A home-built solar heater

A couple of summers ago Frank Rohrbach built a solar hot-air panel from scratch and attached it to a south-facing wall on his house in this eastern Pennsylvania town.

Rohrbach used standard materials freely available from hardware stores and lumberyards for a total expenditure of around $250. His homemade panel worked so well that all winter long, according to his friends, ''he wore a smile as warm as the hot air the thing was producing.''

Some suggest he was even a little smug about it.

The 4-by-8-foot panel (it can be mounted horizontally or vertically) obviously won't heat the whole house, but it does contribute significant amounts of ''free heat'' whenever the sun shines. It keeps the living room toasty and in the process saves some 30 gallons of fuel oil during a typical winter heating season.

Rohrbach built the heating panel from a set of Rodale build-it-yourself plans. He is, in fact, the ''guinea pig'' that the Rodale organization, publishers of Organic Gardening magazine, always uses to test new plans it produces.

Given a set of plans, he was told to go ahead and build the panels while Ray Wolf, who authored the plans, watched. If Rohrbach had any difficulty the plans would be made clearer.

The Rodale plans include two fans to move the air. While the panels will operate through convection alone (hot air rises and enters the room at the top of the panel while drawing in cold air at the bottom), this was found to be far less cost-efficient. Two fans, running an average of 7 hours a day for five months of the year, will cost only $3 a year to run with electricity at 6 cents a kilowatt-hour.

Like all good designs, the Rodale panel has a summer venting mode that prevents the panels from pouring heat into the home when it is not wanted.

The very detailed plans list all the needed materials down to the smallest nail or screw. Step-by-step instructions in the booklet are directly keyed to the six pages of pull-out blueprints. Moreover, the design is such that only hand-held tools are needed for construction and installation.

The solar air-heater plans ($14.95) are available at hardware stores or from Rodale Plans, 33 East Minor Street, Emmaus, Pa. 18049.