not use a drum that has contained any toxic materials. Some toxic substances cannot be washed away.

  2. Once the drum is clean and dry, paint the exterior with a flat-black paint made to adhere to metal surfaces.

 

CONSTRUCT THE CONCRETE-BLOCK FRAME

  1. Site your system where it can receive as much solar radiation as possible and is not shaded by trees or other structures. The drum itself should face due south and should be tilted. (See NOTE in step 12 for more about this).
  2. To build the walls of the frame, mark off an area 5 foot x 5 foot square. For a secure foundation, dig a trench three or four inches deep and lay the first square of blocks in the trench. You need to lay some blocks lengthwise and some crosswise to fit the dimensions. Pack the soil around them.
  3. Add a second layer of blocks atop the first.
  4. Set the third row of blocks on only three walls (west, north and east) to begin the incline of the west and east walls (see Figure 1).

    Starting on the west wall, set the first block about 1/2 block-length in from the south end. Continue setting blocks around the north and east walls, ending 1/2 block-length in from the south end.

  5. Set a fourth row of blocks, decreasing another 1/2 block-length at the south end of the east and west walls, and so on, until the frame is seven blocks high.
  6. At the sixth level, leave a space in the north wall (back) for the outlet hose and another space in the east wall for the inlet hose.

 

FINISH THE WEST AND EAST WALLS OF THE FRAME

The tops of the west and east walls have a step-like shape. To create a flat incline for the glass cover to rest on, fill in the "steps" with cement.

  1. To make braces to support the cement while it dries, use 1 inch x 6 inch x 5 foot boards (plywood or other wood). Lay one board along the inside face and another along the outside face of one of the inclined walls (lining up the top edge of the board with the top of the wall).
  2. Hold the boards in place with pipe clamps or c clamps -- one at the top of the incline and one at the bottom.
  3. Make a second brace along the other inclined wall.
  4. Use ready-mix cement, mortar or a cement/sand mixture to fill in the step-like spaces, forming a smooth surface along the tops of the inclined walls.

    The finished frame's east and west walls will have a 42-45 degree incline from top to bottom (north to south). (See Figure 1.) NOTE: Depending on the latitude, you may want to adjust the walls' tilt so that the drum can be positioned to receive the maximum sunlight available. A good rule of thumb is to set the drum at an angle equal to the site latitude plus 15 degrees. In the example system, designed for Botswana, Africa (latitude 26 degrees), the drum has a 41-degree tilt (26 + 15 = 41 degrees).

 

CUT AND INSTALL FOIL-FACED INSULATION

Four sections of foil-faced insulation form a "dish" for the drum, sloping from the top of the block frame down into the center of the enclosure.

  1. Cut two sections--mirror images of one another--to fit on either side of the drum as shown in Figure 2 . Dimensions are: 4 ft. 11 in. x 2 ft. 6 in. x 3 ft. 5 in. x 2 ft. 11 in. Cut side A-D on an angle as shown.
  2. Cut a third section of insulation to fit under the top of the drum as shown in Figure 3. Dimensions are: 4 ft. 11 in. x 2 ft. 11 in. x 1 ft. 11 in. x 2 ft. 11 in. Cut sides A-B and C-D on an angle as shown.
  3. Cut a fourth section of insulation to fit under the base of the drum as shown in Figure 3. Dimensions are: 1 ft. 10 in. x 4 ft. 11 in. x 1 ft. 10 in. x 3 ft. Again, cut sides A-B and C-D on an angle as shown.
  4. Set the insulation inside the frame so that it slopes from the top of the blocks down into the center of the enclosure.

    You may need to add dirt or sand to support the insulation (and the drum, once it is in place). Remember, the drum must be lower than the concrete frame in order for the glass cover to fit on top.

  5. Use duct tape or reflective tape to hold the sections of insulation together. Don't worry if they do not meet exactly at the center or overlap somewhat. The important thing is to form a curved "dish" to hold the drum.

 

CONNECT THE FITTING, PIPES AND HOSES

  1. Cut a 7-inch section of pipe (to be inserted through the fitting so that it protrudes about 2 inches on either end).

    Before you can insert the pipe, you must use a "rat tail" file or rotary grinder to grind away the ridge inside the 1/2-inch sweat at the top of the fitting (opposite the threaded end).

  2. Solder or sweat the pipe.and fitting at the top (1/2-inch) joint.
  3. Cut a 3-inch section of pipe, insert it in the other (side) 1/2-inch opening and, again, solder the joint.
  4. Use a hose clamp to attach a 3-foot length of garden hose to the pipe at the threaded end of the fitting.
  5. Feed the hose into the drum's 3/4-inch hole and thread the fitting into the hole. (The drum will also have a 1-1/2-inch bung hole. This hole should be plugged.)
  6. Use a hose clamp to attach a second section of garden hose to the pipe extending from the top of the fitting. This hose should be long enough to reach the inlet (cold) water supply.
  7. Connect a third section of garden hose to the other (side) piece of 1/2-inch pipe. This hose should be long enough to reach the outlet (hot) water valve.

 

POSITION THE DRUM AND COMPLETE THE INSTALLATION

  1. Place the drum on top of the insulation so that the fitting and pipes are at the highest point of the drum. (The 1-1/2-inch bung hole should be at the lowest point on the top of the drum.)
  2. Mark where the inlet and outlet hoses need to pass through the insulation and out of the concrete frame. Cut holes and feed the hoses through.
  3. Connect the hose leading out the top pipe to the cold water inlet; connect the hose leading out the side pipe to the hot water outlet.
  4. Open the cold water inlet valve and fill the drum with water. Check for leaks and tighten any clamps if necessary.
  5. Place the glass cover over the frame.

 

OPERATING THE SYSTEM

Your solar batch heater is not under city water pressure, but does need some pressure to move water through it. If you use a second 55-gallon drum, filled with cold water and elevated above the batch heater drum, gravity provides that pressure.

As you open the outlet valve and draw hot water out of the batch heater drum, gravity draws cold water down from the elevated drum. When you close the outlet valve, the drum is sealed. Water in the batch heater drum prevents water in the elevated drum from continuing to flow down.

Since heat rises, the hottest water collects at the top of the drum. That is why the outlet hose extends only a few inches into the top of the drum -- to draw off the hottest water when you open the outlet valve. The inlet hose extends to the bottom of the drum, so that the cold water coming in does not mix with and cool the hot water at the top.

Try to schedule your hot water use for late afternoon and early evening when water in the batch heater will be hottest.

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This document is FSEC Publication FS-36, provided for the Energy Resource CD-ROM by the Florida Energy Extension Service, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Publication date: May 1994. First published: 1988.