A really hot day. And Larry Mack, the Aircon Man, tools down Cedar Lane in his blue-and-white van. He's got a cigarette on his lip, a vial of Advil on the dash and a milk crate full of service manuals on the floor. "This woman," he comments on his next job, "says her electric bill doubled. The air conditioning never turns off."

A cold one, for a change.

He pulls up in front of a huge brick house, straps on his leather tool belt and rings the bell. The owner opens the door. Delight spreads across her face as she realizes who it is: Aircon Man, the savior of sweltering (and, in this case, freezing) humanity. "What's going on," she explains, eagerly handing Mack a $334.31 electric bill, "is it's running all the time. Constantly."

Expertly he fields her angst. Gently he peppers her with a few pointed questions. "First," he then tells her calmly, heading out the back door, "I'll check the electric meter."

"It's been over a month now," she mutters, not so calmly, "and it never kicks off. I'm suffering from too much cold."

Within a minute or so, Mack returns and heads quickly, wordlessly for the basement, a determined expression on his face. He is apparently already on the scent of something. And indeed, in the gloom of the damp, concrete-smelling basement the mystery is solved.

"Ah ha!" exclaims Aircon Man, pointing.

The furnace is running. A Sisyphean standoff. The furnace is heating while the AC is cooling.

"My God," says the woman, sheepishly. "They're fighting each other."

Mack, 36, is a pleasant, redheaded man with a comfortable day's growth of beard who's been in the Aircon business 17 years, the last two for Davis & Davis of Kensington. He enjoys immensely the independence this kind of work affords him. Once he's out in his truck, it's a battle of wills between him and the broken-down units. He schmoozes with customers, becoming a favorite of some, and has learned to be tolerant of human foibles.

It's mostly been a mild summer, not too hot, and that's been bad for business. John Davis, who founded the medium-sized Davis & Davis 25 years ago with his brother, Berk, says calls for service have been running 30 to 35 a day. That's about half the level for a truly hot summer and just barely enough to keep the company's seven "service technicians" working regular hours. The good news has been that customers have been receiving immediate service.

The most common problems, according to Davis, involve low Freon levels and clogged drains. Freon is a substance that flows through air conditioning systems, becoming a gas within a coil inside the house where it picks up heat from air being blown across it by a fan, then condensing into a liquid as it circulates to the outside unit where its heat is expelled into the atmosphere.

"There are also thermostat problems, electrical problems like the relays and capacitors," says Davis, a folksy fellow in a black baseball cap. "A lot of times people will run over the thermostat wire outside the house with the lawn mower, or the dog will chew it up. I had a man with a poodle recently who chewed it up, it shorted the electric out and burned out the transformer and two or three parts. Cost him $300."

He shakes his head.

"A lot of times," he says, "mice get in these air conditioners. They get in there and build a nest, then the contactor won't click in. We've had snakes in units. Another common thing is hornets will nest in air conditioners. The debris will clog the works, and when you turn your thermostat on nothing happens."

So you call Aircon Man.

"One time I went out on a call," Davis continues, "and we had a bat stuck in the flue pipe. We had to get a sheet to catch it. I think one of the funniest things I remember was the guinea pig in the ductwork. That was a two-day ordeal. He was running back and forth and making a lot of noise. Finally they put some feed in there and caught him."

Davis and his brother have 28 employees and about 20,000 regular customers, most of them people who have bought air conditioning or heating units from their firm over the years. They've lived through recessions and adjusted to technological changes, including the blossoming of the heat pump (an air conditioner in reverse) as a heating source.

"Weather, in our business, is the key element," says Davis. "When there's a prolonged hot spell, sales and service calls go up. We've really had mild summers recently. The weather just seems milder and milder," he says, pained.

After educating Furnace Woman (it cost her $50, the Davis & Davis minimum for a service call), Mack heads to confront a more typical problem.

"Lady calls saying her unit is not cooling," he explains, having received the assignment over his two-way radio. "The fan is running all the time. It's older equipment."

He lights up a Marlboro for the short trip, pulls up in front of a modest, $250,000, two-bedroom Bethesda rambler, and is greeted by a delighted Carol Thiele, the owner, who had no idea that the Aircon Man would arrive so quickly.

"I usually don't like to have the air conditioning on at all, but when it's humid you can't work," says Thiele while Aircon Man is down in the basement checking out the coil. A research scientist in childhood cancer at the National Institutes of Health, Thiele is home, part of the time, writing a book chapter on her subject.

"I don't have any energy," she continues. "I try to go as long as possible without putting it on, but when I do I feel stupid because it feels so much better. Literally, I can feel my energy level return." But recently, it's just felt warm in the house even with it on, even though the fan has been blowing.

"Okay," says Mack, emerging from the depths, "here's the problem. The air conditioning coil, half of it is completely clogged with dirt. I can pull the sheet metal off the ductwork and clean it."

"Rounded off," she says directly, "what's the damage? Is it just a cleaning problem?"

Can't say yet, he tells her. Got to get that ductwork off, clean up the coil, then run the unit and see if there's anything else wrong. "The refrigerant appears ample," he says reassuringly, "although in there your blower wheel is also dirty. That could use cleaning. Anyway, the coil is where I'd start. That's the major problem. Could be a couple hours at the most."

At $18 every 15 minutes, or $72 an hour.

She mulls it, gives him the go-ahead, and Mack gets a small vacuum cleaner and other equipment from his van before going back into the basement to begin work.

Thiele isn't unhappy, feeling that at least now she has located a competent technician.

"Last year," she says, "I called this other company and they said you need a new air conditioner. They said you can get one through the gas company. I was immediately suspicious. 'Why the gas company?' I asked. 'Because you need a new heater too,' they said. One is always a little nervous about what some of these people try to sell you."

She had turned that offer down, and the repairman had put more Freon into her system and left. Now, Mack emerges once again from the basement to inform her that there is, perhaps, too much Freon in her system -- which has tended to mask her real problems. He won't know for sure until he does some more checking.

"Great," she says.

And so it goes, the precious quarter-hours ticking by as Mack works assiduously in the basement and Thiele returns to her writing room.

In the end, the job costs her $223. Mack cleans the coil -- removing huge chunks of dust and dirt that had kept the metal out of contact with the air passing by it -- and also installs a new fan control device ("$75 for that one part," she moans).

But overall, Thiele is happy with the result. A few days later, she reports that "It's very cool in here now. It works well, and the house is quiet a lot more because the fan shuts off now when it should."

Considering that much of the work involved cleaning, she speculates that "theoretically, it's something I could have done if I was willing to pull it all apart."

"No way," says Mack over the phone a few days later when informed of her speculation. "It was tough getting access to the area that needed to be cleaned, and a customer just wouldn't know what the problem was. And the replacement part, she couldn't have done that herself, not knowing which one to put in, which size."

The one thing that people can do for themselves, he says -- and that, surprisingly, few do -- is to change the air filter regularly.

"Preventive maintenance," he says. "It's simple."