Q - I know my old furnace is not extremely efficient, but I can't afford a new furnace now. Will adding an inexpensive automatic flue damper make it more efficient? Are there any other benefits from it? J. L.

A - Adding an automatic flue damper to your old furnace should lower your utility bills. It also reduces chilly drafts in your house, so you will be more comfortable. With fewer drafts, you may be able to set your thermostat lower and save substantially more.Although a flue damper doesn't actually improve the operating efficiency of your furnace, it greatly reduces the losses when the burner shuts off (called "off-time" losses). Off-time losses occur when heated room air is continually sucked into the warm furnace burner area and out the flue after the furnace burner shuts off each cycle.

These losses can be quite significant, especially if you have made your house more energy efficient and airtight with caulking, insulation, etc. Your furnace is then oversized and the off-time losses are increased.

A flue damper is designed to automatically close the furnace or water heater flue opening when the burner shuts off. This blocks the flow of heated room air up the flue. When the burner comes on again, it automatically opens to allow the exhaust gases to flow out the flue.

A flue damper can also save additional energy because, with the air flow blocked, the furnace heat exchanger doesn't cool down as fast when the burner shuts off. This leaves more residual heat to warm your home.

There are different types of flue dampers. The simplest and least expensive type uses temperature-sensitive metal shutter blades. When the burner comes on and the blades get hot, they flex to an open position.

When the burner goes off, the blades cool down and flex closed again.

Electrically-operated flue dampers use a tiny precision motor to open and close a damper baffle. Although the circuitry can be somewhat complicated, it basically opens when the burner lights and closes when the burner shuts off. There are safety switches so the gas or oil won't start flowing to the burner until the damper baffle is in the open position.

The simple metal shutter blade vent damper should be used only on gas furnaces and water heaters. The electrically-operated type can be used on either gas or oil systems.

You can write to me for UTILITY BILLS UPDATE No. 216 listing manufacturers of and product information about the types of vent dampers and a chart to estimate the savings for various areas of the country. Write to James Dulley, The Deseret News, 6906 Royal Green Dr., Cincinnati, OH 45244. Please include $1 and a self-addressed and stamped business-size envelope.

Q - We are about to select a new bathroom cabinet with built-in lights. Is there much difference in the electrical usage between fluorescent and incandescent bulbs? T. H.

A - Fluorescent lights use only about one fourth as much electricity as a standard incandescent light bulb for the same amount of light output. In most area of your home, fluorescent lights are the best choice.

However, in your bathroom, you may be better off with standard incandescent light bulbs. Even though incandescent bulbs cost more to operate, bathroom lights are not on for very long. For applying makeup, the quality of the light is important. Although there are special fluorescent lights made to approximate standard light appearance, they may not be adequate for you.