A window AC freezing up in Sun Valley, and why the customer decided to replace it

What we found

In Sun Valley, a window unit was freezing up. Its copper tubes were starting to frost over, even though the blower motor ran and everything else worked.

We don’t remember the brand, and none is visible in the photo.

What the photo shows

In the photo at the top of this page, the unit’s cover is off, and you’re looking at the end of its indoor coil. Several copper tubes are coated in white frost, along with the copper fitting where they join. The bends on the end of the coil beside them are still bare copper.

How a window AC makes cold air

A window AC is a complete air conditioner in one box. The compressor and the outdoor coil sit in the half that faces outside, and the indoor coil and the blower face the room. Refrigerant boils inside the indoor coil, which makes the coil cold, and the blower pushes room air across it.

That coil is supposed to run cold but above freezing. Moisture from the room air condenses on it and drains away as water. When the pressure inside the coil drops too far, its temperature falls below freezing, and the moisture turns to frost instead.

Why would the copper frost up with the blower running?

A coil usually ices for one of two reasons. Either too little air crosses it, as with a clogged filter or a blower that isn’t running, or too little refrigerant moves through it.

On this unit the blower was running and everything else worked. That pointed to the refrigerant side, where there are two suspects: a low refrigerant charge or a restriction in the system.

Low charge or a restriction

Nothing inside a sealed system uses refrigerant up. If the charge is low, some of it has leaked out somewhere.

A restriction is a partial blockage that lets too little refrigerant through. Window units usually meter refrigerant through a capillary tube, a long copper tube with a very small bore. A blockage in it, or in a strainer ahead of it, chokes the flow.

Either way, the indoor coil gets less refrigerant than it needs. The pressure inside it drops, and the coil frosts.

How it shows up at home

As a coil ices up, it passes less and less air, so the unit runs and the room doesn’t get cool. With the front grille off, you may see frost or ice on the coil or on the copper tubes at its end, like in the photo. When the unit shuts off, that ice melts and the water has to go somewhere, so a frozen unit can drip more than usual.

Why we don’t repair window units

A window unit’s refrigerant circuit is sealed at the factory, with no ports to connect gauges to. Finding a leak or clearing a restriction means recovering whatever refrigerant is left, cutting into the sealed system, brazing, pulling a vacuum and recharging. That’s hours of skilled work on a machine that sells new for a few hundred dollars.

So we don’t repair window units. Since this was a window unit, the customer decided to replace the whole unit.

What you can check on a window unit

If your window unit ices up, switch it off, or to fan only, and let the ice melt. Don’t chip at it. Then unplug the unit and take off the front grille to get at the filter. Wash it if it’s dusty, and let it dry before it goes back in.

Make sure curtains or furniture aren’t blocking the front of the unit, and that nothing blocks the outdoor side. If the coil frosts up again with a clean filter and the blower running, the cause is likely on the refrigerant side, the same pattern as this Sun Valley unit.

Don’t open the cabinet, and don’t try to add refrigerant. Handling refrigerant requires EPA Section 608 certification.

What we offer for window units

For window and through-the-wall units, we offer replacement: we size and install a new unit, or install one you already bought. See window AC replacement for how that works. We published this job as a case study to explain what frost on a window unit’s copper tubes means.

CA BEAR registered (#A 50636), EPA Section 608 Universal certified, $1M insured.