The aging brick rowhouse in Petworth was far from a model of energy efficiency. Exterior doors had gaps around them -- in one case so large that sunlight streamed in from around the edges. Cold air penetrated the uninsulated walls of the laundry room and seeped in around the edges of a skylight.

None of this was a big deal to Gadi Ben-Yehuda and his wife, Jenna, when they bought the house this year. But that has changed in recent months. When the couple saw government forecasts saying home heating costs could soar by more than 40 percent this winter, they decided they needed to do something.

"We were terrified," said Gadi Ben-Yehuda, 32.

So the couple spent about $250 on insulation, weather stripping and other items to stop the leaks. "We're really going through the house and making sure everything is sealed up," he said.

Mounting homeowner panic extends far beyond the Ben-Yehudas and their formerly drafty house. People all over the Washington area and other cold-weather regions of the country are taking similar and urgent steps in fear of higher winter heating bills, according to retailers, contractors and manufacturers.

They're buying programmable thermostats that allow them to automatically reduce temperatures during the day and in the middle of the night. Some are plunking down big money to replace drafty, single-pane windows with more efficient models that do a better job of keeping cold air from seeping inside. Many are replacing aging, inefficient furnaces that consume far more power than newer models on the market.

For instance, furnace manufacturer Carrier Corp. said its most efficient units accounted for 42 percent of furnace sales in November, compared with 32 percent during the same month last year.

Companies that specialize in making homes more energy efficient say they have been overwhelmed with calls from new customers. A company called NSpects Ltd. in Chantilly, which tests homes to determine where leaks are occurring, is booked through January. Normally at this time of year, customers would be able to schedule an appointment in about a week, said Lee O'Neal, the company's president.

So far, the winter has been relatively mild. Efficiency advocates expect that when the big winter heating bills hit -- perhaps in January, around the same time as holiday shopping bills -- even more people will race to figure out how to use less energy. "People usually get more motivated when they get that first terrible bill," said Rozanne Weissman, director of communications and marketing for the Alliance to Save Energy in Washington.

Making houses more efficient can significantly lower bills. Installing insulation and sealing leaks can save up to 20 percent on heating bills, according to the Alliance. Installing appliances that carry an "Energy Star" label can result in savings of up to 30 percent compared with an inefficient model that's being replaced, according to the group. In many cases, advocates said, the items will more than pay for themselves in energy savings.

People with furnaces fueled by natural gas and heating oil have the most to worry about this winter. The latest forecast from the Energy Department predicts that, on average, households around the country using natural gas will spend 41 percent more this winter than during the same time last year. The forecast calls for those using heating oil to pay, on average, 27 percent more than last year. Electricity-powered heating costs are expected to go up far less, by about 5 percent.

Prices for home heating oil and natural gas have been rising for some time, the result of strong demand and tight supplies. Hurricanes Rita and Katrina this fall heavily damaged production of oil and natural gas in the Gulf of Mexico, and some of that production remains damaged, which means there's even less supply on the market, helping boost prices.

Because of concerns about energy supplies this winter, President Bush has called on Americans to conserve. The Energy Department is trying to persuade people to make their homes more efficient and turn down their thermostats. Department officials have traveled to a dozen states since October to preach conservation.

Retailers, including big chains and locally owned stores, say customers have been buying more items to make their homes efficient.

At Bill's True Value Hardware & Garden Center in Arlington, customers have been snapping up weather stripping and other plugs for air leaks, said owner Bill Ploskina. Lowe's Cos. reports increases in sales for insulation, weather stripping, portable heaters and wood stoves.

Home heating contractors also report being busier than normal this time of year.

"We're probably as busy as we've ever been this time of year replacing furnaces," said Tom Croker, president of Arlington Heating and Air Conditioning. "People are concerned about operating costs . . . They're taking out 20-year-old and 25-year-old furnaces that are 55 percent efficient and putting in furnaces that are 80 or 90 percent efficient."

The highest-efficiency furnaces can cost close to $5,000.

Contractors also have been busy installing new windows.

Among those who had windows replaced is Tony Akinsete, 33, who lives near Takoma. With his old windows, cold air seeped in -- especially on windy days -- which caused his gas bills to spike. Akinsete recently decided to do something about the problem because of increasing gas costs.

So two months ago, Akinsete replaced six flimsy windows in his rowhouse with high-efficiency, triple-pane models. Akinsete said he spent about $5,000 and figures he will recoup the cost in lower bills over the next few years.

"I'm seeing the immediate effects right now," he said. "I can come in now and tell my home is consistently warm."

In addition to finding ways to prevent heat from escaping, homeowners are doing more to regulate the length of time their furnaces are on.

Manufacturers of programmable thermostats -- including Honeywell, White-Rodgers and Hunter -- report surges in sales. Some companies say new lines of touch-screen thermostats that are easier to program than older models are popular.

Thermostat vendors also say they have seen far more customer interest in recent months. A San Carlos, Calif., Web retailer, ThermostatShop.com says its sales of programmable thermostats are up about 20 percent since September compared with the same period a year before. "When people call, they complain about higher prices, and they want to control that," said Arash Saffarnia, owner of the company.

David Robins and his wife, Victoria, who live in a 65-year-old house in Pittsburgh, recently spent about $100 for a programmable thermostat and its installation.

Last year, the couple would try to remember to turn off the heat before leaving their house. But they would sometimes forget and wind up with hefty bills.

Because of concerns about higher prices this year, they bought a thermostat that turns the heat on and off automatically. "It's kind of the same effect as if you leave home and aren't sure you shut the stove top off," said Robins, 34. "If nothing else, it just gives you the peace of mind."

In an effort to figure out how to save money on heating bills, people are flocking to classes on the topic. Lowe's and Home Depot have offered popular workshops at stores around the country.

In the District, the city's Energy Office, which runs a program that assists low-income residents with their energy bills, has been running packed classes. Because of rising energy costs and increased demand, the agency added a Saturday class designed to appeal to residents from a range of income levels.

Last week, in a cramped room at the District's Reeves Municipal Center, instructor Darnell Heard explained to 23 people how to install weather strips, sheets of plastic that insulate windows and reflectors that are inserted behind radiators.

"Once you're done with all this, you're going to feel an immediate difference in your house," Heard told the group.

Several of those in the class said they planned to follow the advice.

Donna Kelly, 50, of Anacostia, said she wanted to cut down on gas bills that reached more than $300 a month last winter. She said lots of air blows through doors and windows.

"I've got a lot of cracks and air seeping in," Kelly said. "I know once I make all those changes, I should save a great deal."