Your furnace reliably takes care of your home's heating needs, day and night—until the moment that it doesn't. With any mechanical system that works so hard and so often, issues are to be expected.
When your furnace turns on but not enough heat comes out, sometimes there are simple, low-cost solutions. Once you exhaust those possibilities, you'll want to call a professional.
Thermostat Incorrectly Set
The thermostat is likely the first place you checked, but it's worth checking a second time. If the thermostat is set to ON, it prompts the furnace blower to turn on, whether or not a heating cycle is occurring. The ON setting tells the furnace to blow air, much like a room fan.
Solution: Reset Thermostat to AUTO
The solution is to turn the thermostat to AUTO. Also, make sure that the thermostat setting is high enough that it will prompt the furnace to turn on when the room temperature drops low enough.
Air Filter Is Dirty
All warm air that enters your home through the heating registers must first pass through one point: the air filter. A dirty or otherwise obstructed air filter causes your furnace to work harder to do the same job.
Solution: Change Air Filter
You'll know if the air filter is dirty because fuzz, hair, lint, and other physical debris will cling to it. Even a filter that isn't fuzzy will be dirty if its color is light- to medium-gray.
During the cold season, make sure that you're well-stocked with properly sized air filters. Change out the old air filter for a clean one.
Furnace filter locations vary by brand and model. It might be located in a sheet metal box called a blower compartment, or there may be a V-shaped slide in the upper part of the blower compartment. There might also be a slide along the side of the furnace where the filter is located.
Air Filter Is Installed Backwards
Even if the air filter is clean, it might have been installed backward. An air filter that's pointed backward will behave much like a dirty filter: It limits airflow and makes the furnace work harder.
Solution: Turn Filter Around
Turn on the system so that air is flowing. Open the blower compartment. You'll be able to determine the airflow direction on the furnace by the way the filter is clinging against one side of the compartment as air is flowing.
Arrows or Vs on the paper or plastic frame of the air filter should be pointing in the airflow direction. If not, slide the filter out and turn it around.
Some Registers Are Too Remote
If the furnace turns on but there is no heat in certain heating registers, the reason might be that those registers are too far from the furnace. When other heating registers feel overly hot, this is especially indicative of a remote register problem.
Heat is at its hottest in the first foot or two as it travels to your home's various heating registers. After that, temperatures drop. Remote registers will be the coldest. As if that weren't enough, intermediary registers siphon off heat meant for remote registers.
Solution: Install Smart Covers
Replace the register vent covers in each room with battery-powered, motorized, WiFi-equipped smart covers that open and close according to room need. Covers will close more often in rooms that receive too much heat, diverting that heat to underserved rooms. The result should be balanced heating throughout the house.
Price per cover ranges from $90 to $110.
Ductwork Is Leaky
The metal ductwork leading from the furnace to the heating registers should be a closed system all along the way. But because ducts are seamed together, gaps can develop in those joints. Duct tape or foil tape fixes may have come loose.
Solution: Repair Ducts
Any gap in the ductwork leading to the registers will incorporate colder air, and this colder air ends up in your living areas. Sections of accessible ductwork can be joined together again. Patches that may have loosened from old holes can be patched again with duct tape or foil tape.
Even HVAC ductwork in good condition can be affected by the cold if they run through a cold basement, attic, or crawlspace. Consider insulating the ducts with thin insulating foam. Better yet, replace your existing ducts with thermal insulated ductwork.
Pilot Light Is Out or Gas Is Not On
The standing pilot light on the furnace must be on at all times so that the burner can light up and produce heated air. The gas line should be turned on, as well.
Solution: Turn on Gas and Pilot Light
Most older furnaces with an AFUE rating of 80-percent or less will have a standing pilot light. Re-light the pilot light with a kitchen lighter.
While it's rare for a gas line to have been turned off, it's good to check for this. Look for the metal gas line leading to the furnace. A single plastic or metal knob should be turned so that it is parallel to the gas line.
When to Call a Professional
Other than starting the gas flow or changing out the filter, problems located at the source—the furnace itself—should be handled by licensed, qualified HVAC professionals. Ductwork can be difficult to work with, too, since sheet metal must be cut and drilled. It's often best to have HVAC pros work on the ducts, especially if the ducts are not easily accessible.