If your gas furnace is blowing cold air, a number of factors could be to blame — many of which you may address on your own, without the immediate help of a service professional. Here’s an explanation of some of the most common problems and solutions associated with gas furnace failure.
The Thermostat is Not Functioning Properly
First, check to be sure that your thermostat is set to “heat” and not “cool,” and ensure that the fan is set to “auto.” Then, check to be sure that the thermostat is set to a temperature high enough to trigger the furnace to emit heat. If your thermostat is properly set, ensure that it is getting power — check batteries, fuses and circuit breakers — and ensure that the interior components are clean. Dust and debris inside the thermostat can cause it to malfunction. Fortunately, this is usually an easy fix. In some cases, a malfunctioning thermostat may require adjustment or replacement.
The Furnace is Off
Ensure that your furnace is getting power. Check fuses and circuit breakers and ensure that the emergency power switch to your furnace has not been mistakenly turned off.
A Reset is Required
If your furnace has a reset button (there’s a good chance it’s a red button located on the front, side or back of your furnace), try pressing it. If the furnace powers up, you should soon have heated air cycling through your home. Sometimes, pressing the reset button is all you’ll need to do to get your furnace working properly.
The Door Cover Is Open
It is possible — particularly if you’ve recently changed your furnace filter — that the door to your furnace has been left open or closed improperly. If this is the case, properly shutting the door cover on your furnace may fix the problem.
The Filter is Dirty
Dirty furnace filters are one of the most common causes of a furnace failure. A dirty furnace filter can limit the airflow through your furnace and cause it to overheat. If you suspect a dirty air filter, change the filter, press the reset button (if your furnace has one) and see whether your furnace begins to emit heated air. Change your furnace filters regularly — pros recommend changing them once a month — to ensure that your furnace continues to work properly. Unaddressed, dirty furnace filters can cause extensive, and expensive, damage to your home’s heating system.
The Pilot Light is Out
If you have an older furnace with a standing pilot light, it’s possible that the pilot light has been extinguished. First, confirm that the light is out. Then, turn the gas valve to the off position and wait five minutes. If there is a reset button on your furnace, turn the valve to the pilot position and hold a flame to the dedicated pilot opening while pushing the button for about 30 seconds. (If the pilot doesn’t light on the first attempt, wait a few minutes and repeat the procedure, this time depressing the reset button for 45-60 seconds.) Once the pilot is lit, turn the gas valve back to the on position. If the pilot light won’t stay lit, it’s possible that there is an issue with the thermocouple or ignition system. Call an HVAC professional for assistance.
Your Heating Registers Are Closed
Keeping too many heat registers closed can cause heat to build up in your furnace, forcing air out of duct leaks and requiring the blowers to compensate for the added pressure. Essentially, blocking heating vents causes your furnace to work harder and longer than it is intended to work. Further, it forces air pressure to build up in your air ducts, which may both cause and worsens airflow leaks — particularly at joints and seams. Pros recommend keeping at least 80 percent of your heating registers open to prevent unnecessary air from building up in your furnace.
The Flame Sensor Needs Cleaning
The flame sensor safeguards your furnace against the unsafe burning of fuel, and a dirty flame sensor may cause your furnace to function improperly. Typically, the flame sensor comes in the form of a rod that can be found near the back of the furnace, right in the path of the burner. To clean the sensor, turn off power to the furnace at the breaker box. Then, if possible, remove the sensor from the furnace. Scrub the sensor gently to remove dust and other residues. Finally, reattach the sensor and resume power to the furnace.