THE TEMPERATURE outside hovered below 20 degrees. Inside the seven-bedroom, four-bath house, on two levels overlooking the Potomac River, the furnace was not running. Yet the house was as warm and cozy as Grandma's home a generation ago.

This was the setting for the recent "housewarming" former Interior Secretary and Mrs. Stewart L. Udall held at their Mclean home. The Udalls, long active in environmental causes, are just one of the families these days returning to the wood-burning fire for heat. But they well may among the more spartan practitioners of the ritual.

So far this year the Udalls have not burned a dollar's worth of oil. Last winter, before the advent of the stoves, the Udall's oil bill was a mere $33 for the coldest and snowiest winter in local memory.

But they don't expect any oil bills this year, though the thermostat is kept at 35 degrees to turn on if necessary "so that the pipes don't freeze," Udall said.

A neighbor, Robert Sarich, provides the wood for the Udalls. On Sarich's many acres are several tall oak and tulip poplar trees that have been felled by storms. Udall is welcome to all the logs he can cart away. He splits the logs himself to the size needed for stoves and fireplace.

The Udall invitation, showing a horse and cart drawing a load of firewood, said:

"Stewart and Lee Udall invite you to join them for a housewarking to celebrate the installation of their wood-burning stoves and their declaration of independence from OPEC."

Two Scandinavian wood-burning stoves plus a fireplace blazed with logs. One stove and the fireplace were in the living room and the other stove in a corner of the kitchen. It isn't used for cooking, only heat.

Both stoves had been installed with elaborate brick hearths and backwalls by the host, who by day is a lawyer. By night Udall writes articles on saving energy.

Udall laid the bricks in the floor and against the walls where the stoves are installed, so no wandering spark can ignite anything but the intended logs and kindling.

The energy-conscious Udalls carry their keep-warm-yourself even into the bedroom. It was cold when guests left their wraps there. But the Udalls say they abhor even the suggestion, of an electric blanket or heater.

The Udalls bought their stoves from Washington's Acme Stove Co. The Norwegian stove in the living room cost $700. The smaller Danish one in the kitchen cost $275. Udall installed the stoves himself.

They start the wood fires in the morning, stoking them at night. During the week, Udall is at his law office and his wife, Lee, is at her job with the National Council for Traditional Arts in Washington. Over the weekend they keep the fires going day and night when both are at home. Their four sons and two daughters are all grown and away from home most of the time.

It all reminded me of those days at my grandmother's home in the mountains of Tennessee when we would race from the fireplace in the living room through the cold halls to the beds with piles of blankets upstairs.

At least the Udalls are not taking baths in tin tubs pulled up before the fireplace or wood burning stoves as people did around the turn of the century. (The Udalls cheat a bit and use electric heaters in the bath. They didn't say what their electric bill was.)

After a buffet served in the dining room, at the (perhaps questionably named) housewarming, Stewart Udall proposed a toast to "neighborliness, thrift and economy."

Bob Sarich, formerly of the Department of Commerce, and former Secretary of Labor W. Willard Wirtz and their wives were there studying the new methods of conservation. And so was Rep. Henry S. Reuss (D-Wi.).

If everyone was cold, they were too polite to say.