Thermostat Fan Runs But Heat Won't Start? Solutions Here

It can be extremely frustrating when your thermostat fan is running but no heat comes out of the vents on a cold winter day. This issue usually indicates a problem with your home's heating system, but fortunately, there are a number of ways homeowners can troubleshoot and fix the problem with simple tools and a little mechanical know-how.

Follow our guidance below to methodically diagnose the issue step-by-step and get your home warming up again quickly.

Diagnosing Why Fan Runs But No Heat

Check Thermostat Settings and Batteries

Before investigating issues with your heating system, it helps to start with a simple check of the thermostat settings and batteries:

• Verify that the thermostat is set to "heat" mode and that the temperature is set higher than the current room temperature. Also check that the "Auto Fan" setting is selected.
• Examine the thermostat's batteries to make sure they have adequate power. Replace batteries if they show low charge.

Inspect Air Filter and Vents

A dirty air filter or blocked vents can prevent sufficient heated airflow. Conduct the following checks:

• Remove the air filter and check if it's dirty or clogged. Replacement is recommended every 3 months or as needed.
• Make sure vents in each room are open and unobstructed by furniture or objects. Vacuum vent openings to remove any trapped dust or debris.

Examine Thermostat Wiring

Faulty wiring connections between the thermostat and the furnace or heat pump can disrupt communication and operation:

• Carefully check that thermostat wires are properly secured at both ends--loose wire contacts are a common culprit.
• Verify wires are matched correctly according to the wiring diagram labeling to avoid mix-ups.

Rule Out Tripped Breaker Cutting Power

It's possible the circuit breaker controlling the furnace or heat pump has tripped, cutting power. Here's how to evaluate:

• Locate the furnace breaker and flip it to the full "Off" position, then back "On" to reset.
• Listen for a humming sound indicating the furnace blower is active and receiving power.

Gas Furnace Repairs

Fix Igniter to Light Burners

The igniter is an extremely important furnace component responsible for lighting the gas burners. Try these igniter fixes:

• Remove and inspect the igniter visually. Clean with steel wool if dirty. Replace igniter if damaged.
• Consult furnace manual for steps to test igniter resistance values ensure it's operating in normal range.

Repair Flame Sensor If Damaged

The flame sensor is a safety device that signals the gas valve to shut off if no flame is detected. To troubleshoot:

• similarly clean flame sensor with steel wool and reinstall.
• If cleaning doesn't help, replace sensor entirely.

Check Fuel Line Valves and Pressure

Verify gas supply lines and pressure to rule out any issues preventing burner ignition:

• Confirm that all manual gas line shut-off valves are in the open position.
• Inspect gas piping and connectors for damage, blockages, or leaks.
• Use a manometer to check that both inlet and manifold gas line pressures are in the expected range.

Examine Heat Exchanger and Burners

It's also important to check the core furnace components:

• Use a flashlight to look for cracks in the heat exchanger which can prevent proper operation.
• Remove debris if burners are visibly dirty; use a wire brush to gently clean surface.

Heat Pump Troubleshooting

Recharge Refrigerant If Low

With heat pumps, insufficient heat is often caused by low refrigerant levels. Keep in mind:

• Refrigerant slowly depletes over months and years of use.
• Only a certified HVAC technician can access and recharge the sealed, pressurized refrigerant system.

Defrost Frozen Outdoor Coils

It's common for ice to accumulate on the outdoor coils in cold weather, impeding performance:

• Use a hair dryer to safely melt ice if present, then dry coils fully.
• Ensure defrost sensors are undamaged and working properly after.

Fix Broken Thermostat Sensor

If the thermostat sensor fails, the unit can't properly gauge indoor heating needs:

• Replace sensor if heating runs continuously without reaching set temp.
• Test continuity across sensor wire pins to confirm it's defective before replacing.

When to Call a Professional

While many "no heat" issues can be DIY repairs, it's best to call a technician for complex electrical issues, gas line repairs, refrigerant system recharge, or heat exchanger cracks. HVAC pros have the skills, certification, tools, and parts to solve tricky heating problems you can't easily tackle on your own as a homeowner. Don't take chances with gas leaks, electric shock hazards, or carbon monoxide risks.