Why Your AC Works but the Furnace Does Not: Common Causes and Solutions

It can be frustrating when your air conditioner functions properly, but the furnace fails to produce heat. This issue often arises during seasonal transitions, leaving homeowners confused about whether to call for repairs or attempt troubleshooting themselves. Understanding why the AC works but the furnace does not is essential for diagnosing problems related to your HVAC system efficiently and safely. This article explores common causes, diagnostic tips, and potential repair solutions to help homeowners and HVAC professionals address this issue effectively.

Cause	Description	Possible Solution
Thermostat Settings	Incorrect thermostat mode or temperature setting	Verify and adjust thermostat to heating mode with correct set temperature
Ignition Problems	Failure of electric igniter or pilot light	Inspect and replace faulty igniters or relight pilot lights safely
Blower Motor Issues	Blower motor not running limits warm air distribution	Check blower motor function and replace if defective
Gas Supply Interruptions	Gas valve closed or gas supply issues to furnace	Contact utility provider or technician for gas line inspection
Limit Switch and Safety Controls	Safety switch tripped, preventing furnace operation	Reset limit switches or replace faulty sensors

Common Reasons Why Your Furnace Won’t Start But AC Does

Since air conditioning and furnace systems often share components like the blower fan and thermostat, proper operation of one but not the other suggests a targeted problem in the heating function. The most frequent root causes are thermostat malfunctions, ignition failures, and safety control issues specifically related to the furnace operation.

Thermostat Configuration Errors

Incorrect thermostat settings are one of the simplest yet overlooked reasons a furnace doesn’t start. If the thermostat remains set to “cool” mode or a temperature below the current indoor temperature, the furnace will not activate. Verifying that the thermostat is set to “heat” mode and the temperature setpoint is above room temperature is an important first step in diagnosis. Programmable thermostats require correct scheduling to initiate heating cycles.

Ignition System Failures

Furnaces rely on either an electronic igniter or a pilot light to initiate combustion. Ignition problems are a leading cause of furnaces not producing heat. Common ignition issues include a broken igniter, dirty pilot assembly, or a failed flame sensor, which prevents the furnace from firing safely.

Modern furnaces use hot surface igniters made from silicon carbide or silicon nitride capable of glowing red-hot to ignite the gas. If cracked or damaged, the igniter must be replaced. In pilot light models, flames extinguished by drafts or dirt accumulation need relighting and cleaning. Flame sensor problems that falsely detect no flame cause the system to shut down for safety.

Blower Motor Not Running

The blower motor propels heated air throughout the home. If the blower motor or its capacitor fails, even a furnace that ignites will not circulate warm air. Multiple components require inspection: motor condition, power supply, capacitor state, and blower wheel alignment. Furnace fan issues often prevent heat delivery and can also impact airflow during cooling cycles.

Diagnostic Tips For Furnaces That Fail To Heat

Systematic troubleshooting helps isolate the furnace failure cause despite the AC running fine. Utilizing the following checklist can improve troubleshooting accuracy and save unnecessary service calls.

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Check Thermostat Settings: Confirm the thermostat is in heating mode and temperature is properly set.
Inspect Furnace Control Board Lights: Many furnaces have indicator lights that flash error codes revealing ignition or sensor faults.
Examine Pilot Light or Igniter: Visually inspect or test the igniter for glowing or pilot light presence.
Listen for Blower Motor Activity: Ensure the blower fan turns on after furnace ignition.
Check Gas Supply: Verify the main gas valve is open and detectable flame safety sensors are clean.
Reset Safety Switches: High limit switches or flame rollout switches may trip, requiring reset or replacement.

Potential Repairs and When To Call A Professional

While some furnace repairs can be handled by knowledgeable homeowners, it is crucial to prioritize safety and call licensed HVAC technicians for gas line, combustion system, or electrical repairs. Incorrect handling can cause hazardous conditions such as gas leaks or carbon monoxide exposure.

Thermostat Calibration And Replacement

If the thermostat is faulty or unresponsive, recalibrate or replace it to ensure reliable heating control. Upgrading to a smart thermostat can also help prevent future configuration errors.

Igniter And Flame Sensor Maintenance

Dirty or defective igniters and flame sensors require replacement or cleaning. These components are relatively inexpensive but critical for furnace operation. Ignition troubleshooting often cures no-heat conditions.

Blower Motor Repair Or Replacement

Blower malfunction may require lubrication, capacitor replacement, or motor swap. Air circulation is vital for comfort and furnace efficiency.

Gas Valve And Line Inspection

Only qualified technicians should inspect, troubleshoot, or repair gas lines and valves to prevent leaks and maintain safety compliance.

How HVAC System Design Allows AC But Not Furnace Operation

Split systems use shared components for cooling and heating but have distinct circuits and safety controls for each function. The air conditioner uses a refrigerant cycle and compressor, while the furnace relies on combustion and a gas valve. This design means a fault in the furnace’s ignition or gas supply won’t affect the AC’s electric-powered compressor and fan operation. Understanding this separation helps clarify why one system can work while the other fails.

Preventative Maintenance To Avoid Furnace Failures

Regular HVAC maintenance reduces the risk of furnace breakdowns and ensures both heating and cooling systems work efficiently. Recommended practices include: