Build a Solar Water Heater: An Intregal Passive Solar Water Heater

Image Gallery The author's "test-bed" — a tilted, three-tank integral passive solar water heater, provided his family with 70% of their total hot-water needs during a full year of monitoring. David Bainbridge

For the do-it-yourselfer searching for an inexpensive, easy-to-build solar water-heating system, the integral passive solar water heater (IPSWH, pronounced ipswah ) is a dream come true. All you need to get going on this down-to-earth water warmer is a discarded electric water heater tank rescued from the local dump, a homemade plywood box to house it in, a can of flat black paint, a sheet or two of used window glass or clear plastic, a few common plumbing fittings and some pipe and insulation. Combine all that with some spare hours of satisfying sawing, hammering and wrench-turning, and you'll have an ongoing supply of hot water provided virtually free from that friendly furnace in the sky.

But before we get into the nitty-gritty of integral passive solar water heaters, let's run through a quick review of the basics of solar heating for those who may be new recruits to this wonderful world of free energy.

We'll be discussing solar collection systems for heating purposes — not for charging photoelectric cells or for other power applications — and there are only two basic types: active and passive. The essential difference between them is the use of external power: While active solar heating systems employ fans or heat pumps to circulate the Btu they gather, passive setups don't. As their name defines them, integral passive solar water heaters work on the latter principle, and that lack of power dependency and resultant energy savings is one of the IPSWH's greatest selling points.

Passive solar heaters can be subdivided into two classes: units in which the functions of heat collection and storage are separate, known as thermosiphon flat-plate systems, and arrangements that combine collection and storage into one integrated unit, namely, integral passive solar water heaters.

Since the flat-plate passive solar water heater is the predominant type in use today, most folks think of such collectors as being the best available for solar water heating. But in fact, for many uses, especially owner-built applications, IPSWH's outshine their flat-plate competition in almost every way — including ease and economy of installation, reliability and higher resistance to freezing.

How Solar Water Heaters Work

The design of all IPSWH's is based on a tank (or a series of tanks) painted flat black to absorb heat from the sun and then transfer the tapped Btu to the water stored within. IPSWH's are sometimes called batch heaters, because the heart of the system is the "batch" of water stored in the tank(s). To increase heat collection and reduce heat loss, a combination collection/storage tank is enclosed in an insulated box covered on the south-facing side and top with a glazing material, usually glass or molded plastic.