AC Not Working?
12 Things to Check
Before Calling an HVAC Company

The most common cause is a dirty air filter restricting airflow. A clogged filter starves the system, freezes the coil, and prevents proper cooling. Replace the filter, then check the outdoor condenser for debris and confirm the thermostat is set to Cool. If those don’t fix it, you may have low refrigerant or a frozen coil — both of which need a technician.

Yes. If your AC is running but not cooling — especially if you see ice on the refrigerant line — turn it off at the thermostat. Continuing to run a frozen or low-charge system can damage the compressor, which is the most expensive component to replace.

Yes — and it’s the most common cause of “no cool” calls. A clogged filter restricts airflow across the evaporator coil. Reduced airflow drops the coil temperature below freezing, ice forms, and once the coil is iced over, no heat exchange can happen. The system runs, but it doesn’t cool. Replacing a 1-inch filter on time (every 30–90 days) prevents most of these calls.

For a standard 1-inch pleated filter: every 30–90 days. Homes with pets, allergies, or heavy use lean toward the 30-day end. Deeper filters last longer — a 4-inch media filter typically goes 6–12 months. Always check the filter monthly during cooling season; visual inspection beats the calendar.

This usually means the system is undersized, the heat load is too high (poor insulation, leaky ducts, sun exposure), or the condenser can’t dump heat efficiently when outdoor temperatures peak. Cooler night air gives the condenser a break, so the same equipment can keep up. Start by cleaning the outdoor unit and shading it if possible.

Likely culprits: blocked or closed vents, undersized ductwork to that room, return air imbalance, or a long duct run that loses cooling capacity. Walk the room and confirm vents are open and unobstructed. If it’s a consistent issue, a balance test by an HVAC pro can identify whether the duct system needs adjustment or insulation.

A properly sized system on a hot day runs in cycles of 15–30 minutes and lowers temperature about 1°F per 15 minutes once the house has stabilized. On a 95°F day with a 75°F set point, expect 1–2 hours to fully cool a previously warm home. If your system runs continuously and never reaches set point, you have a capacity or airflow problem.

No. Refrigerant doesn’t deplete from use — a sealed system that’s low has a leak. Adding refrigerant without finding and fixing the leak is illegal in most jurisdictions and just delays the real repair. Always insist on leak detection as part of a refrigerant service.

For most residential systems, MERV 8 to 13 is the sweet spot. Higher MERV traps more particles but also restricts airflow more. Older systems and units with marginal duct sizing may struggle with MERV 13. If you’re not sure, MERV 8–11 is a safe default; upgrade to MERV 13 if you have allergies, pets, or wildfire smoke concerns and your system can handle it.

Musty smells usually come from microbial growth on the evaporator coil or in the drain pan — a sign of poor drainage or a dirty coil. A burnt smell can mean overheated wiring, a failing motor, or burnt-out dust on the heat strips. Burnt smells are a “turn it off and investigate” situation, not a “wait and see.”

Short-cycling has many possible causes: oversized system, low refrigerant, a faulty thermostat or thermostat location near a vent, a dirty coil, or a bad capacitor. A properly sized system should run for at least 10–15 minutes per cycle. Persistent short-cycling damages the compressor and needs a technician.

Yes — gentle rinsing with a garden hose (low pressure, top-down, fins-out-to-in) is a great way to clean a condenser. Don’t use a pressure washer; it’ll bend the aluminum fins. Always shut off power at the disconnect before any cleaning.

10–15 years is typical for the outdoor condenser; the indoor air handler/furnace often lasts 15–20. Lifespan depends heavily on maintenance — and the single biggest maintenance factor is filter changes. A neglected filter shortens equipment life by years.

Generally no. A full cover traps moisture and invites rodents and pests. A small cover over just the top to keep falling leaves and ice out is fine, but the sides should remain open. Modern units are built for outdoor exposure.

Most common reasons: a dirty filter (the EPA notes restricted airflow can meaningfully raise energy use), a dirty outdoor coil, low refrigerant, hotter outdoor temperatures, or aging equipment losing efficiency. Start with the filter and the outdoor unit — both are free fixes.

Only with a generator sized for the AC’s starting amp draw (typically 2–3x the running amps) and a properly installed transfer switch. Central AC units draw heavy startup current and can damage undersized generators. Window units and mini-splits with soft-start kits are easier to run on portable generators.

The condensate drain line is likely clogged. The evaporator pulls moisture from the air; that water drains via a small PVC line. When algae and dust clog the line, water backs up into the drain pan and overflows. Vacuum the drain line from the outdoor termination with a wet/dry vac to clear it.

An AC only cools. A heat pump cools in summer and heats in winter by reversing the refrigerant cycle. Troubleshooting for cooling is essentially the same — the items on this page apply to both. Heat pumps in heat mode have additional considerations (defrost cycles, auxiliary strips) that don’t apply to summer no-cool calls.

The rule of thumb is one ton per 400–600 sq ft, but the real answer comes from a Manual J load calculation. Symptoms of an oversized unit: short-cycling, high humidity, cold spots. Symptoms of undersized: long run times, can’t reach setpoint on hot days. If you’re replacing a system, insist on a Manual J — not just “same size as the old one.”

General rule: if the system is 10+ years old, uses R-22 refrigerant, and the repair quote exceeds $1,500 — replacement usually wins on total cost of ownership. New systems are 20–40% more efficient than units installed 15 years ago, and the energy savings often pay for the install delta within 7–10 years.