Furnace System GuideA furnace that will not heat can have a simple cause or a dangerous one, so the order of your checks matters. Safety comes first, and the guidance below is tied to dated EPA and ENERGY STAR sources. Anything that could not be verified against a live source is left out.
Safety First: Stop on Any Gas or Carbon Monoxide Concern
Before any troubleshooting, rule out a safety emergency. If you smell gas or suspect carbon monoxide, do not investigate the furnace yourself. Leave the area and call your gas utility or a licensed professional from a safe location.
The EPA lists leaking chimneys and furnaces, and back-drafting from furnaces, as sources of carbon monoxide in the home. It describes symptoms including headaches, dizziness, confusion, and nausea, and warns that carbon monoxide can be fatal at very high concentrations.
Because carbon monoxide has no smell you can rely on, treat symptoms and any alarm as real. Only continue with the checks below once you are confident there is no gas or carbon monoxide concern.
Check the Air Filter and Airflow
Once safety is confirmed, the filter is the first thing to check. ENERGY STAR advises inspecting, cleaning, or changing air filters once a month, and a badly clogged filter can choke a furnace enough to affect operation.
ENERGY STAR notes that a dirty air filter makes the system work harder to distribute air and can damage equipment, leading to early failure. It also states that airflow problems can reduce a system's efficiency by up to 15 percent.
Replace a dirty filter and make sure supply and return vents are open and unblocked. Restoring airflow is a common fix and it protects the furnace from further strain.
Check the Thermostat and Power
If airflow is fine, move to the controls. Confirm the thermostat is set to heat and set above the current room temperature, and that its batteries are fresh if it uses them. A thermostat set to cool or to a low setpoint is a frequent false alarm.
Next confirm the furnace has power. Check that the furnace switch is on and that the circuit breaker has not tripped. A tripped breaker can indicate an electrical fault, which ENERGY STAR warns can cause unsafe operation and reduce the life of major components.
If a breaker trips again after you reset it, stop and call a professional rather than repeatedly resetting it. A repeating trip is a sign of a fault, not a nuisance.
When to Call a Professional
If the furnace still will not heat after checking safety, the filter, airflow, the thermostat, and power, the problem is likely inside the sealed combustion or control system. Those parts are not homeowner-serviceable.
ENERGY STAR says a contractor should check gas connections, gas pressure, burner combustion, and the heat exchanger. The EPA recommends having a trained professional inspect, clean, and tune up the central heating system annually.
Call a licensed technician for burner, ignition, gas-valve, or heat-exchanger problems, and for any repeated breaker trip or suspected carbon monoxide. Getting a pro on these items protects both the equipment and the people in the home.