Why is my air conditioner not cooling my house?

Most no-cool calls trace back to three things: the thermostat isn’t actually set to Cool, a dirty filter is choking the airflow, or the outdoor condenser isn’t running at all. Check those three before calling for service. A tripped breaker or a flipped disconnect switch by the outdoor unit accounts for a good share of the rest.

Is the thermostat actually set to cool the house?

Check the mode first, not just the number. A thermostat left on Fan Only will blow air all day and never cool anything, because the compressor outside never gets the call to run. Some programmable thermostats also override a manual setting on their own schedule, so a temperature that looks right can still be getting reset by a program running underneath it.

A dim or blank display is the other quiet failure. Battery-powered thermostats go dark with no warning, and a dead unit can’t call for cooling even though the vents look normal at a glance.

One thing that won’t help: cranking the set point down to 60 to cool the room faster. Central air removes heat at a fixed rate no matter how low the number goes, so a 60-degree setting cools the living room at the same pace as a 72-degree one. It just runs longer before it shuts off.

Could a dirty air filter be causing this?

Yes, and it’s the single most common cause of a no-cool call. ENERGY STAR puts it plainly:

A dirty air filter will cause your heating and cooling system to work harder to distribute air throughout your home. This can increase energy costs and damage your equipment, leading to early failure.

Pull the filter and hold it up to a light. If daylight doesn’t show through it, air isn’t either, and the system is straining to pull room air past it. ENERGY STAR’s maintenance checklist recommends checking filters monthly and replacing them at least every 3 months. A standard one-inch filter in a normal household usually lines up with the 90-day interval, sooner with pets or heavy dust.

Two other filter problems can look identical from the vents: a filter with a MERV rating too fine for the blower, and a filter installed backwards. Both restrict airflow the same way a clogged one does, even brand new.

Is the outdoor condenser unit actually running?

Walk out to the unit. The fan on top should be spinning and the unit should be humming, not silent. A silent unit on a hot day usually means one of two things: the breaker, or the disconnect.

The breaker lives in the main electrical panel, usually labeled AC, condenser, or outdoor unit, on a two-pole breaker rated 30 or 40 amps depending on the size of the system. The disconnect is the small gray box mounted on the exterior wall next to the condenser, with a pull-out block or a switch inside. Either one can trip or get left off after a repair, a storm, or a breaker panel that got worked on for something unrelated.

If the fan spins but the unit still isn’t cooling, check for anything blocking airflow around the coils: grass clippings, cottonwood fluff, a fence or shrub planted too close. ENERGY STAR notes that dirty coils reduce the system’s ability to cool the house and make it run longer to do less, which shows up on the bill before it shows up as a repair.

Why is there ice on the refrigerant lines?

Ice on the copper lines or the indoor coil means airflow across the coil has dropped low enough that the coil itself falls below freezing. A dirty filter is the usual cause, followed by closed supply vents, a weak blower motor, or a low refrigerant charge.

Turn the system off at the thermostat and switch the fan to On, not Auto, so warm air keeps moving across the coil while it thaws. That usually takes 2 to 4 hours. A coil melting faster than the pan can drain is also a common reason an AC leaks water inside. Running the compressor with an iced coil pushes liquid refrigerant back to the compressor instead of vapor, which is the fastest way to turn a $150 filter problem into a compressor replacement.

Once it’s thawed and dry, change the filter, open any closed vents, and try the system again. If it ices back up within a day, the airflow problem hasn’t been fixed, or the cause is inside the refrigerant system, not the filter.

When does this mean I need to call a professional?

If the thermostat’s set right, the filter is clean, and the outdoor unit is running with clear coils and it’s still not cooling, the remaining causes sit inside the refrigerant system: a low charge from a slow leak, a failing capacitor that won’t let the compressor start, or a compressor at the end of its life. None of those are things a homeowner can diagnose by eye.

Refrigerant is also the one part of this list that isn’t legal to handle without training. Section 608 of the Clean Air Act puts federal certification requirements around who can purchase, handle, and recharge refrigerant, and bans venting it into the air. A can of refrigerant from the hardware store doesn’t fix a leak, and adding refrigerant to a system that’s already low just delays the same call by a few days.

A note from Kempt

The three checks above take five minutes, but knowing which filter size and MERV rating the system takes, and when it was last changed, is the part that’s easy to lose track of between summers. Kempt keeps that history on file for the house, so when the air stops feeling right there’s already a record of what’s clean, what’s due, and what to check first before anyone reaches for the phone.