Nothing’s worse than a house without heat on a cold night. Chances are, if the temperature has unexpectedly plummeted inside your home, you may have an issue with your furnace. Before you figure out the problem, it helps to learn a little about how your home heating system might work.
How Does a Furnace Work?
The furnace produces heat by burning natural gas or heating oil. When the thermostat is turned on, a pilot light or electronic igniter lights the fuel in a combustion chamber, which begins to warm the heat exchanger, located just above the combustion chamber. A blower fan directs air across the heat exchanger, and when the temperature reaches a certain point, the fan comes on and blows the heated air out into the heating ducts and into the interior of the house. Cooler air cycles back through return ducts into the furnace, and the process starts all over.
What to Do if Your Furnace Only Blows Cold Air
When that doesn’t happen, there are a few things that could have gone wrong. Walk through the steps below to help you determine why your furnace is blowing cold air. And then if you do need to call in a pro, check out this guide on furnaces that will give you a good overview of how much repairs or a new furnace will cost.
Check the thermostat
If your furnace only blows cold air after you’ve turned on the heat, start your troubleshooting at the thermostat. Double-check that it’s set to “Heat” and “Auto,” not “On,” which may only run the blower fan, especially if you have central air-conditioning as part of your HVAC system. Give the furnace a few minutes for the heat to circulate. Then once you’ve determined that there’s no change, turn up the temperature by five degrees to see if the furnace will produce heat.
It’s a good idea to also see if the thermostat’s batteries need to be replaced; it won’t work if the batteries are dead. Another thing to try: If you have an older thermostat, remove the cover and blow or brush off any dust that may have accumulated inside its interior. If everything checks out, but it still blows cold air, try calling a pro to run a diagnostic.
Test out the furnace
If the thermostat checks out, the next thing to check is the furnace itself. However, you’ll need to know if you have an oil- or gas-fired furnace, as well as whether you have an older or newer model, since they each work differently.
If You Have an Oil-Fired Furnace:
Most oil-burners are controlled by a fan and a limit switch. In older furnaces, the devices are set manually; in new models, the controls are electronic and automatic. When the thermostat calls for heat, the burner turns on. When the heat exchanger heats to a predetermined temperature, 150 degrees for example, the fan and limit switch turns on the blower fan. The furnace will continue to produce hot air until the thermostat reaches its temperature setting and turns off the furnace. If the fan continues to blow, but the burner is no longer producing heat, the fan and limit switch has malfunctioned.
If you have an older furnace, find the switch box. Check to see if the auto/manual switch is stuck; it should be in the “auto” position (the “manual” setting will make the blower run continuously). Pull off the cover; if the dial is showing an on-off range of 100-150 degrees, then adjust the lower pointer to below 100. This should cause the cycle to begin again. But if it shuts off, there’s a problem, and you’ll have to get a professional to evaluate the problem.
If your furnace is a newer model with an automatic fan and limit switch, a bad circuit board may be signaling the blower to run without turning on the burner. Unfortunately, there’s not much you can do except call a furnace technician.
The fan and limit switch will also override the burner if the incoming air supply is constricted, which can be caused by a dirty air filter. Pull out the air filter. (It’s usually located on the intake side, tucked between the return ducts and the furnace itself.) Check its condition and replace it if necessary. It’s recommended to replace air filters every three months or so.
If You Have a Gas-Fired Furnace:
Start by investigating the storage tank to make sure that you haven’t run out of gas, and that the supply valves are fully opened. If you smell gas, call your gas provider immediately and leave the house.
Air filters are next on the list. Just as in the oil-fired systems, a clogged air filter can block air from entering the system and cause the furnace to shut down. Be sure to replace them with the appropriate filter model. If neither of those fixed the furnace, the problem may have to do with its ignition process.
If you have an older gas furnace, there will be a pilot light to start the combustion, so look to see if there’s a flame. There’s usually a small access port where you can view the pilot light flame. If the flame is out, follow the printed directions that are usually posted near the pilot light to re-light the flame. If you can’t re-light the pilot, call a service professional.
If you have a newer gas furnace with an electronic ignition, reset it by turning off the unit, then turning it back on. If you still get an error code, there may be a problem with the igniter or an interruption in the gas line, in which case, it’s best to call a professional.
Seal any leaks
If you have access, it’s a good idea to see if there are any holes, gaps, or loose connections in the heating ducts themselves that could introduce cold air into the system. In older systems especially, the joints between ducts may have loosened or rusted. In a pinch, you can wrap duct tape around the leaks, but a better solution is to apply a water-based air duct sealant, (also called mastic), which is a goopy substance that hardens to stop air leaks.
Schedule a Yearly Inspection
It’s smart to have your furnace serviced annually. That way, a professional will come and change the filters, clean the furnace thoroughly, and make sure the sensors, burners, and blowers are all in good working order so you never have to worry about troubleshooting the problem again. It’s a small investment that will help keep your heat on when you need it most.