Most HVAC complaints reduce to a handful of symptoms - not heating, not cooling, tripping the breaker, short-cycling, or noise. This section starts from what you're noticing and points toward the likely cause, rather than starting from a part name you may not know yet.
If you already know which component failed, Repair Costs is organized by part instead of symptom. Start here if you only know what the system is doing.
Not heating or not cooling at all
Rule out the simple causes first: thermostat set incorrectly or with a dead battery, a tripped breaker, or - for a furnace - a pilot light or ignition fault. Past that, "not heating" and "not cooling" usually trace back to a failed component from the Repair Costs list - most often the thermostat, capacitor, or compressor.
Blowing air, but not hot or cold enough
Different cause than not running at all. A dirty air filter restricting airflow is the most common and cheapest cause. Beyond that: a refrigerant leak (AC side), a failing compressor, or - for a furnace - an ignition or heat exchanger problem.
System runs constantly or won't turn off
Called short-cycling when it happens in rapid bursts rather than running long and stopping. Can point to an oversized system (see Types & Options on correct sizing), a thermostat placement or wiring problem, or a refrigerant issue. Worth diagnosing rather than living with - constant cycling wears components faster.
Tripping the breaker
Do not just keep resetting it. A breaker that trips repeatedly is protecting the system from a real electrical fault - commonly a failing capacitor, compressor, or a wiring problem - and running it anyway risks further damage.
Unusual noise
- Squealing or screeching - often a belt or motor bearing issue.
- Banging or clanking - often a loose or broken part inside the unit.
- Clicking on start or stop - can be normal, or can point to an electrical control issue if it is new or getting more frequent.
- Hissing - can indicate a refrigerant leak, which is also a performance and environmental issue worth addressing quickly.
Noise that is new, getting louder, or paired with reduced performance is worth a professional look rather than waiting it out.
Leaking or pooling water
Usually a clogged condensate drain line or a full condensate pan - common, and usually inexpensive to fix. Left alone it can cause water damage, so it is worth addressing promptly even though the underlying cause is often minor.
Common questions
My system is blowing warm air from the AC. What's wrong?
Most commonly a dirty filter restricting airflow, low refrigerant from a leak, or a failing compressor. Filter first - it is the cheapest thing to rule out yourself.
Is it normal for a furnace to click when it starts?
A single click at ignition is typically normal. Repeated clicking without the burner lighting, or clicking that has become more frequent, is worth having checked.
Should I keep resetting a tripped breaker?
No - repeated tripping means the breaker is doing its job protecting the system from a real fault. Get the cause diagnosed before running it again.