When the furnace is blowing cold air, but the thermostat is in the heat setting, there is no question that the furnace is not functioning properly. The question instead is, why is the furnace not working correctly? Unfortunately, many reasons can cause a furnace to malfunction, but fortunately, once the issue has been identified, repairs are often manageable.
Condensation Line Blockage
Many newer furnaces are designed as high-efficiency models. As part of their energy-saving design, they have condensation drain lines that route excess moisture away from the system.
If these lines become blocked for any reason, the excess moisture will back up into the furnace instead of flowing out, which can essentially cause the burners to stop working. Without active burners, the unit is unable to produce heat. The blockage must be removed so that the moisture can drain out to restore the burner function and, thus, return the flow of warmed air.
Pilot Light Failure
If you have a gas furnace and you are experiencing this issue, more than likely, the pilot light is to blame. The pilot light is somewhat of an ignitor that initiates the heating process within a furnace, so if the light is not working correctly, the unit will not blow warm air.
An issue with the furnace can cause the pilot light not to ignite, or the light might ignite but fail to remain lit. While a homeowner can reset a pilot light, typically, if the core of the issue with the light is not addressed, you will repeatedly deal with this issue. It is best to have the light repaired or determine whether it needs to be replaced.
Flame Detector Obstruction
If you have a gas furnace but you do not have a pilot light, the source of the problem could be the flame detector. As its name suggests, this component's function is to detect when there is an active flame within the unit.
Dirt and dust can collect on this detector, which can minimize its ability to detect flames. When the flame is not detected, the system will immediately begin blowing cold air, which is what you might be experiencing. Only a repair professional can clean the detector correctly, so do not attempt it yourself.
If your furnace is not blowing warm air, be sure to contact a technician at your earliest convenience to have the issue diagnosed and addressed. Speak to a company like B & H Heating to learn more.