AC Works but Furnace Does Not: Troubleshooting Heating Failures

The most common reason a home has a working air conditioner but a nonworking furnace is differences in system components, controls, or fuel sources. This article guides readers through systematic furnace troubleshooting, common causes, safety checks, DIY fixes, and when to call a licensed HVAC technician.

Symptom	Likely Cause	Possible Fix
No heat but AC works	Thermostat mode, pilot/ignition, power to furnace	Set thermostat to heat, reset furnace, check breakers
Blower runs but no heat	Ignition failure, gas supply, flame sensor	Inspect ignition, check gas valve, clean flame sensor
Intermittent heat	Thermostat calibration, dirty filters, limit switch	Calibrate thermostat, replace filter, test limit switch

How Central HVAC Systems Differ Between Cooling And Heating

Understanding why the AC can run while the furnace doesn’t starts with basic system differences. In many homes, the air conditioner and furnace share the same blower and ductwork but use separate components for temperature control.

An air conditioner depends on the outdoor condenser, indoor evaporator coil, and refrigerant loop driven by a compressor electrical circuit. A furnace depends on ignition, gas or oil fuel delivery, burners, and safety controls. Shared components can operate while furnace-specific parts fail.

Initial Safety Checks Before Troubleshooting

Safety is paramount. Before inspecting a furnace, turn off power at the thermostat and furnace switch if accessible. If the furnace uses natural gas, verify the gas supply is on and there are no gas odors. If a gas smell is present, evacuate and call the gas utility or emergency services.

Wear protective gloves and eye protection when inspecting components. Avoid touching live electrical terminals. If uncertain about any step, contact a licensed HVAC technician. Never bypass safety switches or tamper with controls beyond basic resets.

Common Causes And Step-By-Step Troubleshooting

Thermostat Settings And Controls

Begin with the thermostat. Confirm the thermostat is set to HEAT and the setpoint is higher than the current room temperature. Some programmable thermostats have separate heating and cooling schedules or a temporary hold mode that overrides settings.

Replace thermostat batteries if the display is dim or unresponsive. For digital thermostats, perform a reset per the manufacturer instructions. If a smart thermostat was recently installed, verify wiring (R, W, Y, G, C) and that the W (heat call) terminal receives the signal.

Power And Electrical Issues

Check the furnace switch and the main electrical panel. The AC working does not guarantee the furnace breaker is closed. Locate and reset any tripped breakers and check any inline fuses near the furnace control panel.

Inspect the furnace door switch or access panel interlock; many furnaces won’t run if the access panel isn’t seated correctly. If the furnace has a separate switch on a nearby wall, confirm it’s in the ON position. Electrical continuity and power delivery are frequent culprits.

Pilot Light, Ignition, And Flame Sensors

Older furnaces use a standing pilot light. If the pilot is out, relight it following the label’s instructions or the owner’s manual. If the pilot won’t stay lit, a faulty thermocouple or gas valve might be at fault.

Modern furnaces use electronic ignition systems (hot surface ignitor or direct spark). A cracked or burned-out ignitor, or a failing flame sensor, can prevent burners from igniting. Visual inspection often reveals a white or very dark cracked ignitor or a dirty flame sensor. Cleaning the flame sensor with fine emery cloth can help, but replacement is required for a cracked ignitor. Ignition and flame detection issues are a top cause of no-heat conditions.

Gas Supply And Gas Valve Problems

Verify the gas supply to the home and the furnace’s gas shutoff valve are open. If other gas appliances work, the supply is likely fine. A stuck or electrically failed gas valve can prevent burners from receiving fuel even when the furnace attempts to ignite.

Gas valve failures and regulator issues require a trained technician because they involve gas safety and precise adjustments. If the furnace repeatedly tries to ignite and then locks out, this pattern may indicate a gas valve or control board problem. Stop troubleshooting if gas odor is detected and call for emergency assistance.

Airflow Restrictions And Blower Problems

A running blower with no heat could mean the burners are not firing, but it could also indicate restricted airflow that triggers high-limit switches to shut burners down. Check and replace dirty filters first; clogged filters are a common, easily fixed cause of overheating and cycling.

Inspect the blower motor and belt (if equipped). Some systems use variable-speed motors controlled by the furnace control board. If the blower runs but the system still has no heat, the problem likely lies with the ignition, gas, or burner assembly rather than the blower itself.

Blocked Vents, Flue, And Combustion Air

Blocked exhaust flues or combustion air inlets can cause the furnace to fail safety checks and shut down. Birds, nests, or debris at the flue termination are common outdoor issues. High-efficiency furnaces with PVC venting can be blocked by snow or ice in winter.

Inspect vent terminations and remove obstructions. If the furnace detects poor draft or combustion flow, a pressure switch or draft inducer failure may be indicated. Keep vent paths clear year-round.

Control Board And Limit Switch Failures

The furnace control board orchestrates ignition sequences and safety interlocks. A faulted control board, or blown capacitors, can prevent heat calls from producing ignition. Error codes may flash on the control board LED; consult the furnace manual to interpret these codes.

Limit switches shut off burners if the heat exchanger gets too hot. If a limit switch frequently opens, investigate airflow, dirty heat exchangers, or failing blower motors. Replacing a faulty limit switch without addressing the cause can result in repeated failures.

Diagnostics: What To Observe And Record

Accurate diagnosis begins with careful observation. Note whether the blower runs, whether there are any ignition attempts, whether LEDs show error codes, and whether there are unusual noises such as clicking, humming, or banging.

Record thermostat behavior, time to lockout after ignition attempts, and any pattern of intermittent operation. These details help a technician pinpoint the issue and can speed up repairs. Photos of wiring terminals and error codes can be helpful for remote diagnostics.

DIY Fixes Versus Professional Repairs

Some fixes are safe for a homeowner: replacing the thermostat batteries, changing air filters, resetting breakers, clearing vent obstructions, and cleaning a flame sensor. These steps often restore heating quickly.

Do not attempt gas valve replacement, ignition system repair, combustion testing, or heat exchanger inspection without a licensed professional. These tasks carry safety risks including fire, carbon monoxide, and gas leaks. When in doubt, call a certified HVAC technician.

When To Call A Technician Immediately

Call for professional help if there is any gas odor, visible cracks in the heat exchanger, repeated ignition lockouts, evidence of carbon monoxide (alarms or symptoms), or if control board error codes persist after basic resets.

Also contact a technician when complex components like burners, gas valves, draft inducers, or the heat exchanger need service. Licensed technicians can perform combustion analysis, safety checks, and component replacements to code.

Preventive Maintenance To Avoid Future Failures

Regular annual maintenance keeps furnaces reliable. A professional tune-up includes cleaning burners, inspecting ignition components, testing safety controls, checking heat exchanger integrity, and verifying venting and combustion performance.

Homeowners should replace filters every 1–3 months, keep vents and returns unobstructed, and schedule a spring or fall inspection. Routine maintenance reduces emergency repairs and extends equipment life.

Cost Expectations For Common Repairs

Repair costs vary widely by region and system. Simple tasks like replacing a flame sensor or thermostat batteries are low cost. Ignitor replacement typically runs moderate; gas valve or control board replacements are higher cost. Heat exchanger replacement is often expensive and may justify system replacement depending on age.

Obtain multiple quotes for major repairs and ask for itemized estimates. Consider age, efficiency, and lifetime repair history when deciding between repair and replacement. High-efficiency furnace replacement can deliver long-term savings in some cases.

Key Safety And Carbon Monoxide Considerations

Furnaces that burn fuel produce combustion gases that can include carbon monoxide (CO). Install CO detectors in sleeping areas and on each floor. Test detectors regularly and replace batteries per manufacturer guidance.

If the furnace emits soot, produces strong odors, or causes CO detector alarms, shut off the furnace and ventilate the area, then call emergency services and an HVAC professional. CO risks require immediate attention.

Frequently Asked Questions

Why Does The AC Work But The Furnace Won’t Ignite?

Because AC uses a refrigerant cycle and an outdoor unit while the furnace depends on burners, ignition components, and fuel delivery. Shared components like the blower may work while furnace-specific components fail.

Can A Dirty Filter Prevent The Furnace From Turning On?

A severely dirty filter can cause overheating and trigger limit switches, leading the furnace to shut down burners. Replace filters regularly to prevent airflow-related shutdowns.

How Long Should A Furnace Try To Ignite Before Locking Out?

Ignition sequences vary by model; many furnaces will attempt ignition a few times over 30–90 seconds before entering lockout and requiring manual reset. Check the manual for model-specific behavior and error codes.

Is It Safe To Reset The Furnace Control Board Frequently?

Occasional resets after addressing the underlying issue are acceptable, but frequent resets indicate a persistent fault that requires professional diagnosis. Repeated resets can stress components and mask safety problems.

Resources And Next Steps

For model-specific instructions, consult the furnace owner’s manual or the manufacturer’s website. For electrical or gas issues, contact a licensed HVAC technician or local utility. Keep maintenance records and service history to inform repair decisions.

Homeowners who document symptoms, error codes, and previous fixes can reduce diagnostic time and costs when calling a professional. Timely action protects safety and comfort during heating season.