Hvac heater not working

When your HVAC heater stops working, systematic diagnosis can identify the root cause and determine if you need professional repair. This diagnostic guide walks you through checking the most common failure points in heating systems, from simple thermostat issues to complex component failures.

What You’ll Need

• Digital multimeter
• Flashlight
• Screwdriver set
• Wire strippers
• New batteries for thermostat

Diagnostic Steps

Step 1: Verify Thermostat Operation and Settings

Check that the thermostat is set to heat mode and the temperature setting is at least 5 degrees above current room temperature. Replace batteries if the display is dim or blank, as low battery voltage can prevent proper operation. Verify the fan setting is on auto rather than off, and check for any error codes or unusual displays. Test the thermostat by raising the set point significantly to see if the system responds.

Step 2: Inspect Circuit Breakers and Electrical Supply

Check the main electrical panel for tripped breakers serving the HVAC system, including both the indoor air handler and outdoor unit circuits. Reset any tripped breakers by switching them fully off then back on. Verify power at the furnace disconnect switch and ensure it hasn't been accidentally turned off. Use your multimeter to confirm 240V power supply at the unit's electrical disconnect.

Warning: Turn off power at the circuit breaker before checking electrical connections or components.

Step 3: Examine Air Filter and Airflow Restrictions

Remove and inspect the air filter for excessive dirt buildup that could cause system shutdown due to overheating protection. A completely blocked filter can trigger high limit switches and prevent the furnace from operating. Check all supply and return vents for obstructions like furniture, curtains, or closed dampers. Ensure the blower compartment door is properly closed, as most units have safety switches that prevent operation with open panels.

Step 4: Check Ignition System and Flame Sensor

For gas furnaces, observe the ignition sequence through the sight glass during a heat call – you should see the hot surface ignitor glow red or hear the spark ignitor clicking. If the ignitor glows but no flame appears, check gas supply valve position and verify gas pressure at the meter. Clean the flame sensor with fine steel wool if the burner lights briefly then shuts off, as contaminated sensors cannot properly detect flame presence.

Warning: If you smell gas at any point, shut off the gas supply immediately and contact a qualified technician.

Step 5: Test Blower Motor and Capacitor Function

Listen for unusual noises from the blower compartment such as grinding, squealing, or clicking sounds that indicate motor bearing failure. Check the blower capacitor for signs of swelling, leaking, or burn marks on the top. Use your multimeter to test capacitor microfarad rating against the label specifications. Verify blower wheel spins freely when power is off and isn't hitting any obstructions.

Warning: Capacitors store electrical charge and can shock you even with power turned off – discharge properly before handling.

Step 6: Inspect Safety Controls and Limit Switches

Locate the high temperature limit switch, usually mounted on the heat exchanger, and check for manual reset buttons that may have tripped. Test the pressure switch on induced draft furnaces by checking continuity across terminals with the unit running. Examine the rollout switches near the burner compartment for any that have triggered due to improper combustion airflow. Document any switches in the open position as these indicate safety issues requiring professional attention.

Frequently Asked Questions

Why does my furnace turn on but no heat comes out?

This typically indicates a problem with the gas supply, ignition system, or heat exchanger. The blower may run while the burners fail to light, often due to a faulty ignitor, gas valve, or flame sensor. Professional diagnosis is needed to identify the specific ignition component failure.

What causes a furnace to keep cycling on and off?

Short cycling usually results from restricted airflow due to dirty filters, blocked vents, or blower problems. Oversized equipment, faulty flame sensors, or malfunctioning limit switches can also cause rapid cycling. Check the air filter first, then examine the thermostat location for heat sources affecting its operation.

How do I know if my heat pump is in emergency heat mode?

Emergency heat mode is indicated by a light or message on your thermostat, often labeled as AUX or EM HEAT. The system will run continuously and your electric bill will increase significantly since backup electric resistance strips are operating. This mode bypasses the heat pump and should only be used temporarily during equipment failure.

Why does my heater work intermittently?

Intermittent operation often points to electrical connection problems, failing contactors, or temperature sensors providing inconsistent readings to the control board. Loose wire connections, corroded terminals, or capacitors beginning to fail can cause sporadic operation. The issue may worsen over time until complete failure occurs.

When should I call a professional instead of troubleshooting myself?

Contact a technician immediately if you smell gas, see damaged electrical wiring, or hear loud mechanical noises from the equipment. Safety control failures, cracked heat exchangers, and refrigerant leaks require professional expertise and specialized tools. Most combustion and electrical component repairs need proper licensing and certification to perform safely.