There are several reason why a central heater might blow cold instead of warm air and these include a blown out pilot light, an overheated furnace or its thermostat is set to "On." These three issues are the most common causes of why a central heater will blow cold air and the ways to fix these issues are relatively simple.
The following are the three common reasons why a central heater blows cold air and how to troubleshoot and fix the issue.
Blown out pilot light
A pilot light is a small flame that is constantly lit within a furnace and is on standby to light up the furnace when the homeowner turns on the heater. Follow the instructions that came with the heater's manual in relighting the pilot light. If the pilot light refuses to light up, check if the gas valve is set on "On" or if the pilot light needs cleaning.
An overheated furnace also causes cold instead of warm air to come out of the vents. This is because once the furnace gets too hot, the high temperature triggers the limit switch that turns off the burners but keep the blowers running to cool the furnace before shutting down. Dirty furnace or air filters are the common causes of overheating furnace and cleaning them out will usually solve the problem.
Thermostat set on "On"
If the setting of the thermostat is set on "On," the blower will keep on releasing cold air. Adjusting the setting to "Auto" will solve this simple issue.