Federal mortgage loan subsidies of up to $5,000 may be available this spring for buyers of new or substantially rehabilitated houses with solar heating features. But the impact may be limited because the $125 million authorized by Congress last year for the program must also cover loans for energy conservation improvements.

Passive solar design relies on architectural features of a house to collect, store and distribute the sun's energy. So-called active solar heating uses some other form of energy to pump water through a solar collector and distribute heat to a house. Federal policy for encouraging solar use in houses had been aimed at active systems but lately has turned toward the less expensive and more easily designed passive systems.

Solar experts attending the convention here last weekend of the National Association of Home Builders said they hoped loans made through the Solar Bank being set up in the Department of Housing and Urban Development would spur interest among builders. Department of Energy officials and others here said that including passive solar designs in a house is not difficult and can make houses more attractive to potential buyers interested in energy saving.

Once the loan program is in operation this spring, builders can reserve up to 20 commitments from local lending institutions in any one county; individual buyers will also apply to local lenders.

The loan subsidies can be used for reduction in the principle or the interest of a loan on a passive solar house. To qualify, a house must have:

A solar collection area: Transparent glass or plastic facing within 30 degrees of south to admit direct sun rays.

A absorber: A hard, generally dark surface directly exposed to the winter sun that receives solar radiation and converts it to heat.

A "storage mass" for heat: Concrete floor slabs and walls, masonry walls, ceramic tile floors and water tanks are often used, HUD says.

Heat distribution method: Direct radiation, convection of air currents and low-power fans are described as common methods.

Heat regulation controls: Overhangs, awnings, insulated shutters and similar methods.

Subsidy amounts will depend on the size and effectiveness of the entire system. HUD officials say they expect most loans during the fist year to go to new single-family houses with passive solar systems.

Currently more than 20 states have tax credits or subsidies for passive solar houses. The builders association hopes that members can turn to solar remodeling projects, among other rehabilitation work, to tide them over the current housing slump, which has driven many builders out of the new-home market.

Robert San Martin, deputy assistant secretary at DOE for solar and conservation programs, said 30,000 to 40,000 homes in this country currently have passive solar elements. He said that number is expected to double this year.