Why Your Ac and Furnace Run at the Same Time: Causes, Diagnosis, and Fixes

When An Air Conditioner And Furnace Run Simultaneously, It Can Signal System Misconfiguration, A Heat Pump In Transition, Or A Wiring Or Thermostat Issue. This Article Explains Common Causes, Step-By-Step Diagnostics, And Practical Fixes To Help Homeowners Identify Why AC And Furnace Run At The Same Time And What To Do Next.

Symptom	Likely Cause	Immediate Action
Both Systems Operating Together	Thermostat Fan Set To On / Incorrect Wiring	Set Fan To Auto / Check Thermostat Wiring
Furnace Runs With Cooling	Heat Pump In Emergency Heat Or Dual Fuel Changeover	Check System Mode / Call Technician
Short Cycling Or Noise	Control Board Or Relay Fault	Power Cycle / Service Call

How HVAC Systems Normally Coordinate

Residential HVAC Typically Uses One Primary System For Heating And One For Cooling, Coordinated By A Thermostat That Controls Modes And A Fan Setting. In Normal Operation, The Furnace Should Only Run During Heating Calls, And The Air Conditioner Should Only Run During Cooling Calls.

Modern Systems May Include Heat Pumps Or Dual Fuel Arrangements Where A Compressor And Furnace Work Together Under Specific Conditions, But Both Systems Running Continuously Is Usually Abnormal.

Common Reasons AC And Furnace Run At The Same Time

Thermostat Fan Set To “On” Instead Of “Auto”

If The Thermostat Fan Is Set To On, The Furnace Blower Will Run Continuously Whenever The System Has Power, Even When The AC Compressor Is Active. This Still Allows The Outdoor Unit To Cool The Home But Keeps The Furnace Blower Moving Air.

Heat Pump In Emergency Or Auxiliary Heat Mode

When A Heat Pump Cannot Meet Heating Demand Or Detects A Fault, The Thermostat May Engage Emergency Heat Or Auxiliary Heat, Which Turns On The Furnace While The Heat Pump (or AC mode) Also Runs For Defrost Or Transitioning, Causing Overlap.

Dual Fuel System Changeover

In Dual Fuel Systems, A Heat Pump Handles Mild Heating While A Gas Furnace Kicks In For Very Cold Conditions. During The Changeover Period, Short Overlap Can Happen, Especially If The Thermostat Has A Delay Gap Or Misconfigured Settings.

Incorrect Thermostat Wiring Or Configuration

Miswired Thermostats Or Incorrectly Configured Equipment Settings Can Send Simultaneous Signals To Cooling And Heating Relays, Causing Both The AC Compressor And Furnace Blower Or Burner To Run At The Same Time.

Stuck Relays, Control Board, Or Short Circuits

Mechanical Relays Or Electronic Control Boards Can Fail, Causing Contacts To Stick Closed And Run Multiple outputs At Once. A Stuck Relay Can Keep The Furnace Running Even When Only A Cooling Call Exists.

Damper Or Zoning System Issues

In Zoning Systems, A Faulty Motorized Damper Or Control Panel May Direct Some Zones To Call For Heat While Others Call For Cooling, Creating Simultaneous Operation In Central Equipment.

How To Diagnose Why AC And Furnace Run Together

Check Thermostat Mode And Fan Settings

Set The Thermostat To Cool And Fan To Auto. Observe Whether The Furnace Runs Immediately Or Only When The Fan Is On. If Switching Fan To Auto Stops The Furnace, The Fan Setting Was The Cause.

Inspect Thermostat Wiring And Equipment Type

Turn Off Power And Remove The Thermostat Faceplate To Verify Wiring Colors And Terminal Connections Match The System: Y For Cooling, W For Heat, G For Fan, O/B For Heat Pump, And C For Common. Mismatches Or Extra Wires Can Indicate A Wiring Problem.

Listen For Relays And Sequence Of Operation

When Calling For Cooling, Listen At The Furnace For Clicks Or Continuous Hum From A Relay. Note If The Burner Ignites Or The Blower Runs Before The Outdoor Compressor Starts. This Timing Helps Identify Whether A Relay Or Control Board Is Faulty.

Check For Heat Pump Defrost Or Emergency Heat

If A Heat Pump Is Present, Confirm The Thermostat Is Not In Emergency Heat Mode. Inspect Outdoor Unit For Defrost Cycles; During Defrost, The Indoor Furnace May Activate Briefly to Maintain Comfort.

Review Zoning Panel And Dampers

For Zoned Systems, Inspect The Zoning Control Panel For Error Codes And Check Dampers For Proper Positioning. A Damper Error Can Make The Controller Call For Heat In Part Of The House While The AC Serves Another.

Practical Fixes Homeowners Can Try

Set Fan To Auto

Change The Thermostat Fan Setting From On To Auto. This Prevents The Furnace Blower From Running Continuously During Cooling Cycles While Keeping Whole-House Air Circulation Under Control.

Reset Or Power Cycle Thermostat And HVAC

Power-Cycle The Thermostat And HVAC Equipment By Turning Off Breakers For 30 Seconds And Restarting. This Can Clear Minor Control Glitches Or Stuck Relay States.

Verify Thermostat Mode And Equipment Type Settings

On Programmable Or Smart Thermostats, Ensure The Equipment Type Is Set Correctly (Heat Pump Vs. Conventional). Update Settings In The Thermostat Menu If Necessary To Prevent Mixed Signals.

Replace Faulty Relays Or Thermostat

If Relays Are Stuck Or The Thermostat Sends Conflicting Signals, Replace The Faulty Component. Installing A Modern, Compatible Thermostat Often Resolves Coordination Issues Between Heat And Cool Outputs.

Schedule Professional HVAC Diagnosis

If Wiring Issues, Control Board Failures, Or Zoning Problems Are Suspected, Call A Licensed HVAC Technician To Run System Diagnostics, Test Control Voltages, And Repair Or Replace Damaged Components.

When Simultaneous Operation Is Normal

Certain Scenarios Make Both Systems Operate Together By Design. Heat Pump Defrost Cycles Temporarily Start The Furnace To Keep Indoor Temps Stable While The Outdoor Coil Defrosts.

Dual Fuel Systems May Also Engage The Furnace Briefly During Transition Periods Or High Heating Loads, Which Is Normal If It Occurs Sparingly And Follows Manufacturer Timing Guidelines.

Safety And Efficiency Considerations

Continuous Simultaneous Operation Increases Energy Use And Can Cause Wear On Equipment. Monitoring Energy Bills And Watching For Unusual Noises Or Short Cycling Can Help Catch Problems Early.

Never Attempt Gas Valve, Burner, Or High-Voltage Control Repairs Without A Qualified Technician. For Electrical Or Gas Components, A Certified HVAC Professional Ensures Safe And Compliant Repairs.

Maintenance Steps To Prevent Future Problems

• Replace Air Filters Regularly To Reduce Load On Both Systems And Prevent Dust From Causing Control Issues.
• Schedule Annual Tune-Ups For Both Furnace And AC To Keep Controls, Relays, And Sensors In Good Condition.
• Test Thermostat Settings Quarterly And Update Batteries Or Firmware For Smart Models.
• Inspect Zoning Dampers And Motors Annually To Avoid Malfunctions That Cause Conflicting Calls.

When To Call A Technician

Contact A Licensed HVAC Technician If The Furnace Runs With Cooling After Changing Fan To Auto, If The Thermostat Wiring Appears Correct But The Issue Persists, Or If There Are Error Codes On The Zoning Panel Or Thermostat.

Immediate Service Is Recommended When There Is A Gas Smell, Burner Ignition Issues, Or Repeated Short Cycling, As These Indicate Safety Or Reliability Concerns.

Key Takeaways For Homeowners

• Set Fan To Auto First—This Fixes Many Cases Where The Furnace Blower Runs During AC Operation.
• Verify Thermostat Mode And Equipment Type To Avoid Mixed Signals Between Heat And Cool.
• Recognize Normal Scenarios Like Heat Pump Defrost And Dual-Fuel Changeover That Can Briefly Run Both Systems.
• Replace Faulty Components And Schedule Professional Service For Electrical, Gas, Or Zoning Control Issues.

For Persistent Problems, Document When Both Systems Run (Time Of Day, Thermostat Settings, And Any Error Codes) To Provide Clear Information To A Technician And Speed Up Diagnosis And Repair.