AC Not Working in a Heat Wave? What to Do Right Now

A dead air conditioner during a heat wave is genuinely urgent — not because the equipment can't wait, but because heat is dangerous. Before you pay an after-hours premium, run the quick checks below. Several of them fix the most common no-cooling calls in a few minutes.

First, protect people. In extreme heat, get anyone elderly, very young, or medically vulnerable somewhere cool — a neighbor, a mall, a cooling center. Drink water. Heat illness is the real emergency here.

Do these checks first

1. Check the thermostat. Set it to COOL and drop the target well below the room temperature. Replace the batteries. Make sure it's not stuck in "fan on" or a schedule.

2. Check both breakers. Central AC often has two circuits — one for the indoor air handler, one for the outdoor condenser. Look for a tripped breaker (sitting between on and off). Flip it fully OFF, then ON. If it trips again, stop — that's an electrical fault for a pro.

3. Replace the air filter. A clogged filter is the number-one cause of weak or no cooling. It chokes airflow and can freeze the coil solid. If the filter is dirty, swap it.

4. Check for a frozen coil. If you see ice on the refrigerant lines or indoor unit, turn the AC OFF and set the fan to ON. Let it thaw for a few hours. Running a frozen system can wreck the compressor. A freeze usually means a dirty filter or low refrigerant.

5. Clear the outdoor condenser. Shut off power to the unit. Clear leaves, grass, and debris from the fins. Make sure there's at least two feet of clearance around it. Gently hose off a caked-over coil (fins only, no pressure washer).

6. Check the condensate drain and float switch. A clogged condensate line can trip a safety float switch that shuts the system down. Clear standing water in the drain pan if you can reach it.

If it's still not cooling, you're likely looking at low refrigerant, a bad capacitor, a failed compressor, or a contactor — all pro repairs.

When to DIY vs. call a pro

The checklist above is all homeowner-safe: thermostat, breaker, filter, clearing the condenser, thawing a frozen coil.

Call an HVAC pro when:

• The breaker trips again after you reset it.
• The outdoor unit hums but the fan won't spin (often a failed capacitor).
• You suspect low refrigerant — hissing, ice, or weak cooling. Refrigerant work legally requires EPA certification.
• The compressor won't start or shuts off quickly.
• Cooling doesn't return after a filter change and a full thaw.

Refrigerant is not a DIY job — it's federally regulated, and handling it without certification is illegal and dangerous. Leave sealed-system work to a licensed tech.

What's usually wrong when the checklist doesn't fix it

If the safe checks don't bring cooling back, here's what a tech typically finds — most of it beyond DIY:

• Failed run capacitor. The most common single failure. The outdoor unit hums but the fan or compressor won't start. A cheap part, but diagnosing and replacing it safely is a pro job (capacitors hold a charge).
• Low refrigerant / a leak. Weak cooling, ice on the lines, or hissing. Refrigerant is federally regulated — a tech has to find the leak, fix it, and recharge to spec.
• Bad contactor. The relay that starts the condenser wears out and pits.
• Frozen or dirty coils. Restricted airflow or low charge freezes the evaporator; a caked condenser coil can't shed heat.
• Failing compressor. The most expensive failure. On an older unit, it often tips the decision toward replacement.

Knowing the likely cause helps you sanity-check the quote you're handed.

What it costs

| Job | Typical 2026 range | |---|---| | After-hours diagnostic call | $150–$400 | | Emergency AC repair | $150–$800 | | Capacitor replacement | $150–$450 | | Refrigerant recharge + leak fix | $250–$1,500 | | Compressor replacement | $1,200–$2,800 | | Full central AC install | $3,900–$12,000 |

Emergency HVAC calls run 1.5 to 2 times the daytime rate. If the unit is 12+ years old and needs a compressor or major refrigerant work, replacement often makes more sense — see our central AC installation cost guide, or estimate your local repair cost.

Staying safe in the heat while you wait

HVAC techs are slammed during heat waves, so you may wait hours or a day. Beat the heat safely:

• Close blinds and curtains on sun-facing windows to block radiant heat.
• Run fans to move air; a box fan in a window at night pulls in cooler air.
• Hydrate constantly and skip alcohol and heavy meals.
• Stay on the lowest floor — heat rises.
• Take cool showers and use damp cloths on the neck and wrists.
• Relocate anyone vulnerable — elderly, infants, pregnant, or medically fragile — to a cooled space like a library, mall, or cooling center.
• Know the warning signs of heat illness: heavy sweating that stops, dizziness, nausea, confusion, or a very high body temperature. Confusion or fainting is a 911 call.

Prevent the next breakdown

• Change the filter every 1–3 months in cooling season.
• Book a spring tune-up before the first heat wave — techs are booked solid once it's 95 degrees out.
• Keep the condenser clear of plants, debris, and clutter.
• Rinse the outdoor coil a couple of times a season.
• Watch for early warning signs — weak airflow, warm spots, unusual noises — and address them before they strand you.

Our HVAC maintenance guide has the full schedule, and the summer maintenance checklist covers cooling-season prep.

FAQ

Why is my AC blowing warm air? Common causes: thermostat set wrong, a dirty filter, a frozen coil, low refrigerant, or a tripped breaker. Work the checklist above before calling.

Should I run my AC if the coil is frozen? No. Turn it off, run the fan to thaw it, and find the cause (usually a dirty filter or low refrigerant). Running frozen can destroy the compressor.

Is a broken AC an emergency? The heat can be. If temperatures are dangerous or anyone in the home is vulnerable, treat it as urgent — and get to a cool place while you wait.

How do I stay safe without AC in a heat wave? Hydrate, use fans, close blinds, stay on lower floors, take cool showers, and relocate vulnerable people to a cooled space. Watch for heat-illness symptoms.

Repair or replace my AC? If it's 12+ years old and facing a compressor or major refrigerant repair, replacement usually wins. See the central AC cost guide.

Why does my AC freeze up? Ice on an AC almost always means restricted airflow (a dirty filter or blocked return) or low refrigerant from a leak. Turn it off, thaw it fully, change the filter, and if it refreezes, call a pro to check the charge.

My outdoor unit is running but no cold air inside — what's wrong? Could be a frozen coil, a failed indoor blower, low refrigerant, or a thermostat issue. Work the checklist first; if cooling doesn't return, it's a service call.

How long can I run without AC in extreme heat? That depends on the temperature and who's home. Healthy adults can manage with fans, shade, and hydration, but vulnerable people should relocate to a cool space — heat illness is a genuine medical risk.

Cost figures are 2026 national averages for general information only, not quotes. Your price depends on your specific job, home, and location. Always get a written estimate before authorizing work. In extreme heat, prioritize getting vulnerable people to a cool place and watch for signs of heat illness — call 911 for symptoms like confusion, fainting, or a very high body temperature.