Part IV: Solar Water HeatingChapter 6. Simple and Batch Water Heaters
Batch (Breadbox) Water Heater for Warm Climates
A painted tank in an insulated box that preheats domestic water where nights stay above freezing.
- Difficulty
- Intermediate
- Cost
- $150–350
A batch heater, sometimes called a breadbox, is an insulated box with a glazed front and a tank of potable water inside. The sun heats the tank directly. There is no pump and no separate collector. Cold water goes in when someone opens a tap, and preheated water leaves toward the regular water heater.
It is a warm-climate device. If the nights at your site drop below freezing, this box is the wrong project. A single hard freeze can burst a tank or a pipe, and a glazed box that "usually" stays warm will not save you on the night it does not. For freezing climates, read the system choice guide and use a layout that drains or uses antifreeze.
Where freezes do not happen, a single tank of 30 to 40 gallons behind 8 to 12 square feet of glazing is a practical preheater for a small household. It will not produce a full day's showers by itself in marginal sun. It will take the chill off the water the main heater would otherwise heat from the ground temperature, and that is the savings.
Difficulty
- Level
- Intermediate
- Time
- Two weekends, plus plumbing time and a permit wait if your town requires one
- Skills
- Basic carpentry and potable-water plumbing. Relief valves and mixing valves are not optional extras.
Cost estimate
$150–350
A used glass-lined tank and bargain twinwall can land near the low end. A new tank, a tempering valve, a temperature-and-pressure relief valve, and new glazing land near or above the high end. Do not save money by skipping the valves.
| Piece | Typical cost |
|---|---|
| Potable tank, 30–40 gal | $0–180 |
| Box lumber, insulation, glazing | $60–120 |
| Tempering valve and T and P valve | $40–80 |
| Pipe, isolation valves, bypass | $30–60 |
Materials
| Item | Qty | Notes |
|---|---|---|
| Glass-lined or otherwise potable pressure tank | 30 to 40 gallons | A retired electric water heater can work if the lining is sound and every unused port is plugged with a rated fitting. Do not use a tank that held anything but drinking water. |
| Temperature and pressure relief valve | 1, rated for the tank | On the tank, with a discharge pipe the full size of the valve outlet, aimed where a scalding discharge cannot hit anyone. |
| Thermostatic mixing valve | 1 | On the outlet to the house, set near 120 degrees F, so a stagnant sunny day cannot send 160 degree water to a shower. |
| Insulation | Enough to line the box | Foil-faced polyiso or mineral wool. Keep foam off the hot tank surface if the foam's rating is below the temperatures you expect. |
| Glazing | The face of the box | Twinwall polycarbonate or tempered glass. Annealed glass in a door people can bump is a bad failure mode. |
| Isolation valves and a bypass | 1 set | So the house still has water when you drain the box for service. |
| Flat black high-heat paint | For the tank | Only if the tank's existing coating is not already a durable dark finish. Do not paint over a label you need for the relief-valve rating. |
How the water moves
Pipe the box as a preheater. Street cold water enters the tank low. Water leaves the tank high and goes to the cold inlet of the existing water heater. The existing heater is the backup and the place where you can still hold a stored temperature high enough to limit bacterial growth. The solar tank will sit at whatever the day produced, including lukewarm stretches in cloudy weather. That is a reason to keep the backup heater in the circuit rather than drinking straight from a tank that spent three days at 95 degrees F.
A bypass around the solar tank lets you isolate it without shutting the house down. Ball valves on both sides of the tank, and a valve in the bypass, are the usual set. Do not put a valve between the tank and its relief valve, and do not put a valve in the relief discharge.
Tilt the glazed face toward the winter sun, roughly at your latitude, if you can do that without aiming the glazing at a tree. A vertical box on a south wall still works and sheds rain more easily. It collects less in summer and less around noon in winter than a tilted face. For a preheater, that compromise is often worth it.
Paint or select a dark tank, insulate every side that is not glazed, and keep the glazing close enough that you are heating the tank and not a large volume of air. Seal the box against rain and against insects. Leave the tank fittings accessible.
Heat, pressure, and the valves that make it legal to operate
A dark tank behind glass on a clear day can exceed 160 degrees F if nobody draws water. That is a scald temperature. A thermostatic mixing valve on the outlet, downstream of the tank and ahead of the fixtures, is how you keep a sunny afternoon from becoming an injury. Set it near 120 degrees F unless a local health rule tells you otherwise, and test the outlet with a thermometer. The valve is a mechanical part. It will fail someday. The relief valve is the backup for pressure and for temperature at the tank, not a substitute for the mixing valve at the shower.
The tank must have a temperature-and-pressure relief valve sized and rated for that tank, installed in an opening the tank maker intended, with the probe in the water. The discharge pipe runs downhill, is the same nominal size as the valve outlet, has no valve and no plug, and ends where a sudden dump of hot water will not hit a person or freeze the pipe shut. If you do not know how to do that, this is the moment to hire a plumber. The rest of the wooden box is not the dangerous part.
Pressure in a closed piping system rises as the water heats. The house may already have an expansion tank on the water heater. Adding a solar tank does not remove that need. If the municipal supply has a check valve or a pressure-reducing valve, the system is closed and needs a place for expansion. That is a plumbing-code question, and it is cheaper to ask than to discover it as a drip from the relief valve every sunny day.
Many towns require a permit for potable plumbing. Call before you cut the first pipe. Some places allow a homeowner to do the work on their own house and still want the inspection.
Climate limit, in plain words
If you have seen frost on the grass, do not build this and "just drain it when you remember." Pipes in the box freeze when the tank is still liquid. A glazed cover loses heat all night. The honest options in a freezing climate are a system that drains itself when the pump stops, or a closed loop with antifreeze. Both are in the next chapter. This page is for places where freezing is not part of the weather.
Summer overheating is the matching problem in a climate that never freezes. If the house is away for a week, either cover the glazing or make sure the relief valve, the mixing valve, and the expansion arrangement are already correct. Do not leave a garden hose as the only overheat plan.
Decide the climate with a winter of memory
If a freeze is possible, stop and use a drainback or closed-loop plan. If it is not, pick a south wall or a ground spot that the site survey says is sunny, close to the existing water heater.
Choose a potable tank and its valves
Buy the relief valve and the mixing valve with the tank, not later. Confirm the tank pressure rating and that every port has a rated plug or fitting.
Build the insulated box
Glaze one face, insulate the others, and support the tank so the fittings are not carrying the weight. Flash the box like a small roof.
Pipe it as a bypassable preheater
Cold in low, hot out high, then the mixing valve, then the existing heater. Isolation valves on both sides. No valve on the relief line.
Get the inspection your town requires
Then fill the tank, check for leaks, and open a hot tap until the solar tank has been flushed of air and debris.
Measure a sunny day
Read the tank outlet before the mixing valve and the water after it. The second number should sit near the valve's setting even when the first number is much hotter.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/Projects/WaterHeating/water_heating.htm
- https://www.builditsolar.com/Projects/WaterHeating/ThermosyphonDIY/ThermosyphonDIY.htm
- https://www.builditsolar.com/Projects/WaterHeating/StockTankBatchHeater/StockTankBatchHeater.htm
- https://www.builditsolar.com/Projects/WaterHeating/SolarHorseTank/SolarHorseTank.htm
- https://www.builditsolar.com/Projects/PoolHeating/pool_heating.htm
- https://www.builditsolar.com/Projects/PoolHeating/DIY/SimplePoolHeat.htm
- https://www.builditsolar.com/Projects/WaterHeating/slingshower.htm
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.