How to Build a Passive Solar Water Heater

By David Bainbridge
Updated on May 16, 2023
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by Solar Components Corporation
A single-tank batch “breadbox” heater.

How does a solar water heater work? Explore the various types of solar water heaters. Reduce your reliance on fossil fuel emissions by building a passive solar water heater.

How Batch Water Heaters Work

A few basic principles and considerations govern the design, installation, use and maintenance of batch heaters. IPSWHs are called batch heaters because the heart of the system is the “batch” of water stored in the tank(s). The basic batch heater design is based on a tank or a series of large diameter pipes or tanks. The tank is painted flat black or coated with a selective surface that absorbs solar heat easily and transfers it to the water stored inside. To increase heat collection and reduce heat loss, the tank is enclosed in an insulated box covered on the south-facing side or top with a glazing material.

In the standard batch heater, cold water enters the solar collection tank through a side inlet near the bottom or through a “dip tube” that enters at the top and discharges cold water near the bottom. The heated water exits through an outlet near the top where the water is warmest, then moves to a backup conventional heater (fueled by gas, electricity or wood) where it’s heated all the way to the desired temperature. Batch heaters use waterline pressure for circulation, eliminating the need for expensive pumps and controls. During the summer months, or where it is warm and sunny year-round, the backup heater often can be turned off and bypassed, with the batch heater providing all your hot water needs. In Davis, Calif., my three-tank heater provided all our hot water for nine months.

The relatively low temperature of the collector minimizes expansion and contraction of components, reducing wear and tear. The mass of the water in the system helps protect it from freezing and extreme high temperatures that can harm materials, which leads to long system life. Research in Europe suggests that up to 20 percent of the water in the collector can freeze without damaging the tank, but keep in mind pipes are still at risk unless protected or drained.

How to Build a Passive Solar Water Heater

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