Mary Fischer of Potomac has boarded up her fireplaces to avoid drafts. Rugs are stuffed under the doors. Plastic sheets cover the exterior of the windows. Sweaters are worn indoors and electric, blankets warm the beds. Still, moans Fischer, the oil bills are "terrible."

Washington's winter has proved a bonanza for the area's cold weather merchants. Sales of furnances, heaters, firewood and long johns seem to be booming.

On the coldest day so far, a woman answering the phone at Bethesda-Chevy Chase Tree Experts said the men in the firm were all outside splitting wood. "Why, I've had 25 calls this morning alone," she said. "Our sales (at $75 a cord) have doubled in the past two weeks. We ran out of split wood. So now we're selling it as fast as they can split it. Things are as busy as they were during the energy crisis."

More than one merchant compared today's cold spell to that winter of 1973-74, when motorists had to endure long lines as gas stations.

"Last Monday morning (when the temperature plummted to zero) was even more active than during the energy crisis," said Al Hodges, of Hodges Home Insulation and Improvement Inc. in Arlington. The intervening winters were clement, he said, and people but off doing anything. "Now they're panicked."

Hodges said he has had six times as many calls this year as last. Homeowners usually have to wait two weeks to have their houses insulated; currently the wait is three to four weeks, he said.

"People who put off insulating until they could get an insulation tax credit have now spent twice as much on fuel as they would have saved," he added.

At the Acme Stove Co., manager Jerry Taylor predicted that the worst is yet to come.

"We're extremely busys now," he said, "but just wait for the rush when it warms up a bit." People don't venture out to buy heaters when it's that cold, he explained. Acme's big sellers this year are wood heaters, stoves with automatic temperature controls that give up to 12 hours of heat on one load of three or four logs. The stoves sell for $145 to $300.

Fedders Central Air Conditioning & Heating reports a 50 per cent increase in replacements for gas heating units. Since there is a moratorium on the installation of new gas units, Fedders reasons the cold weather is responsible for putting so many furnaces out of commission this winter.

This big item at Honeywell is the clock thermostat that automatically lowers the heat at night or during the day, if the resident is absent. Honeywell's Chronotherm adjusts the differential between thermostat and thermometer by turning off the heat before the room overheats.

"You can't get a heating contractor to even talk to you," said Hechinger's executive vice president, echoing the sentiments of the homeowners who flock to his stores to do repairs themselves. Right now, said Philip Mansfield, the big sellers are insulation stripping, electrical heat tape and pipe wrap to prevent freezing. They are also selling a lot more axes, sledge hammers and wedges than normal as people take to chopping firewood. Pessimists are buying kerosene fire heaters.

With homeowners responsible for their sidewalks, stocks of sand and ice salt have been nearly depleted at local stores. The Wisconsin Avenue Hechinger's sold out of its stock of several hundred 10-pound bags in four hours the first day. More has been ordered, but Mansfield fears its supply of 400 tons may not last the winter, with sand selling at 20 times its normal rate. Those who can't get sand or rock salt have taken to using chemical fertilizer, which is gritty and creates some heat, and even kitty litter.

And, with a cold house and icy walks, you can take cold comfort at the thought that clothing stores have been doing a hot business in winter wear. In the first 13 days after the snap began, Woodward & Lothrop sold three and a half times the number of children's boots it normally sells all winter. Men's rubber overshoes sold out in one day. Also sold out: men's flannel pyjamas, thermal underwear and earmuffs.