The air conditioner Works While The Furnace Does Not Is A Common Home HVAC Issue That Can Be Caused By Electrical, Gas, Control, Or Maintenance Problems. This Guide Helps Identify Likely Causes, Step-By-Step Diagnostics, Safety Precautions, Typical Repair Costs, And Preventive Maintenance For U.S. Homes To Restore Reliable Heating.
| Symptom | Likely Causes | Quick Check |
|---|---|---|
| Thermostat Runs Cooling But Not Heating | Thermostat Wiring, Mode Setting, Or Power Issue | Set To Heat And Raise Temp; Check Batteries |
| Blower Runs For AC But No Heat | Ignition/ Pilot, Gas Supply, Flame Sensor, Control Board | Listen For Ignition Click; Check Gas Valve |
| No Blower At All | Blower Motor, Limit Switch, Fan Relay, Breaker | Check Breaker, Filter, And Furnace Switch |
| Short Cycling Or Shutting Off Quickly | Flame Sensor, Overheat, Venting Problem | Inspect Filter And Vents; Note Error Codes |
How Heating And Cooling Systems Share Components
Understanding Why The Air Conditioner Can Work While The Furnace Does Not Starts With Recognizing Shared Versus Independent Components. Most Split Systems Share The Thermostat, Blower Motor, And Some Electrical Circuits, While The Outdoor Condenser, Reversing Valve (Heat Pumps), And Gas Ignition Systems Are Distinct.
The thermostat Sends 24V Control Signals To Both Systems. The Blower Motor Is Often Used For Both Heating And Cooling Air Distribution. The Furnace Contains Unique Components For Heat Generation, Including The Ignition System, Gas Valve, Flame Sensor, And Heat Exchanger.
Common Reasons The AC Works But The Furnace Fails
1. Thermostat Or Control Settings
A thermostat set to “Cool” or “Auto” with incorrect temperature settings can appear to work for cooling while not calling for heat. Faulty thermostat wiring, a failed internal relay, or dead batteries can prevent the heat call from reaching the furnace.
2. Power And Breaker Issues
Dedicated breakers or switches sometimes control the furnace separately. A tripped breaker, blown fuse, or an emergency switch (furnace power switch near the unit) can disable the furnace while the AC remains powered by a different circuit.
3. Ignition And Pilot Problems
Modern furnaces Use Electronic Ignition (Hot Surface Ignitor Or Intermittent Pilot) While Older Units Use Standing Pilots. A Failed Ignitor, Faulty Pilot, Or Malfunctioning Ignition Control Module Will Prevent Flame Establishment, So The Furnace Won’t Produce Heat.
4. Gas Supply Or Valve Failure
If Natural Gas Or Propane Supply Is Interrupted—Due To An Off Valve, Meter Issue, Or A Faulty Gas Valve—The furnace Cannot Ignite. The outdoor AC uses electricity, so it can run when the furnace’s gas supply is the problem.
5. Flame Sensor And Combustion Safeties
A Dirty Or Faulty Flame Sensor Causes The Furnace To Shut Off Shortly After Ignition For Safety. Limit Switches Or Overheat Safeties Can Also Prevent Burner Operation If The Furnace Detects Unsafe Conditions.
6. Control Board Or Relay Failures
The Furnace Control Board Coordinates Ignition, Gas Valves, And The Blower. A Failed Control Board, Faulty Relays, Or Corroded Connectors Can Allow cooling functions That Bypass The Furnace Controller To Work While Heating Commands Fail.
7. Blower Motor And Fan Issues
In Some Cases, The Blower Motor Runs During Cooling But Not During Heating If The Heat Relay Or The Furnace’s Fan Limit Switch Is Faulty. This Will Make It Seem Like The Furnace Isn’t Working Even Though The Cooling Fan Operates Independently.
Step-By-Step Troubleshooting Checklist
Follow These Steps In Order To Narrow Down The Cause. For Safety, Turn Off Power Before Inspecting Internal Components And If Unsure, Contact A Licensed HVAC Technician.
- Verify Thermostat Settings: Set Thermostat To Heat, Raise Setpoint, Replace Batteries, And Confirm Mode Is “Heat.”
- Check Breakers And Switches: Inspect The Furnace Breaker, The Outdoor Unit Breaker, And The Furnace On/Off Switch Near The Unit.
- Listen For Ignition: When The Thermostat Calls For Heat, Listen For Clicking Or The Ignitor Glow. No Sound Suggests Ignition Or Control Issues.
- Inspect Air Filter: A Dirty Filter Can Cause Overheating And Limit Switch Trips; Replace If Dirty.
- Look For Error Codes: Many Furnaces Blink An LED On The Control Board—Record The Blink Pattern And Reference The Manual.
- Check Pilot/Flame: For Standing Pilot Units, Ensure The Pilot Is Lit. For Electronic Ignition, Inspect The Ignitor For Cracks Or Discoloration.
- Test Gas Supply: Confirm Furnace Gas Valve Is Open And Other Gas Appliances Operate. If No Gas, Contact Utility.
- Examine Disconnects And Wiring: Ensure 24V Transformer Wires Are Secure And That There Is Voltage To The Gas Valve When A Heat Call Is Made (Professional Task Recommended).
- Reset The System: Turn Off Power For 30 Seconds Then Restore—This Can Clear Some Control Board Faults.
When To Stop And Call A Professional
Stop Troubleshooting And Call A Licensed HVAC Technician If There Is Any Suspected Gas Leak, If The Furnace Smells Of Gas, If There Is Repeated Short Cycling, Or If The Home Has Carbon Monoxide Alarms Triggered.
Hire A Professional For Complex Electrical Tests, Gas Valve Replacement, Heat Exchanger Cracks, Control Board Replacement, Or When Diagnostic Codes Indicate Internal Component Failures. Certified Techs Can Also Provide Proper Combustion Testing And Carbon Monoxide Safety Checks.
Typical Repairs And Estimated Costs
Costs Vary Widely By Region, Unit Age, And Labor Rates. The Following Are U.S. National Estimates For Common Furnace Repairs.
| Repair Type | Typical Cost Range |
|---|---|
| Thermostat Replacement | $100–$400 |
| Ignitor Replacement | $150–$400 |
| Flame Sensor Cleaning/Replacement | $75–$200 |
| Control Board Replacement | $300–$1,000 |
| Gas Valve Replacement | $200–$600 |
| Blower Motor Repair/Replacement | $250–$1,200 |
| Heat Exchanger Replacement | $1,000–$3,000 (Often Replaces Unit) |
These Estimates Include Parts And Labor; Emergency Or After-Hours Service Can Cost Significantly More.
Safety Checks And Carbon Monoxide Considerations
A Furnace That Fails To Ignite Properly Or That Has A Cracked Heat Exchanger Can Create Carbon Monoxide Risks. Homes Should Install And Maintain CO Alarms On Each Level And Near Sleeping Areas.
Never Attempt To Test For Gas Leaks With An Open Flame. If A Gas Smell Exists, Evacuate The Home, Call The Gas Utility Or 911, And Avoid Electrical Switches. Only Licensed Technicians Should Repair Gas Components.
Preventive Maintenance To Avoid Future Failures
Regular Maintenance Keeps Furnaces Reliable And Can Prevent Cleanings, Sensor Failures, And Safety Trips. Recommended Annual Tasks Include Filter Replacement Every 1–3 Months, Professional Tune-Up With Combustion Analysis, Cleaning Or Replacing The Flame Sensor, And Inspecting The Venting System.
Replace Aging Thermostats With Programmable Or Smart Models To Improve Diagnostics And Reduce Misconfiguration. Seasonal System Switchovers Are A Good Time To Test Heating Readiness Before Cold Weather.
Diagnostic Tools And Tips For Homeowners
Useful Tools Include A Simple Multimeter For Checking 24V Thermostat Voltage, A Magnet Or Screwdriver For Resetting Some Switches, And A Mirror/Flashlight For Visual Inspections. Read The Furnace Manual For LED Blink Codes And Manufacturer Guidance.
Document Any Error Codes, Noises, Or Behavior Patterns When The Technician Arrives To Speed Diagnosis. Photograph Wiring Labels And Terminal Connections Before Disconnecting Anything.
Common Myths And Misconceptions
Myth: If The AC Works, The Furnace Must Be Fine. Reality: Shared Components Can Mask Furnace Failures; Ignition, Gas, Or Combustion Parts Are Independent And Can Fail Without Affecting Cooling.
Myth: A Dirty Filter Only Affects Cooling. Reality: A Clogged Filter Can Cause Overheating And Limit Switch Trips In Heating Mode, Shutting The Furnace Down Quickly.
FAQs
Why Does The Thermostat Show Heat But Furnace Ignites Then Shuts Off?
Short Cycling Often Indicates A Dirty Flame Sensor, Overheat Condition Due To Restricted Airflow, Or A Faulty Limit Switch. Recording The Timing Helps Technicians Diagnose The Fault.
Can A Furnace Blower Run For AC But Not Heat?
Yes. The Furnace Blower Relay Or Fan Limit Control May Not Engage In Heating Mode If The Control Board Or Relay Is Faulty, Even If The AC Calls For Fan And Runs The Motor.
Is It Safe To Try Fixing The Furnace Myself?
Simple Tasks Like Replacing Filters Or Thermostat Batteries Are Safe. Do Not Attempt Gas Valve, Combustion, Or High-Voltage Electrical Work. Hire Licensed Professionals For These Repairs.
Maintenance Checklist For A Reliable Furnace
- Replace Air Filter Every 1–3 Months.
- Schedule Annual Professional Tune-Up Before Winter.
- Test Carbon Monoxide Alarms Monthly.
- Inspect Venting And Chimney For Blockages.
- Clean Or Replace Flame Sensor During Service.
- Verify Thermostat Settings Seasonally.
- Keep Area Around Furnace Clear And Accessible.
Following This Systematic Approach Helps Homeowners Narrow Down Causes When The AC Works But The Furnace Does Not, Ensures Safety, And Guides When To Call An HVAC Professional.
For Persistent Issues Or If Any Gas Or Carbon Monoxide Concerns Exist, Contact A Licensed HVAC Technician Immediately.