Oil vs. Gas: Which is better?

I have an old coal-burning furnace which has been converted to oil but have been told it is very inefficient and I should buy a new one. Is there any rational way I can decide which will cost me less money to run over the next 10 to 15 years: Oil or natural gas? We live in a two-family, 10- room house with a large attic and basement. It has aluminum siding over some kind of insulating material. The fuel cost for the last 12 months was about $950. Joan B. Mangum Staten Island, N.Y.

The difference between gas and oil is not in efficiency.

An oil furnace is only about 2 percent higher in efficiency than a gas furnace when it comes off the factory floor, an expert tells me. The reason is that the oil system has a power burner which has a smaller hole while the gas burner has a bigger hole -- and you're losing air up the stack all the time the burner is off.

Thus, the differences between the two are not in efficiency. Why not go out and buy a new oil furnace in the spring, advises the heating expert.

"I believe he will find an improvement right away," he adds.

However, there is the ever-present risk of an oil shortage in the future. Who can predict what will happen? Natural gas was said to be in short supply a few years ago and now there is a glut.

You might also ask the contractor about putting in a smaller stack and blocking up the old one. The old coal-burning furnace probably had a large flue which you no longer require.

You can write to the Consumer Information Center, Pueblo, Colo. 81009 and ask for a copy of a 24-page booklet, "Home Heating." The number is 048G and it costs Furnace," No. 605G is free.

While you're at it, you might want to ask for a copy of the Consumer Information Catalog, a free pamphlet which lists scores of publications available to the public, either free or for a nominal charge.

Vent dampers in oil furnaces will save energy but some people have complained about odors. It probably depends on the ventilation in the house.

"We had this problem in some field studies we did," says a government engineer, "and we think there might be a drip from the oil burner down into the hot firebox after it has been shut off. Those vaporization fumes could work their way up into the house."

Ask a local heating contractor about a flue damper.