Why is My Furnace Blowing Cold Air?

“Why is my furnace blowing cold air?” is a question heard often at Basset Home Services. It’s a legitimate question. After all, aren’t furnaces supposed to blow warm air?

Well yes, that is what they are designed to do. So when someone asks me “why is my furnace blowing cold air?” I know that they have a malfunctioning furnace. Often it is a simple problem to correct. But it could also indicate a more serious problem and can be taken care of by our Colorado Springs heating experts.

If your furnace only blows cold air when you set your thermostat to heat, it is trying to tell you that something is amiss. Yeah, I know. Furnaces don’t talk. But they do communicate. They usually “tell” us when something is wrong. By acting unusual or making strange noises, it is their way of telling us “I need attention.”

So, why is my furnace blowing cold air? Basset Home Services is here, ready to help you with your Colorado Springs furnace services. But I’ll dig a little deeper to explain the most common causes as to why is my furnace blowing cold air?

Furnace Control Board Issues

Almost all modern furnaces have an IFC (integrated furnace control) board. And these circuit boards usually have a small LED diagnostic error code light. It will blink a code when something is wrong with the furnace. You can view a photo of a IFC board with a diagnostic light here. These circuit boards are usually mounted on the blower housing behind the blower compartment door.

Furnace Error Code

The furnace should also have a diagnostic error code chart. This chart will indicate via a blinking LED light why the furnace is not functioning properly. The chart is usually located on the inside of the blower door panel, but sometimes you will find it on the front of the door panel. The chart should look similar to this furnace panel error code chart.

If you’re asking, “why is my furnace blowing cold air?” you will most likely see an error code indicating an open limit or roll-out switch. So what exactly does this mean?

Faulty Limit Switches

Limit switches and roll-out switches are safety controls designed to shut off the gas to the furnace in the event of overheating. When a high-limit temperature or roll-out switch trips, it is because the furnace is becoming dangerously hot and needs to cool down. The open limit circuit should turn off the burners yet keep the blower running in order to cool the furnace down.

Some roll-out switches have a manual reset button (seen here). Some are auto-reset, as are almost all high-temperature limit switches. Auto reset limit switches are supposed to reset after the furnace cools down, although I find that often times they don’t. Sometimes tapping on the front of the switch with a small tool (such as the end of a screwdriver) will reset it.

Other Common Reasons Why a Furnace is Blowing Cold Air

So what causes these switches to trip? If you find that you have an open roll out or limit switch, here are the most common causes that occur.

Dirty or Plugged Air Filter(s)

If your furnace filter(s) become plugged with dirt or pet hair, not enough air can get through to the blower. This results in the heat exchanger becoming too hot, which causes the high-temperature limit circuit to open. When this happens, the burners shut off while the blower motor continues running until the furnace cools down.

If the limit switch resets on its own, the furnace will short-cycle, blowing warm air followed by cold air. If it does not automatically reset, the furnace will continue to blow unheated air into the home.

Solution:
Replace the filter(s) and reset the limit switch if necessary.

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Failed Igniter or Dirty Flame Sensor

Some model furnaces will activate the blower if the igniter fails to light the burners. A dirty flame sensor can cause the burners to ignite and then shut off before the blower activates, which can result in overheating of the heat exchanger.

Solution:
Remove and clean the flame sensor and reset the limit switch if necessary. If the igniter has failed, it is recommended to call a professional to replace it.

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Dirty or Plugged AC Evaporator Coil

If your furnace is paired with a central AC system and the evaporator coil becomes dirty or plugged with animal hair (a common issue), airflow through the furnace becomes restricted. This restriction can lead to overheating.

Most AC evaporator coils are difficult to access and may need to be completely removed to be properly cleaned.

Solution:
Call a professional to correct this problem. Never operate your furnace without filters in place, and ensure filters are properly sized and fit snugly to prevent dirt and debris from bypassing the filter and entering the blower or AC coil.

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Open Roll Out Switch

An open roll-out switch can indicate a serious issue. Roll-out switches are designed to trip if flames roll out or drift from the heat exchanger, which may signal a faulty heat exchanger and a potential carbon monoxide hazard.

Roll-out switches are typically mounted above the burners or on the side of the burner compartment. Many furnaces have more than one roll-out switch.

Solution:
If you experience this issue, it is strongly recommended to call a professional to diagnose the cause.

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Dirty Blower Motor or Blower Wheel

If the blower motor or blower wheel becomes too dirty, airflow is reduced, causing the high-temperature limit switch to open. These components cannot be properly cleaned with a vacuum alone.

The blower assembly must be removed from the furnace and disassembled for thorough cleaning.

Solution:
Regular preventive maintenance performed by a heating contractor will prevent this issue, along with routine filter replacement and using properly sized filters.

For more information or if you need to schedule a service on your furnace in Colorado Springs, you may contact Basset Home Services today.