Furnace adjustment could take some chill out of your heat bill

Easy-to-get information from the Department of Energy, local heating contractors, and rising fuel bills have made homeowners energy-wise and cost-conscious about heating their homes.

In addition, many homeowners are calling on their local utilities for energy audits these days.

Among other things, an energy auditor may recommend that a furnace be ''derated'' and a ''flue restrictor'' installed.

''I've heard of automatic flue dampers and how they're supposed to work,'' one homeowner says. ''But furnace derating and flue restrictors?''

Licensed heating contractors define ''derating a furnace'' as making an adjustment to scale down the amount of gas going through the burner in a furnace. A flue restrictor is a device used to slow the flow of hot gases up the flue.

Both refer to operations of gas-fired, forced-air furnaces.

Many of these furnaces are oversized for the houses they heat. Before the energy crunch, it was not unusual for a home builder to put in a larger furnace than was needed. Perhaps a 100,000-btu furnace would be placed in a house that only required an 80,000-btu model.

An oversize furnace runs in short bursts and allows excess heat to flow up the chimney.

A furnace that is just the right size for the house runs for longer periods in cold weather, and it provides better humidification and air-convection currents. If a house has an oversize furnace, energy auditors may suggest both a ''flue restrictor'' and ''derating.''

The restrictor varies in cost, depending on its size, and looks just like a normal flue damper. But once the installer makes the correct adjustment for the particular furnace and flue, the restrictor doesn't open and close like a damper , because the restrictor doesn't have any moving parts. The restrictor's function is simply to reduce (restrict) the rate at which hot gases can flow up and out the chimney, thus retaining heat longer in the furnace.

If an energy auditor recommends the installation of a flue restrictor, he will at the same time call for a gas-flow adjustment in the furnace. And a qualified, licensed heating contractor should make this adjustment through the burners.

Making an older furnace more heat-efficient is a job for an expert. Rarely should a homeowner rely on the claims of the manufacturers of furnace devices.

The Department of Energy says the best idea is to have a local utility company make the energy audit for a nominal fee. Then take the recommendation of a licensed heating contractor to check out your heating system to determine what is needed in your particular furnace.

Use energy-audit figures as a rough guide for the payback figures; in other words, the number of years of fuel savings required to pay you back for the cost of the new furnace equipment.

One utility company suggests that if the payback period requires more than seven years, the installation devices are not worthwhile.