Gas furnaces use either natural gas or propane to heat an enclosed area. A significant advantage of this method of temperature control is that gas burns much cleaner than the oil used with oil furnaces. Consequently, mechanical problems tend to arise much less frequently with gas furnaces, and when they do, they are generally easier to diagnose and, therefore, faster to correct.
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When a gas furnace fails to produce sufficient heat, or produces no heat at all, the problem is usually the result of a closed control valve, a faulty circuit or fuse, an incorrectly set or non-working thermostat, or a pilot light that's out. To correct the situation, open the control valve sufficiently to allow proper gas flow, adjust or replace the circuit or fuse to restore power, reset or repair the thermostat as required, or light the pilot flame again.
Even when a gas furnace is generating heat, sometimes the temperature or amount of heat is inadequate. Both problems can occur if the blower is blocked or the blower belt is damaged or loose, the burner or filter is too dirty, or the setting on the thermostat is too low. Simply raise the temperature setting in the last case and either clean or restore the appropriate part in the previous cases.
When the blower is clogged, the filter dirty, and the motor or blower overly dry, the furnace can switch on and off too frequently, leading to erratic heating. Fix the problem by cleaning the dirty part or lubricating the blower or motor in the proper port areas. If the irregular behavior is related to a problematic thermostat heat anticipator instead, then reorient the adjustment arm accordingly.
Pilots that won't light are commonly the result of a clogged pilot opening or no gas due to a closed or insufficiently open valve. Clear the blockage in the first case and adjust the gas valve in the second, then try to light the pilot flame again.
If the pilot won't stay lit, check to ensure the pilot flame isn't set too low, the thermocouple nut isn't loose, and the thermocouple itself isn't damaged. Set the pilot flame to at least two inches in the first case, tighten the nut in the second case, or replace the entire thermocouple in the third case. Occasionally, a defective pilot can be the culprit if the furnace uses the electronic kind, in which case a professional will need to make repairs.
Carbon monoxide buildup and gas leaks are potential deadly hazards if not tended to immediately. Most gas furnaces produce a negligible amount of carbon monoxide during normal operation. However, it's essential to have a working carbon monoxide detector to signal if the amount present has reached a harmful level because of a malfunctioning furnace.
Unlike carbon monoxide, which is odorless, gas has a unique and strong smell. Individuals can often detect a gas leak when a furnace malfunctions because they smell the substance. If that happens, exit the building immediately. Make sure to keep all lights exactly as they are, whether on or off, and leave the door open. Once safely outside, stay there and contact the fire department or the gas company for assistance. Do not attempt to repair such gas leaks yourself.