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Common HVAC problems, explained
Sixteen guides covering the cooling and heating faults that come through this line most often: what causes them, what a technician checks, what the repair typically costs, and when it is an emergency rather than an inconvenience.
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Call (855) 321-3194How do I work out what is wrong with my HVAC system?
Start with what the system is doing rather than how the house feels. Whether it runs at all, whether air moves, and whether the breaker has tripped each point at a different fault. Pick the guide below that matches, or call (855) 321-3194.
Cooling
Cooling problems
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AC blowing warm air
Warm air from the vents with the AC running: refrigerant, compressor and capacitor causes, with typical repair costs.
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AC leaking water
Water pooling around an air conditioner: condensate drains, overflow switches, frozen coils and pans, with repair costs.
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AC not cooling
Why an AC runs but will not cool: ranked causes, what a technician checks, typical repair cost and when it is an emergency.
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AC short cycling
Why an air conditioner short cycles: oversizing, thermostat placement, airflow and refrigerant faults, with repair costs.
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Clogged condensate drain
Why condensate drains block, what happens when they do, and how to stop it recurring. Typical clearing costs included.
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Frozen AC coil
Why an AC coil freezes: airflow restriction, low refrigerant and blower faults, plus what to do before a technician arrives.
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Low refrigerant signs
How to recognise low refrigerant, why it always means a leak, and why topping up without a leak search wastes money.
Heating
Heating problems
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Furnace blowing cold air
Why a furnace blows cold air: flame sensor dropout, fan settings, limit switches and ductwork, with repair costs.
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Furnace not turning on
A furnace that will not start: ignition lockouts, switches, filters and control faults, with typical repair costs.
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Heat pump not heating
Why a heat pump will not heat: reversing valves, defrost faults, low charge and auxiliary heat, with repair costs.
Either system
Problems that affect both
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Emergency HVAC checklist
What counts as an HVAC emergency, who to call first, what to shut off, and what can safely wait until morning.
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HVAC making noise
Grinding, squealing, banging, clicking and humming from an HVAC system: what each noise means and what it costs.
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HVAC smells burning
Burning smells from a heating or cooling system: which are harmless, which mean shutting it off, and what to do first.
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HVAC tripping breaker
Why an HVAC system trips its breaker: compressors, motors, wiring and sizing, and why resetting repeatedly is a mistake.
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Thermostat not working
A thermostat that is blank, misreading or will not switch modes: power, wiring and C-wire causes, with repair costs.
Can wait
Uneven heating and cooling
Why one room is always too hot or too cold: duct losses, balancing, return air and zoning, with typical costs.
Safety first
Is this a call now, or a call the utility first?
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- No cool air during a heat advisory
- No heat with outdoor temperatures below freezing
- Burning smell from the vents
- Water pooling at the air handler
- Ice on the line set
- Breaker tripping on every cycle
Call (855) 321-3194 and give your ZIP code.
Shut it off and call the utility or 911 first
- You smell gas: leave the building, then call the gas utility or 911
- Smoke or sparking at the equipment
- A carbon monoxide alarm is sounding: leave and call 911
Reference: CPSC recalls and US Fire Administration heating fire safety.
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Written and fact-checked by the HVAC Emergency USA editorial team. Last updated . Editorial policy.