10 Most Common HVAC Problems and How to Fix Them Before Calling a Technician

HVAC service calls average $100-$200 just for the technician to show up, plus parts and labor for any repairs. But roughly 30% of service calls involve issues homeowners could have resolved themselves in 5-15 minutes. Before calling, check these 10 common problems.

1. System Won't Turn On

Check first: Is the thermostat set to the correct mode (heat or cool)? Is the temperature set below (for cooling) or above (for heating) the current room temperature? Is the thermostat display on? (Dead batteries are a common culprit.) Is the system switch on the thermostat set to "Auto" or "On" rather than "Off"? Check the circuit breaker - a tripped breaker is the most common cause of a completely dead system. Look for both the indoor and outdoor unit breakers. Also check for a power switch on or near the indoor unit (often looks like a light switch) that may have been accidentally turned off.

2. System Runs But Doesn't Heat or Cool

Check first: Is the outdoor unit running? Go outside and listen/look. If the outdoor unit isn't running but the indoor fan is blowing, the outdoor unit may have a tripped breaker, a blown fuse in the disconnect box (usually mounted on the wall near the outdoor unit), or the unit may have shut down on a safety lockout. Try resetting the breaker and the disconnect. If the outdoor unit runs but air from vents isn't warm (heating) or cool (cooling), check your air filter - a severely clogged filter can cause the system to freeze up or overheat, preventing proper operation.

3. Weak Airflow from Vents

Check first: Dirty air filter is the #1 cause. Replace it and see if airflow improves within 30 minutes. Check that all supply registers and return air grilles are open and unobstructed (no furniture blocking them). Make sure the blower fan switch is set to "Auto" not "On" - in "On" mode, the fan runs continuously even when the system isn't actively heating or cooling, which can feel like weak flow.

4. Unusual Noises

Rattling: Usually a loose panel, screw, or component. Check the access panels on the indoor unit and make sure they're fully secured. Also check the outdoor unit for loose screws or debris (sticks, leaves) caught in the fan. Squealing or screeching: Often a worn belt or bearing in the blower motor. This needs professional attention but isn't an emergency - schedule a service call. Banging or clanking: Can indicate a loose blower wheel, broken component, or compressor issue. Turn the system off and call a technician - running with a loose component can cause further damage.

5. System Short Cycling

If your system turns on and off every 5-10 minutes without completing a full cycle, check the air filter (severely restricted airflow causes overheating and safety shutdown), the thermostat location (if it's near a heat source like a lamp, it reads incorrectly), and whether the system is oversized for the space (a chronic issue from improper installation). If the filter is clean and the thermostat location is fine, a technician needs to diagnose the issue.

6. Water Leaking Around Indoor Unit

Check first: The condensate drain is likely clogged. The AC produces water as it removes humidity, which flows through a drain line. When the line clogs (usually with algae or debris), water backs up and overflows. Clear the clog by pouring a cup of white vinegar or bleach solution into the drain line access point (usually a PVC pipe near the indoor unit). If the pan is full, use a wet/dry vacuum to clear it. Regular maintenance (monthly vinegar flush) prevents this issue.

7. Thermostat Not Matching Room Temperature

Check first: Is the thermostat getting direct sunlight, near a heat source (lamp, oven), or near a drafty area? Any of these can cause inaccurate readings. Also check the thermostat level - a tilted thermostat with a mercury sensor (older models) reads incorrectly. If you have a smart thermostat, check that the software is up to date and sensor calibration is correct.

8. Ice on Outdoor Unit or Refrigerant Lines

Ice formation usually indicates restricted airflow (dirty filter) or low refrigerant. Turn the system off and switch the fan to "On" to melt the ice. Replace the air filter. If ice returns after running for a few hours with a clean filter, you likely have a refrigerant leak that needs professional repair.

9. Bad Odors from Vents

Musty/moldy smell: Usually mold or mildew in the ductwork, on the evaporator coil, or in the condensate drain pan. Clean the drain pan, flush the drain line, and schedule a professional coil cleaning. Burning smell: At the start of heating season, a brief burning smell (dust burning off the heat exchanger) is normal. A persistent burning smell indicates a potential electrical issue - turn off the system and call a technician. Rotten egg smell: If you have a gas furnace, this could indicate a gas leak. Leave the house immediately and call your gas company from outside.

10. Uneven Temperatures Between Rooms

Some rooms hot, others cold? Check that all vents are open and unobstructed. Check for ductwork disconnections in the attic or crawlspace (a disconnected duct dumps conditioned air into unconditioned space). Ensure return air grilles aren't blocked. If the issue persists, it may indicate ductwork design problems, inadequate insulation in certain rooms, or the need for a zoning system or supplemental mini-split.