If your furnace is running but the air coming from the vents feels cold, the system may not be completing the heating cycle properly. Sometimes the cause is simple, such as a thermostat setting or dirty filter. In other cases, the furnace may have an ignition problem, flame sensor issue, airflow restriction or another problem that is shutting the burners off.
The first step is figuring out whether the furnace is actually producing heat.
Check the thermostat fan setting
One of the simplest causes is the thermostat fan setting. If the fan is set to ON instead of AUTO, the blower may run even when the furnace is not actively heating. That means air can continue moving through the ductwork between heating cycles, and that air may feel cool. Set the fan to AUTO and see whether the problem stops.
The furnace may not be lighting
If the burners never ignite, the blower may eventually move unheated air through the house.
Possible causes include:
A failed igniter
A gas valve problem
A gas supply issue
A pressure switch problem
A control board fault
Another ignition problem
If the furnace keeps attempting to ignite and fails, it may eventually go into lockout.
The burners may light and then shut off
A furnace can also begin heating normally and then lose the flame after only a few seconds. A dirty or failing flame sensor is a common cause. The flame sensor confirms that the burners are actually lit. If the furnace does not receive the proper signal, it shuts off the gas supply for safety. The blower may continue running after the burners go out, which can make the furnace seem like it is blowing cold air.
A dirty furnace filter can cause overheating
A clogged furnace filter can restrict airflow through the system. When the furnace cannot move enough air, heat can build up inside the unit. The high-limit switch may then shut the burners off while the blower keeps running to cool the furnace down. During that period, the air coming from the vents may feel cool or lukewarm. Check the filter and replace it if it is visibly dirty.
The furnace may be overheating for another reason
A dirty filter is not the only thing that can cause overheating.
Other possible causes include:
- Closed or blocked registers
- A dirty blower wheel
- A weak or failing blower motor
- Restricted ductwork
- An overly restrictive filter
- Other airflow problems
If the furnace keeps overheating, the underlying problem should be corrected rather than allowing the system to repeatedly trip the high-limit switch.
A pressure switch or venting problem can stop the burners
Gas furnaces use safety controls to verify that combustion gases are venting properly.
If the pressure switch does not prove correctly, the furnace may stop the ignition sequence or shut the burners down.
Possible causes include:
A blocked intake or exhaust pipe
A weak inducer motor
Damaged pressure-switch tubing
A clogged condensate drain
A faulty pressure switch
Another venting problem
The blower may still run even though the burners are no longer producing heat.
A clogged condensate drain can affect a high-efficiency furnace
High-efficiency furnaces produce condensation during normal operation. If the condensate drain, trap or tubing becomes clogged, the furnace may shut down part of the heating cycle. This can interfere with pressure-switch operation and prevent the burners from staying on. If the blower continues running afterward, the vents may continue blowing cool air.
The blower may run after the burners shut off
Some cool air at the end of a heating cycle is normal. After the burners shut down, the blower often continues running briefly to move the remaining heat out of the furnace and into the ductwork. As the furnace cools, the last bit of air from the vents may feel cooler. That is different from a furnace that blows cold air throughout most or all of the heating cycle.
The air may feel cooler than you expect
Not every furnace produces air that feels extremely hot at the register. Modern high-efficiency, two-stage and variable-capacity furnaces may operate at lower output for longer periods. The air may feel warm rather than hot. The better question is whether your home is reaching and maintaining the temperature set on the thermostat. If it is, the system may be operating normally. If the furnace is blowing cool air and the house is not warming up, there is likely a problem.
Could the blower motor be causing the problem?
Yes.
A blower motor that is running at the wrong speed, operating intermittently or beginning to fail can affect both airflow and furnace temperature. Poor blower performance can also cause the furnace to overheat, which may shut the burners off. A technician can test the blower motor and determine whether it is contributing to the problem.
When should you call for furnace repair?
You should have the furnace inspected if:
It blows cold air throughout the heating cycle
The burners never light
The burners light and then quickly go out
The furnace keeps shutting itself down
The furnace repeatedly goes into lockout
The house does not reach the thermostat setting
The furnace is overheating
Replacing the filter does not solve the problem
If you smell gas, leave the area and contact your gas utility or emergency service from a safe location.
Need help with a furnace that is blowing cold air?
If your furnace is blowing cold air, the cause could be as simple as a thermostat setting or dirty filter, or it could involve the igniter, flame sensor, burners, airflow, blower motor or another furnace control.
Roberts Brothers Comfort technicians can inspect the system, determine why your furnace is not producing warm air and make the necessary repairs. We provide expert furnace repair service throughout Ohio’s Miami Valley region including Casstown, Covington, Fletcher, Piqua, Rosewood, Saint Paris, Sidney, Tipp City, Troy, Urbana, West Milton and all the surrounding areas.
