Heating Repair

Heating Repair

Why Your Furnace Won't Start (And What's Actually Wrong)

Your furnace stopped working. Maybe it won't turn on at all. Maybe it lights but shuts off. Maybe it runs but doesn't heat. Maybe it's making a sound that wasn't there before.

Something's wrong, and you need to know what it is and whether to fix it. This page covers the most common heating failures, how we diagnose them, and when repair makes sense versus replacement.

Before You Call: Quick Checks

Some "breakdowns" have simple causes. Try these first:

Check Your Thermostat

• Mode should be "Heat" (not "Cool" or "Off")
• Temperature should be set above current room temperature
• Check batteries—a blank screen usually means dead batteries

Check Your Filter

A severely clogged filter restricts airflow enough to overheat the furnace. The high limit safety switch shuts it down. Pull your filter—if it's completely packed with dust, replace it, wait 30 minutes for the furnace to cool, and try restarting.

Check Your Circuit Breakers

Look for a breaker labeled "Furnace," "HVAC," or "Air Handler." If it's tripped (middle position), flip it fully off, then on. If it trips again immediately, you have an electrical problem requiring service.

Check Your Gas Supply

If you have other gas appliances, check if your stove lights or water heater works. If nothing gas-powered works, the issue might be your gas supply, not your furnace—call your gas company.

Check the Furnace Switch

There's usually a switch on or near your furnace that looks like a light switch. Sometimes people accidentally turn it off. Make sure it's on.

Common Furnace Problems

Problem #1: Furnace Won't Ignite

Symptoms: Thermostat calls for heat, inducer motor runs (you hear humming), no ignition, system shuts down after a few minutes. May retry ignition several times.

Most likely causes:

• Failed ignitor (35% of calls): The hot surface ignitor glows orange to light the gas. Ignitors crack or burn out after years of thermal cycling (average life: 4-7 years).
• Faulty flame sensor (25%): The flame sensor detects whether gas ignited. A dirty or failed sensor shuts gas off because it "doesn't see" the flame.
• Gas valve issue (15%): The gas valve controls gas flow to burners. Electrical failures or stuck valves prevent gas delivery.
• Pressure switch failure (10%): Pressure switches verify the inducer is running before allowing ignition. Failed switches or blocked sensing tubes prevent ignition.

Problem #2: Furnace Lights But Shuts Off Quickly

Symptoms: Burners ignite successfully, flame burns for 3-10 seconds, system shuts down, may retry and repeat the cycle.

Most likely cause: Dirty flame sensor (60% of these calls)

This is the #1 service call for furnaces—and it's often preventable with maintenance. The flame sensor is a metal rod that sits in the flame. Buildup on the sensor prevents it from detecting the flame, so the control board thinks there's no flame and shuts off gas.

Problem #3: Blower Won't Run (No Air Movement)

Symptoms: Burners light and run normally, heat exchanger warms up, no air comes from registers, system may overheat and shut down on high limit.

Most likely causes:

• Failed blower motor (30%): Motors burn out from age, overheating, or bearing failure. Motor may hum or produce burning smell.
• Failed capacitor (25%): Capacitors help motors start. A weak or failed capacitor prevents the blower from starting even though the motor is functional.
• Failed control board (20%): The control board sends signals to run the blower. Board failures can prevent blower activation.

Problem #4: Furnace Runs But Doesn't Heat Well

Symptoms: System runs normally, air comes from registers but isn't hot enough, house can't reach set temperature, system runs constantly.

Most likely causes:

• Dirty air filter (30%): Restricted airflow reduces heat transfer. Replace your filter—this is on you, not a service call.
• Dirty blower wheel (20%): The squirrel cage blower accumulates dust that reduces airflow.
• Ductwork issues (20%): Disconnected ducts, closed dampers, or leaks waste heated air before it reaches living spaces.
• Gas pressure issues (10%): Incorrect manifold pressure reduces heat output.

Problem #5: Strange Noises

• Boom or bang on startup: Delayed ignition. Gas accumulates before igniting. Repeated delayed ignition can crack heat exchangers—address promptly.
• Squealing or screeching: Belt issues (older systems) or motor bearing failure.
• Grinding or metal-on-metal: Blower wheel rubbing housing, failed bearings. Turn off and call for service—continued operation causes more damage.

Heat Pump Heating Problems

If your heating comes from a heat pump, different issues apply:

Heat Pump Not Heating (Blows Cold/Cool Air)

• Reversing valve stuck: Keeps heat pump in cooling mode instead of heating
• Low refrigerant: Indicates a leak in the system
• Defrost board failure: Outdoor unit can't defrost properly

Emergency Heat Running Constantly

Your heat pump has auxiliary/emergency heat strips for when outdoor temps are too cold for efficient heat pump operation. If these run constantly, the heat pump may be malfunctioning, the thermostat may be set incorrectly (emergency heat mode), or the outdoor unit may be blocked or frozen. Warning: Emergency heat costs 2-3x more to operate than heat pump mode.

Frozen Outdoor Unit

Light frost is normal during cold, humid conditions. Heavy ice completely encasing the unit is not normal—possible causes include failed defrost board, low refrigerant, stuck reversing valve, or blocked airflow. Don't chip ice off—you can damage coils. Address the underlying cause.

When to Repair vs. Replace

Repair Makes Sense

• System under 10 years old: Modern furnaces should last 15-20+ years
• Single component failure: First repair suggests normal wear, not system failure
• Repair cost under 30% of replacement: Reasonable investment for remaining life
• System well-maintained: Good maintenance history suggests longer remaining life

Replace Makes Sense

• Cracked heat exchanger: CO safety risk isn't worth patching
• Repair exceeds 50% of replacement: You're buying limited life extension at high cost
• Repeated major repairs: Third significant repair in three years indicates systemic decline
• System 20+ years old: Even successful repair leaves you with aging equipment; modern systems are 20-30% more efficient
• Safety issues: Cracked heat exchanger, persistent CO production, gas leaks—don't repair, replace

Safety: When to Shut Off and Call

Immediate shutdown required:

• Smell of gas: Turn off furnace, leave the house, call gas company from outside
• Carbon monoxide alarm: Leave immediately, call 911 from outside
• Visible flames outside combustion chamber: Turn off immediately, do not restart
• Electrical burning smell or sparks: Turn off at circuit breaker

Furnace not working? Try the quick checks at the top of this page. If still not working, schedule service. We diagnose, you decide. If repair makes sense, we fix it same-day when possible. If replacement makes sense, we'll explain why and provide options.