6 Reasons Why Furnaces Stop Heating Air

dirty hvac air filter

Has your furnace started blowing out cold air or stopped working altogether? Before you worry, the good news is that the solution might be very simple. Below, we’ve outlined some basic troubleshooting steps for you to follow when your furnace loses its “heating power.”

1. Incorrect Thermostat Setting

When troubleshooting, start by checking the simplest things first. For a furnace that’s blowing out cold air, check the thermostat before anything else. If the fan is set to ON, then your HVAC system will continue circulating air throughout your home whether your furnace is heating that air or not. Make sure your thermostat is set to AUTO instead.

2. Clogged Air Filter

A dust-clogged air filter prevents adequate airflow from circulating through your heating system. Without enough airflow, your system can overheat and shut down. If your furnace starts to overheat, the unit’s high limit switch might trip to shut off the burners. This prevents excessively high temperatures from causing the heat exchanger to crack.

Shut your furnace off, and replace its air filter. If the unit still doesn’t work, contact a technician to help you reset it.

3. Tripped Circuit Breaker

Furnaces can trip their respective circuit breakers for numerous reasons. A temporary power surge is a typical cause. However, suppose your furnace continues to trip the breaker after you reset it. In that case, this could indicate an urgent electrical issue that needs attention, such as an overloaded circuit, a short circuit, or a problem with your circuit breaker.

4. Standing Pilot Light Problem (in Older Furnaces)

If you own an older unit, it might still have a standing pilot light. (Manufacturers stopped making furnaces with pilot lights around 2010.) If your pilot light isn’t working, it can’t light the burners in your furnace, and without burners, there’s no way to get hot air.

Sometimes a strong draft can extinguish a pilot light, in which case you only need to relight it. Pilot lights can also have trouble staying lit if the pilot tube is dirty and grimy. If your unit hasn’t had preventive maintenance in a few years, cleaning the pilot tube might solve the heating issue.

5. Faulty Thermocouple or Heat Sensor

If you own an older furnace with a standing pilot light, then it has a thermocouple. If your furnace uses electronic ignition instead of a standing pilot light, it has a heat sensor. Both thermocouples and heat sensors are safety features that shut off gas to your furnace when they sense something is wrong with the ignition process.

  • The thermocouple in an older furnace will shut off the burners’ gas supply if it can’t detect the standing pilot light’s heat. In some cases, this happens because the pilot light has blown out. In other cases, the thermocouple might not be in the correct position right over the pilot light’s flame, causing it to “think” that the pilot light isn’t lit. It could also be that the thermocouple is broken and needs to be replaced.
  • The flame sensor in more up-to-date furnaces will shut off the burners’ gas supply if it detects an issue that’s keeping the burners from igniting. If you have a faulty flame sensor, you’ll most likely need to replace it.

6. Clogged Condensate Line (in High-Efficiency Furnaces)

During the heating process, high-efficiency furnaces create condensate (moisture). This moisture drains outside your home through a pipe called the condensate line. If dust or mold clog the line, or if the condensate pump is broken, the water that needs to drain out has nowhere to go but backward. Therefore, the water will start backing up into your furnace. When this happens, the furnace’s overflow kill switch trips and shuts down your furnace.

To fix this problem, you’ll need to unclog the condensate line or replace the broken condensate pump. If a clog was the problem, you can easily prevent this issue in the future by scheduling routine maintenance for your heating equipment.

Contact Albright's Mechanical Services for quality home heating maintenance, repairs, and replacements in Baltimore and neighboring communities: (410) 834-0148.

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