Air Conditioning Not Cooling: What to Check Before You Call

First hot week of the year, the AC comes on, and the house does not get cool. Here is my order of operations, and the first four are free.

Is the thermostat actually calling for cool

Sounds insulting. It is the first thing I check anyway, because it is right often enough to be worth ten seconds.

Set it to COOL, not AUTO or OFF, and drop the setpoint five degrees below the current room temperature. Fan on AUTO, not ON. Fan set to ON runs the blower continuously even when the compressor is not running, which pushes room temperature air around and makes people think the system is running when it is not cooling.

If it is a battery powered thermostat, change the batteries. Dying batteries cause the strangest intermittent behavior.

Filter

Yes, again. It is the answer to a surprising share of all HVAC calls.

Restricted airflow across the evaporator coil means the coil gets too cold, and eventually it freezes. Once there is ice on it, air cannot get through at all and you get essentially no cooling. Meanwhile the system runs and runs.

Pull the filter. If it is grey, replace it.

Go look at the outdoor unit

Two things to look for.

Is the fan spinning? If the indoor blower is running but the outdoor fan is not, you have a problem out there. Common causes are a failed capacitor or contactor, both routine repairs. Do not stick anything into the fan to help it along.

Is it clear? The outdoor coil rejects heat and it needs airflow. Cottonwood fluff, grass clippings, leaves, a shrub that grew in around it over two summers. Shut the power off at the disconnect and rinse it gently with a garden hose from the inside out. Low pressure. Those fins fold over at almost nothing and once they are flat the airflow is gone.

Give it a couple feet of clearance all around and about five feet overhead.

Is the indoor coil frozen

Go look at the indoor unit. If you see ice on the refrigerant lines or on the coil itself, you found your problem.

Shut the cooling off and set the fan to ON. That runs air across the coil without making more ice and thaws it, usually a few hours. Do not chip at it.

Then figure out why it froze, because it will do it again. The two causes are restricted airflow, which is the filter, a blocked return, or closed registers, and low refrigerant charge, which means a leak.

Now the ones that need me

Water where it should not be. The indoor coil makes condensate and it drains away through a line. If that line plugs, water backs up. Many systems have a float switch that shuts everything down to prevent a ceiling collapse, and that is a system doing exactly what it should. If you find water around the air handler, shut it off and call.

It runs constantly and cools a little but never enough. Often low charge, which means a leak. Refrigerant is not consumed, so a system low on it has lost it somewhere. Topping it off without finding the leak means paying for the same repair again next summer.

Breaker keeps tripping. Reset it once. If it goes again, stop and call. Something is drawing more current than it should and repeatedly resetting a breaker on a fault is how you get a fire.

Hissing, grinding, or a buzz followed by nothing. A buzz then silence at the outdoor unit is very often a failed capacitor. It is a common part and a quick fix.

Smells like something burning. Off, and call the same day.

One that is not a failure

If your system cannot quite keep up on the very hottest, most humid days of the year but it is fine the rest of the summer, that may be correct sizing rather than a problem.

Equipment gets sized for a design temperature, not for the single worst afternoon of the decade. A system sized to handle the extreme is oversized for the whole rest of the season, which means it short cycles, never runs long enough to dehumidify, and leaves you with a house that is cold and clammy.

A properly sized system running long and steady on the worst day is doing its job. If it is falling behind on ordinary eighty degree days, that is different, and worth a look.