Do you have your heating running and suddenly realize that cold air is blowing out of your vents? This issue is somewhat rare with a gas furnace since various mechanisms should prevent the blower from running and circulating air if the furnace’s burners are lit. Any time the burners are lit, the air flowing through your furnace always gets heated, resulting in hot air coming out of the vents. Nonetheless, there are a few potential situations where the blower may run when the furnace isn’t lit, resulting in your HVAC system just circulating cool air and not providing any heating. In this article, we’ll look at what can cause this to happen so you know if you need to enlist the help of a professional furnace services provider.
Incorrect Thermostat Settings
The one time you’ll occasionally have cold air coming out of your vents when your heating is turned on is if the fan setting on your thermostat is turned to “On.” The fan runs alongside your furnace when it is on the “Auto” setting. That means it turns on shortly after the burners light and then runs for up to a few minutes after the furnace shuts off. If you have the fan on the “On” setting, it will run continuously without ever shutting off. This isn’t an issue when the furnace is also running, but it will result in the blower drawing in cold air and pumping it back out of your vents not long after the furnace shuts off.
Control Board or Wiring Issues
While the blower should shut off shortly after the furnace does, there are times when an issue arises that prevents it from turning off when it should. This can happen because the control board malfunctions and doesn’t signal the blower to shut off. There could also be an issue with the wiring between the control board and the blower that leads to the signal not reaching the blower. This most commonly happens due to a loose electrical connection, which can lead to the blower only intermittently turning on and off when it should.
Furnace Overheated
Another time when your heating system may temporarily start blowing cold air is if your furnace overheats before completing a full cycle. Gas furnaces have a safety mechanism known as a temperature limit switch that monitors the temperature of the furnace’s heat exchanger and will cause the furnace to shut down if it gets too hot immediately. This happens because major damage can occur due to the furnace overheating, or the furnace could even end up catching fire.
While the limit switch will always cause the furnace to shut off, the blower will continue running. The reason is that cold air must keep flowing over the heat exchanger to help it cool down more quickly and reduce the chances of any damage occurring. The limit switch monitors the temperature inside the furnace and won’t signal the blower to shut off until the heat exchanger sufficiently cools. That means if your heating system is blowing cold air due to the furnace overheating, the blower should shut off within 15 minutes or so.
If you suspect that your furnace is overheating, you should change its air filter. While overheating can occur for other reasons, it most commonly happens because the air filter is dirty and greatly restricts how much cool air the blower can draw into the furnace.
Broken or Malfunctioning Temperature Limit Switch
One issue when a furnace overheats is that the heat can end up damaging the temperature limit switch and either ruining it or causing it to malfunction. When a furnace turns on, the limit switch monitors the temperature and eventually opens once it senses the heat exchanger is sufficiently hot. The switch opening is then the signal for the blower to turn on and start bringing air into the furnace. The issue if the furnace overheats is that it can cause the limit switch to fail and get stuck in the open position. That means the blower won’t ever shut off since the switch can’t close and signal it to do so. In this situation, the only way you can get the blower to shut off is to switch off the circuit breaker for your HVAC system.
Ductwork Issues
Leaky ductwork or uninsulated air ducts in an attic or crawl space can also result in cold air blowing out of some of your vents. Nonetheless, these issues are something you would’ve typically noticed beforehand since they’d lead to your heating system frequently or constantly blowing cold air in some parts of your house.
If your furnace is blowing cold air or you’re experiencing any other problems with your heating, air conditioning, or electrical system, the experts at Putnam Mechanical are here to help. Our team has over 75 years of combined experience and the knowledge and skills to assist with all of your heating repair, maintenance, and installation needs. Contact us today to schedule a service call in Mooresville or the Greater Charlotte area.
