Even though San Diego typically experiences fairly mild winters, most residents still rely on a furnace to keep their home warm on any of those chillier days. If everything is working correctly, your furnace shouldn’t have to run all that long to fully heat your home. However, there are times when a furnace can stop working, and your HVAC system will suddenly start blowing out cool air. If you notice that your furnace is blowing cold air, here are eight possible reasons why it is happening and what you can do to fix them.

1. Pilot Light Isn’t Working

If you have a gas furnace, the reason it isn’t heating may be because of an issue with the pilot light. If you have an old furnace that has a standing pilot with a constant flame, it may simply be that the flame has gone out and the pilot needs to be relit.

If you can’t get the pilot light to stay lit, it could be that the gas tube is clogged or that the thermocouple is dirty, bent or malfunctioning. It could also be that the pilot control has failed and needs to be replaced. These problems can occur on furnaces that use either a standing pilot light or an automatic pilot.

Some pilot issues are fairly simple to fix on your own. However, if you continue to have problems with the pilot going out or not lighting, you will need to have it inspected by a certified HVAC technician.

2. Dirty or Malfunctioning Flame Sensor

Many newer furnaces often don’t have a pilot light and instead use some other form of ignition. In this case, it will have a flame sensor that detects whenever gas is ignited inside the furnace. This flame sensor often becomes coated with dirt and grime over time. When this happens, it won’t be able to detect that there is a flame. This results in the furnace immediately shutting down as the flame sensor is what opens and closes the valve to allow gas to flow into the unit.

The solution to this problem is to have a technician clean and inspect the flame sensor. If it isn’t working correctly, it may need to be replaced.

3. Clogged Air Filter

Your HVAC air filter can also create issues with your heating system. If you don’t change the filter regularly, it will eventually get so dirty that very little air can flow through it. This blocks air from getting into the furnace, and no heat will be produced. This problem can easily be prevented by replacing your filter every few months.

4. Thermostat Issues

Your thermostat can also malfunction and fail to signal the furnace to turn on, but this issue usually means that no air will come out of your vents. The more common problem is that you have the thermostat set to “On” instead of “Auto.” When set to On, the fan will constantly run, even when the furnace is shut off. As a result, the fan will quickly start to circulate cold air until the furnace kicks back on. This is why you should always leave the thermostat set to “Auto” since the fan will only run when the furnace is on and producing heat.

5. Ductwork Leaks

If your ductwork is damaged or not fully sealed, it can also cause your HVAC system to blow cold air. Any cracks or holes can draw cold air into the ductwork. If the system is leaking enough, it will cool the air inside the ductwork and cause the system to blow out cold air. This problem is often most pronounced when the ductwork runs through colder areas like your basement or attic. The solution to this issue is to have an HVAC technician inspect your ductwork and reseal, repair or replace it.

6. Gas Supply or Electricity Problems

It could also be that there are issues with the gas supply or electricity that are preventing your furnace from turning on. When this happens, the HVAC fan will still run whenever the temperature gets too low. However, since the furnace isn’t heating, the fan will end up blowing cold air. In either case, you will again need to have the problem inspected and repaired by a furnace technician.

7. Clogged Condensate Drain Lines

Newer high-efficiency furnaces require condensate lines to drain water away. These furnaces utilize a secondary heat exchanger that absorbs the vast majority of heat from the hot combustion gases before they are vented out through the flue. This causes the gases to cool quickly and any moisture in the air to condense into water. The water drips down into a drain pan and then flows out through condensate lines into a floor drain.

The heat from the furnace often causes mold and mildew to grow inside the condensate lines. Dirt and dust can also get inside the lines, and both of these issues can lead to the pipe becoming clogged. Most units have a safety switch that will prevent the burner from igniting if the condensate line gets clogged. This ensures that water can’t overflow out of the drain pan and cause issues with leaking and water damage. The only way to overcome this problem is to have a technician clean out the condensate lines, as the furnace won’t run until the drain system is unclogged.

8. Cracked Heat Exchanger

This is by far the worst of all the possible problems and will mean either a costly repair bill or a need to replace your furnace entirely. The heat exchanger is what transfers heat to the air, and it also seals off the combustion chamber to prevent carbon monoxide and other combustion fumes from entering the rest of the HVAC system.

A cracked heat exchanger can potentially be a deadly problem as it can allow carbon monoxide to circulate throughout the home. The problem can cause your furnace to stop producing heat, but oftentimes you actually won’t know that the heat exchanger is damaged until your carbon monoxide detectors start going off. This is why it is essential that you have a professional HVAC technician inspect and maintain your furnace every year. This is also why it is so vital that you have a carbon monoxide detector near your furnace and at least one additional detector on every level of your home.

San Diego’s Home Heating Experts

If your furnace is not working properly and blowing cold air, you can trust the team at Friar's Heating and Air to fix the issue and get your heating system running again. We specialize in heating repairs and maintenance as well as new furnace installations. We also work on central air conditioners, ductless mini-splits and heat pumps. Our certified HVAC technicians can also take care of any thermostat or ductwork issues. In addition, our team installs and maintains indoor air quality equipment, including HVAC UV lights and whole-home air purifiers. If you’re looking to improve your overall comfort and reduce your energy costs, we can also help install a zone-control HVAC system that can do just that. If you need to have your furnace inspected, contact Friar's Heating and Air today.

company icon