Key takeaways
- Short answer: The most common causes are a clogged air filter, a frozen evaporator coil, a failed capacitor, a dirty outdoor coil or low refrigerant from a leak.
- Check the filter and thermostat first
- Ice on lines? Turn the system off
- Outdoor fan not spinning often = capacitor
- Typical hvac cost: $7,000–$12,000 for a combined AC and furnace replacement.
Troubleshoot in this order
- Confirm the thermostat is set to Cool and below room temperature.
- Replace the air filter if it's dirty.
- Check the breaker for the outdoor unit.
- Look at the outdoor unit: is the fan spinning and the compressor humming?
- Look for ice on the copper lines or indoor coil. If present, switch the system to Fan only to thaw it.
- Clear debris and gently rinse the outdoor coil with a hose (power off).
When to call a pro
- The outdoor fan doesn't spin or hums loudly
- Ice keeps coming back
- You hear hissing (possible refrigerant leak)
- The breaker trips repeatedly
Average AC repairs cost about $375, and capacitor replacements $100–$400.
Related questions
Can a dirty filter stop an AC from cooling?
Yes. A clogged filter starves the coil of airflow, which can freeze it and stop cooling completely.
How long does it take a frozen AC to thaw?
Usually 1–3 hours in Fan-only mode. If it refreezes, there's an airflow or refrigerant problem that needs a technician.
Is low refrigerant normal?
No. AC systems are sealed, so low refrigerant means there's a leak that should be found and repaired.
Other hvac questions homeowners ask
Why is my AC running but not cooling?
The usual causes are a clogged filter, a frozen evaporator coil, a tripped outdoor unit, a failed capacitor or low refrigerant from a leak. Replace the filter and check the breaker first; if that doesn't fix it, call a technician.
How much does a new AC unit cost for a 2,000 sq ft house?
Most 2,000 sq ft homes need a 3–4 ton system. A central AC replacement alone typically runs $2,500–$9,000 nationally, and replacing both the AC and furnace costs about $7,000–$12,000.
Is it worth repairing a 15-year-old AC?
Usually not. Use the $5,000 rule: multiply the unit's age by the repair cost. If the result is over $5,000, replacing is usually the better value. A $400 repair on a 15-year-old unit scores $6,000.
How long does an HVAC system last?
Furnaces typically last 15–20 years, and central air conditioners and heat pumps 10–15 years. Long cooling seasons and skipped maintenance shorten those lives.
What size AC do I need for my house?
A rough rule is one ton of cooling per 500–600 square feet, but insulation, windows, climate and ductwork change that. Ask for a Manual J load calculation instead of a square-footage guess.
Should I replace my AC and furnace at the same time?
Often yes when both are over 12–15 years old. A matched system runs more efficiently, and you pay for one installation visit and one permit instead of two.
How much does an AC tune-up cost?
A tune-up typically costs about $75–$200 per visit. Many companies offer maintenance plans that bundle a spring AC and fall furnace check.
Are there rebates for a new heat pump?
Federal energy tax credits changed in 2025, so check current IRS guidance. Utilities and some states still offer rebates for high-efficiency heat pumps, so ask installers which programs apply.
HVAC at a glance
Timeline
Most AC or furnace swaps take one day. A full system with new ductwork takes 2–5 days.
Permits
Replacing an AC, heat pump or furnace usually needs a mechanical permit, and the installer normally pulls it.
DIY or pro?
Filters, clearing debris from the outdoor unit and flushing the condensate line are safe DIY jobs. Refrigerant, gas and electrical work need a licensed technician.
What HVAC repair and replacement costs
Replacing a central AC and furnace together typically costs $7,000–$12,000 installed. An AC alone runs about $2,500–$9,000 and a furnace $2,000–$6,900, depending on size, efficiency and brand. Most single repairs cost $250–$700.