Ice on the indoor unit or the copper refrigerant lines means the evaporator coil has dropped below 32°F, so the moisture it pulls from the air freezes instead of draining away. That happens for one of two reasons. Either not enough warm air is crossing the coil (a dirty filter, a dirty coil, a weak blower, closed vents or blocked ducts), or the refrigerant is running too cold because the charge is low or something restricts its flow. Running the air conditioner on a cool day can do it too. Thaw the coil first, then find out which cause it was.
Thaw it the right way
- Turn cooling off at the thermostat and set the fan to On. Moving room air across the coil melts the ice faster than leaving everything off.
- Protect the floor. Melt water fills the drain pan quickly. Put towels or a shallow tray around the indoor unit and check them every half hour.
- Wait it out. Light frost clears in about an hour; a solid block of ice can take several hours. Do not scrape the ice or aim heat at it.
- Change the filter while you wait.
- Restart and watch. When the coil and lines are dry, turn cooling back on and check the larger copper line after 15 to 20 minutes. Cold and sweating is normal. Frost forming again means the cause is still there.
If the blower does not run on Fan On, you have probably found the cause: leave cooling off and call.
Why a coil turns to ice
In a working system the refrigerant inside the coil boils at roughly 40°F. Warm air from the house passing over the fins keeps the surface above freezing while the coil absorbs heat and the moisture it condenses drips into the pan. Take away some of that air and the coil runs colder until the water on it freezes; the ice then blocks even more air, so the freeze speeds up. Lose some refrigerant and the pressure in the coil drops, and its temperature drops with it. From the outside both causes look the same, which is why a technician measures before adding anything.
Symptom, likely cause, what it takes
| What you see | Likely cause | What it usually takes |
|---|---|---|
| Ice plus a grey, packed filter | Low airflow | New filter, then a coil check |
| Ice, weak air at every vent, clean filter | Dirty evaporator coil or a slow blower motor | Coil cleaning or blower repair |
| Ice, and the blower does not run at all | Failed blower motor, capacitor or control | Blower repair |
| Ice back within a day of a clean filter | Low refrigerant charge from a leak | Leak search, repair, recharge |
| Ice plus a hiss at the indoor unit | Refrigerant leak at or near the coil | Leak repair or coil replacement |
| Ice after running on a cold, foggy day at a low setting | Too little heat reaching the coil | A higher setting, or no cooling on cold days |
| Ice with several vents closed or rooms starved of air | Closed registers, crushed or blocked ducts | Opening vents; ductwork repair |
Where the melt water goes
A frozen coil releases a surprising amount of water as it thaws. The drain pan under the coil and the condensate line have to carry it away, and if the line is partly clogged, the pan overflows. In many Daly City homes with the living space over a ground-floor garage, the furnace sits in the garage with the cooling coil mounted on top of it, so overflow can run down onto the furnace cabinet and its controls. After a thaw, look for water on and around the furnace and check that the condensate line is dripping where it drains. If a float switch shut the system off, have the line cleared before restarting.
Checks after the thaw
- Every supply register and return grille is open and unblocked.
- Duct you can see in the garage or crawlspace is not crushed, kinked or pulled apart.
- The new filter is the right size and seated with no gap, so air cannot bypass it and carry dust onto the coil. An air filter replacement schedule keeps this from happening again.
When it needs a technician
- Ice comes back after a full thaw and a new filter.
- The blower will not run, or runs weakly.
- You hear hissing or see oily residue on the fittings.
- The coil freezes again and again through the season.
- Melt water has reached a ceiling, a floor or the furnace.
A technician checks blower speed and static pressure, inspects or cleans the coil (coil cleaning), and measures the refrigerant charge. If the charge is low, the leak is found and repaired before any refrigerant is added. For a system that runs without ice but still does not cool, see AC not cooling.
A note on cool, foggy days
Most home air conditioners are designed for warm weather. Run one on a cold, damp day with the thermostat set very low and the coil takes in so little heat that its temperature can fall below freezing. If yours iced up on a foggy morning after someone set it far below room temperature, that may be the whole story: thaw it, use a normal setting, and see whether it happens again in ordinary use.