How HVAC Systems Work
Understand normal operation before diagnosing symptoms.
Interactive Troubleshooter
Not sure which symptom guide to start with?
Use the HVAC Troubleshooter to answer a few questions about what the system is doing. It will narrow the likely causes, identify safe checks and point you to the most relevant guides.
Cooling Problems
Air-conditioning symptoms
Start with what you see, hear or feel. The same symptom can have several possible causes, so these pages are guides—not diagnoses.
Cooling
AC Running But Not Cooling
What can cause poor cooling even though the system appears to be running.
Troubleshoot → CoolingFrozen Evaporator Coil
Why the indoor coil can ice over and what should happen before restarting the system.
Troubleshoot → DrainageAC Leaking Water
Common reasons water appears around the furnace, air handler or indoor coil.
Troubleshoot → ControlsAC Short Cycling
Why the system may start and stop too frequently.
Troubleshoot → Outdoor UnitOutdoor AC Unit Not Running
What can prevent the condenser fan or compressor from operating.
Troubleshoot → ComfortHigh Indoor Humidity
Why a home can feel damp even while the air conditioner is operating.
Troubleshoot →Heating Problems
Furnace and heating symptoms
Heating
Furnace Not Starting
Common reasons a furnace may not begin or complete a heating cycle.
Troubleshoot → HeatingFurnace Short Cycling
Why a furnace may repeatedly start, stop or trip on a safety limit.
Troubleshoot → HeatingFurnace Blowing Cool Air
Possible reasons supply air does not feel warm during a heating call.
Troubleshoot → ComfortUneven Heating
Why some rooms remain cold while others are comfortable.
Troubleshoot →Airflow & Controls
Symptoms that affect both heating and cooling
Airflow
Weak Airflow From Vents
Common airflow restrictions and blower-related conditions.
Troubleshoot → ControlsThermostat Not Working
What can cause a blank display or a system that does not respond to thermostat calls.
Troubleshoot → NoiseHVAC System Making Noise
What rattling, buzzing, grinding, squealing and other sounds may indicate.
Troubleshoot → OdorUnusual HVAC Odors
Which odors may be harmless and which deserve immediate attention.
Troubleshoot →
Safety First
Know when to stop troubleshooting
Turn equipment off and seek professional evaluation if you smell gas, see smoke or arcing, hear severe mechanical noise, find significant water near electrical components, or suspect combustion or carbon-monoxide problems.
Related Resources
Continue with the right next topic
Maintenance Center
Prevent common airflow, drainage and equipment problems.
HVAC Parts Dictionary
Identify parts mentioned in troubleshooting discussions.