The next source of fraud for consumers to worry about is solar-energy fraud. And if fraud is not a problem incompetence may be - incompetence in the design, manufacture and installation of solar heating systems.

Some of the systems on the market are substandard, likely to leak or corrode, or not up to the heating job assigned them. A few are outright frauds. And even a good system may be so poorly installed that it doesn't work, or doesn't last.

Congress is considering a tax credit that would lower thecost of solar heating equipment and perhaps bring millions of new customers into the market. Installations are cheaper oon new homes, but many older homes with hot-air heating systems) can be converted to solar. If you're in the market, or think you might be, how do you tell the good systems from the bad?

(1) Don't buy a solar-heating system based entirely on information presented by the salesman. Some systems that look great in the brochures are awful on the roof. Mistakes will cost at least $1,000 or $2,000, and often more.

(2) Do a lot of research before you set foot in a store or allow a salesman across the sill. "Buying Solar," prepared by the federal Office of Consumer Affairs is a must. It contains an enormous amount of technical and financial information about how to various systems work and how to figure whether they'll actually save you any money. "Buying Solar" costs $1.85 and can be obtained from the Government Printing Office, Washington, D.C. 20402.

(3) Have your home tightly caulked and insulated and install storrm windows. Then watch your heating bills for a while. Many families will find that they save so much money just by plugging the cracks that solar systems will no longer be worth the expense. Your local utility may come and check out your insulation at no cost.

Don't buy a solar system without first getting the advice of a heating engineer experienced in residential solar installations. Right now they're about as rare as whooping cranes, but the flock is increasing. Nicholas La Courte, manager of codes and standards at the American Society of Heating Refrigeration and Air-conditioning Engineers, told my associate Linda Rubey that although some people bill themselves as solar engineers, "there really is no such thing."

(5) Avoid buying from a manufacturer who claims his system is 100 per cent efficient. It can't be. Avoid low-cost installations - the odds are that they won't work well. Avoid salesmen who assure you, after looking around your house, that the system will pay for itself within three or four years. That evaluation should be made by an independent engineer. It's not uncommon for the payback period to run 10 to 15 years or more.