When John Fleming and his crew arrive at a house, they know they'll find enough gaps, cracks and holes--through which about half of the home's heating and air conditioning can be lost--to equal the size of a window or more. And they generally can plug at least half of those leaks with a variety of sealants.

"A thermostat, electrical plate, anything that punctures a wall will let air infiltrate into the living space if the wall has access to the attic," said Fleming, president of Constructive Alternatives Inc. in Takoma Park. That can run up homeowners' energy bills as they try to stay warm or cool, he said.

The firm finds the leaks through the use of a powerful fan and "smoke guns." Workers mount the 16-inch diameter fan in the bottom of a home's front doorway and seal the rest of the entrance, fireplace openings, vents and ducts. The howling fan blows a 20-mph wind into the home, forcing air out through all gaps, cracks and holes.

The smoke guns are run along baseboards and sill plates, around windows and doors and elsewhere where air could leak in or out. Whenever the swirling, white smoke is sucked through a hole, the leak is marked for later patching with clear silicone or other caulking material. Fleming said some leaks are not fixed becaue it's not cost effective.

"Chances are air infiltration is the greatest problem in a home," he said. He cited a Department of Housing and Urban Development study that blames air leaks for 55 percent of a home's heating costs and 41 1/2 percent of its cooling bill.

Constructive Alternatives, which primarily designs and builds solar homes, began its leak-plugging service in January and has averaged two homes a week. The two-year-old firm is one of at least two locally that uses the fan--developed by Princeton University scientists in the mid 1970s--to correct leaks. The other company is Potomac Energy Group Inc., in Alexandria.

A couple of years ago, Constructive Alternatives experimented in providing the service with equipment borrowed from the U.S. Energy Department, which funded the fan's development. An Energy Department official said only a few firms in the country offer the service.

Constructive Alternatives' charge for the service varies, but Fleming said it can cost $500 for a day's work and $300 for a second day, including patching materials and labor. Depending on the house, the air leaks can be found within 90 minutes and the rest of the time is spent patching.

While the cost may seem steep, Fleming noted that some people will spend hundreds of dollars on storm windows, although it could take 25 years for them to recoup their investment in energy savings. The air leak plugging service might have a payback of two years, he said. "There's no more cost effective way to save energy," he said.

Fleming does not guarantee how much his work will cut heating and cooling bills because the service is only three months old and a homeowner's energy use may vary from year to year. But he said customers so far have reported they are warmer in their homes and no longer feel drafts.

Bothersome drafts prompted Maureen Kohl to hire Constructive Alternatives to plug her Takoma Park home's leaks. "We could feel a cold breeze around the front and back doors and along the baseboards," she said.

Kohl and her husband had installed some weatherstripping and caulking in the 60-year-old, 1,100-square-foot house. "But it's very time-consuming and labor intensive, and it's worth it to us for them to do it," she said.

She said her family is energy conscious--another reason they bought the service--and it keeps the thermostat low. This winter, she said, the house's gas heating bills averaged $70 a month.

In the Kohls' house, Fleming estimated air was replaced 14 to 15 times an hour--a common rate for homes he has improved. "An ideal home has an air exchange rate of about three-fourths an hour. We'll be able to cut it down to three to four an hour here," he said.

Kneeling in the living room, Fleming pointed to a gap along the baseboard--a typical leak. He ran a smoke gun along the baseboard, and the smoke disappeared into the gap.

"What's happened here is that the floor has settled. This wall is somewhat suspended (by the way the home was built), and you have this quarter of an inch crack, 20 feet long along the baseboard," he said. The leaks will be patched with clear silicone, with the seal checked with the smoke gun.