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Q - I have an older gas forced air furnace and I want to give it a fall checkup. What type of things can I check and maintain myself and what are some common problems to expect? K.S.

A - It is always a good idea to do some minor maintenance each fall. By spending an hour or two, you should be able to cut at least $100 off your annual utility bills. A gas (natural or bottled) furnace should be inspected by a qualified service technician every several years.Your fall checkup should include a change of the furnace filter. It is inexpensive and should be changed every month or two. Switch off the electricity to the furnace. As a safety check, put a few drops of soapy water on all the gas line fittings. Bubbles indicate a small gas leak.

Remove the cover to gain access to the blower. You should lubricate the blower motor and pulley bearings. Just a few drops of oil is adequate. Check the tension on the drive belt from the motor. At the proper tension, the center of the belt should flex about one-half inch. You can tighten it by loosening the motor mount bolt and adjusting it.

Turn up the thermostat to see if your furnace is working. It is normal to have a short delay before the blower starts. There is a temperature sensor that delays starting the blower until the heat exchanger gets hot enough.

If it did not start, first check the pilot light. It may have gone out over summer. Follow the simple relighting instructions usually printed somewhere on the furnace near the pilot light. Make sure you switched the electricity back on. You may also have a faulty thermostat.

Once your furnace is operating, you can check for the proper blower speed by measuring the temperature rise of the air blowing through the furnace. Drill a small hole in the return duct and measure the air temperature. Drill another hole in the warm air duct and measure that temperature.

If the temperature rise is not within specifications, you should change the blower speed. Speeding up the blower lowers the temperature rise, and slowing the blower increases it. The blower speed is usually changed by simply adjusting the diameter of the pulleys.

One common problem is too little heat output. This may be caused by a dirty air conditioner coil, low manifold pressure or a bad thermostat. If the main burner cycles on and off while the blower stays on, the manifold pressure may be too high or the temperature limit switch may be faulty. Yellow burner flames are caused by a dirty orifice or poor ventilation.

You can write to me for UTILITY BILLS UPDATE No. 241 showing a do-it-yourself fall furnace checkup guide and a troubleshooting guide for gas furnaces listing 20 common problems and the proper corrective actions. Write to James Dulley, The Deseret News, 6906 Royal Green Dr., Cincinnati, Ohio 45244. Please include $1 and a self-addressed business-size envelope.

Q - The wood back door on our house is in fairly good condition and the weatherstripping seals well, but it is not insulated. Would it be worthwhile to install a new, better-insulated door? R.S.

A - An old wood door shrinks and grows a lot with weather changes and it is probably leakier than you think. Most of the heat loss is from outdoor air leaking past it, not the heat loss through it. On windy days, check for air leaks holding a candle near the gaps and observing the flame.

If it really is airtight year-round, then the savings from replacing it with an insulated door will not pay back its cost quickly.