Why your AC freezes up in a Texas summer

It is 98 degrees in Tomball, the thermostat says 75, and the house is sitting at 80. You open the air handler closet or climb into the attic and find ice on the copper lines or a furry block of frost on the coil. An air conditioner making ice in a Texas summer feels backward, but it is one of the most common reasons homeowners pick up the phone, and it has a short list of causes.

Why a coil turns to ice

The indoor evaporator coil runs cold on purpose. Refrigerant passing through it pulls heat and moisture out of the house air that blows across it. As long as plenty of warm air keeps moving over the coil, its surface stays just above freezing and the moisture drips off into the drain pan. Cut down the air, or drop the refrigerant pressure too far, and the coil surface falls below freezing. The moisture that was dripping now turns to frost.

Frost is a good insulator, which is the cruel part. Once a layer forms, less air gets through the coil, less heat reaches it, and it gets even colder. The ice builds on itself until the coil is a solid block and very little air moves at all. At that point the vents may blow weakly and the house warms up, even though the outdoor unit is running hard.

The usual suspects

Airflow problems are the most common cause. A filter that has not been changed in months is the first thing to check, because a clogged filter starves the coil of air. Closed or blocked supply vents do the same thing, and closing vents in unused rooms to save cooling is a common habit that backfires. A dirty coil, a dirty blower wheel, a crushed section of flex duct in the attic or a blower motor running below speed can all choke off the air in the same way.

The second cause is low refrigerant. A sealed system should not lose charge, so low refrigerant means a leak. Less refrigerant means lower pressure in the coil, and lower pressure means a colder coil surface that can freeze even when airflow is fine. Adding refrigerant without finding the leak is a short-term patch, and the system tends to freeze again.

Condensate comes into it as well, though in a different way. When ice melts, all that water has to go somewhere. A clogged condensate drain line or an overfull drain pan can overflow into a closet, a ceiling or an attic. Many systems have a float switch that shuts the unit off when the pan fills, which is a useful protection, but it can look like a mysterious shutdown if you do not know it is there.

What to do when you find ice

Do not keep running the air conditioner and hope. Turn the cooling off at the thermostat first. Then set the fan to On, which moves house air across the coil without the refrigerant cycle running and helps the ice melt. Thawing can take a few hours, so be patient.

• Switch the thermostat to Off, then set the fan to On.
• Check the filter and replace it if it is dirty or clogged.
• Look around the air handler and drain pan for water, and wipe up what you can.
• Check that supply and return vents in the house are open and unblocked.

Do not chip at the ice, scrape it, or aim a hair dryer or heat gun at it. The coil fins and refrigerant lines are thin and easy to puncture, and a leak you cause is a far bigger problem than the one you started with.

Why running it frozen makes things worse

Technicians worry about running a frozen system because the compressor, the most expensive component in the outdoor unit, is designed to handle refrigerant vapor. A badly iced coil can send liquid refrigerant back toward the compressor, and that kind of strain shortens its life. Even without that risk, a system that is frozen is not cooling the house, so the electricity you are paying for is mostly wasted.

When thawing does not fix it

If the coil freezes again after you changed the filter and thawed it, the filter was not the cause, and it is time to call. A technician will usually measure the temperature drop across the coil, check the blower and the airflow through the ducts, check the refrigerant charge, and look for a leak if the charge is low. They will also check the condensate drain, since water damage is the part of a freeze-up homeowners tend to discover last.

It helps to tell whoever answers what you found: where the ice is, whether the filter was clean, whether any vents were closed, and how long the system ran before it iced. Those details point a technician in the right direction before they arrive. If you need help with a frozen air conditioner, ASR Mechanical in Tomball can be reached at (281) 351-5354, and its listed hours run from 12:00 AM to 11:30 PM on weekdays and all day on weekends.