As the vice president of Norman Bernstein Management, Sidney Levy is the fellow the hundreds of tenants in his company's apartment complexes would curse if their bedrooms turned unreasonably cold next winter.

Levy is well aware of that fact, and he also knows that it might be difficult for him to obtain a sufficient supply of home heating oil to keep the apartments warm. "Right now," he says, "we feel we can't assure our tenants of oil."

The solution: switch to natural gas.

Many apartment managers like Levy, and homeowners as well, have taken hold of that idea and flooded the local gas utility with calls, asking if they can convert their heating oil systems to natural gas.

In the past month, the Washington Gas Light Co. has been averaging about 100 such requests a day, according to public information officer Paul Young.The calls started coming in, Young says, soon after news stories noted that fuel oil prices were rising.

"Our supplies right now are good," Young says. "Most of it is domestic gas. Gas for many years has been appreciably cheaper than oil or electricity."

Natural gas costs $3.70 per 1,000 cubic feet, up 8 percent from last year. Heating oil, on the other hand, now costs 75.8 cents a gallon, up 50 percent from a year ago.

It is not that easy to make the switch, however.First, the utility company must determine that there is a gas main in the area or that it would be economically reasonable to install one.

And the present is costly. Levy, who wants to convert to gas in three of the 18 apartment buildings his firm manages, says switching is only practical in buildings with 100 units or less. "You have to buy a whole new boiler," he notes. "It costs a considerable amount of money - $20,000 to $30,000. "You're looking at a long-term investment."