The question was whether it really saves fuel to set a home thermostat down to 55 overnight. On the line was Prof. Seichi Konzo, at age 71 retired from the University of Illionois but still much in demand as a consultant.

"Professor," I said. "The Honeywell people calculate that in the Washington area a 10-degree thermostat setback overnight will save about 13 per cent in fuel. Yet one of my readers says he uses more fuel, not less, when he sets his thermostat at 55 overnight. John Muller of FEA says you can set me straight."

"I'll try," the professor said. "What your reader has described can really happen in very cold weather. In some situations you can use more fuel at an overnight setting of 55 than at some higher setting."

"Why does this happen only in abnormally cold weather?" I asked.

"To understand that you must understand what happens when your furnace first turns on, and what changes take place after it has been on for a normal length of time, and then for an abnormal length of time.

"Let me tell you about some things we've learned in studies conducted in our experimental homes. The University built the first of its research residences in 1924, and we've been learning from them ever since.

"To keep the arithmetic simple, let's assume that we're dealing with a gas furnace that produces 1,000 BTU's of heat for every cubic foot of gas it burns. We knew that about 70 per cent of that heat was being delivered to the upstairs registers. What we wanted to find out was what happens to the rest of the heat.

"We learned that about 50 BTU's escape from the furnace into the basement. But that isn't really 'lost' heat because eventually it finds its way up through the ceilings and floors into the house. There is also some leakage from the ducts into the basement, but that, too, isn't really a loss.

"The major loss is what goes up the chimney in what's called flue gas. In round numbers, that's about 250 BTU's. But if the chimney is inside the house, about 50 of that 250 leaks back into the house before it gets outside, and the net loss in a normal operation is 200. In other words, that furnace is operating at 80 per cent efficiency.

"However, when a furnace must work for an unusually long time to bring a house back up to a desired temperature, its efficiency drops very substantially - whether the fuel is gas, oil or coal. As the furnace gets hotter, the heat recovery from it becomes proportionately less. It can no longer transfer heat into the living quarters of the house as rapidly as it is putting out that heat - and so a much greater percentage of the heat begins to go up the chimney."

In most homes, Prof. Konzo told me, the builder installs a furnace that is deemed adequate for the sixe of the house and the climate in which it is located. When temperatures in that locality remain normal, the furnace should function well, and dealing down at night should produce reasonable fuel savings. However, when freak weather hits, all bets are off.

If your builder assumed that Washington seldom sees temperatures below 10 or 15 but your furnace had to cope with temperatures that were actually 20 or 25 degress below that, it may have had to work so hard in the morning to bring back to 65 that you wasted more gas than you saved.

Prof. Konzo's summary was: "Dialing down does save fuel in normal weather, but not when the weather is abnormally cold."

Closing off rooms also helps to conserve energy, he said, but not as much as you might think. "Too much heat leaks into the unused rooms from the rest of the house." And as for saving fuel by burning logs in a fireplace, unless your fireplace is equipped with tempered glass doors that can be shut when you go to bed, your fireplace is no bargain. When there are glowing embers left at bedtime and you must therefore leave your damper open overnight, the heat in your house - and your money - go straight up the chimney.