Why Does the HVAC System Not Blow Hot Air?

Thermostat and Pilot Light

• When heat is turned on for the first time at the beginning of a cold season, the HVAC system may not blow hot air. An often-overlooked reason is because the thermostat is still set on “cool.” Switching the thermostat setting to “heat” easily corrects this problem. Some gas furnaces have pilot lights that must remain burning for the system to generate hot air. If pilot lights go out, they must be re-lit to ignite the burners.

Air Filters

• Some HVAC systems have replaceable air filters, and other systems have cleanable filters. The U.S. Department of Energy recommends cleaning or replacing filters at least every three months. Filters become clogged from trapped dirt, pollen and pet hair, which makes the HVAC system run less efficiently. A severely clogged filter causes a furnace to overheat and triggers an internal safety switch that shuts down the system and doesn’t allow the system to blow hot air.

Burners

• Heater burners must be cleaned periodically to function at peak efficiency. Deposits of soot, debris and rust accumulate inside burners where they clog jets. Your HVAC system will not be able to blow heated air throughout your house because of restricted heat flow. Burners can also move out of alignment, which offsets the direction of the flame and prevents air from heating properly. Burner problems can present hazards, which should be addressed immediately by licensed contractors.

Gas Supply

• If the gas supply is cut off at the meter, or if the gas meter malfunctions, there is no fuel source to ignite the burners. Your furnace will still operate, but it will blow cool air. When gas company employees work on gas mains because of severed gas lines, they must cut off the gas supply at the point of damage. This disables the fuel source to your home and your furnace will not be able to supply hot air.