While your high-efficiency gas furnace is running, you may have realized that your house is not heating up the way it should. After further investigation, you may have found that cold air is coming out of the vents. If so, there are a few potential reasons why your home's furnace is blowing cold air, as well as possible solutions you can try.

1.  Thermostat's Fan Setting Is Set to "On"

If your furnace is blowing cold air, the first thing that you should go check is the thermostat. If the thermostat is set to the "On" position, this could be what is causing the issue. When the fan is set to "On," this means that it will always blow air even if the furnace is off. 

To keep this from happening, switch the setting to the "Auto" position. This should ensure that the fan is only running when the heater is on.

However, if the fan setting is already set to "Auto," there could be one of two issues. First, the thermostat itself could be malfunctioning. In this case, you would need to have it either repaired or replaced by a professional.

Second, the issue may not be with the thermostat at all. In this case, continue on to keep troubleshooting the problem.

2.  Furnace's Condensate Line Is Blocked

Another possible reason why your furnace is blowing cold air is that its condensate line is blocked. When you have a high-efficiency furnace, it keeps its high Annual Fuel Utilization Efficiency (AFUE) rating by condensing the gas within the system.

During this process, condensation is produced and is normally drained through the condensate line. However, because of the heat, constant moisture, and dark environment, mold can start to grow within the line. It can also become blocked with dirt or other grime.

After checking your owner's manual for the location of the line and turning off the furnace, remove the line and check for any possible blockages in it. If you do find any, soak the line in a container full of bleach water, then pull it vertically out of the bucket a few times to flush it. Allow it to dry completely, and replace the line.

If you do not find any blockages, or if flushing the line did not help, there could be another issue at play. Continue on with your investigation.

3.  Filter Is Extremely Dirty

After checking the condensate line and discovering it is not the issue, the next place you should inspect is the air filter. Even if you have changed it recently, it could have become clogged with dust, allergens, and even pet hair.

If the filter becomes extremely dirty, it will block the airflow. For the furnace to heat properly, it must pull in recirculated air. If the air is not flowing freely, it will affect the amount of air that is heated.

Even if the filter does not look dirty, either replace it with a new one if it is a disposable filter or clean it if it is a reusable one. You should also wipe the area around the filter with a damp cloth to remove any excess dust, dirt, and allergens that may be interfering with the airflow.

After changing or cleaning the filter, give it a few hours to see if there is any improvement with the air temperature.

If you have tried to troubleshoot possible problems with your furnace and it is still blowing cold air, there may be another issue causing the problem that needs professional attention. Contact a heating contractor to have them inspect your heating unit so that they can take the necessary steps to fix any issues that they may find.