There are several possible causes for a furnace blowing cold air, so it can be difficult to determine the exact cause. If you notice cooler air blowing from your furnace, make sure to contact us so we can inspect the furnace for you.
One reason for the issue is if the thermostat is set to the on position. If there is cool air sometimes, or the blower of the furnace is constantly running, check the thermostat. If your thermostat’s fan setting is set to on, the furnace blower will constantly run even when your furnace is not heating the air.
Another cause can be if the furnace overheated. If the furnace blows hot air, then cold, and then stops blowing any air eventually, it could be caused by overheating. If so, the limit switch, a furnace safety device, turned the furnace burners off. The blower keeps blowing to cool the furnace down to a safe level. The most common cause for the furnace to overheat is an air filter that is dirty and clogged. The dirty air filter will block airflow, causing the furnace to run longer to heat your home until it overheats. Change the air filter and see if your furnace starts only blowing warm air. Do not ignore the issued, because repeated overheating will hurt the furnace’s heat exchanger, which is costly to fix.
If your gas furnace is only blowing cold air, then the pilot light may be out. Check to see if it is and try these solutions. First, try and relight the furnace pilot light. If the pilot refuses to light, ensure that gas is flowing into the furnace. Make sure the furnace’s gas valve switch is in the “on” position. This is usually parallel and in line with the gas supply pipe. If the switch is on, then check to see if the pilot light is dirty. If it is, clean it to allow gas to come through. If the pilot lights, but will not stay lit, then there is a problem with the thermocouple, the copper rod you see the pilot flame hitting. It needs to be adjusted or replaced.
If there is an issue with your furnace or HVAC system, make sure to schedule an appointment so we can inspect the system for you. This can help you to have an efficient and reliable furnace this winter.