If you see ice on the copper refrigerant line coming out of your indoor unit, frost on the coil behind the filter, or ice forming on the outdoor unit, the system is freezing up. This is not a cosmetic problem. Running the system with a frozen coil can damage the compressor — the most expensive component in the system.
Turn the system off. Then figure out why.
How a coil freezes
The evaporator coil is designed to operate around 40 degrees. Warm air from your house blows across it, the refrigerant inside absorbs the heat, and the coil stays above freezing. If something reduces the amount of warm air reaching the coil, or if the refrigerant pressure drops too low, the coil temperature falls below 32 degrees and moisture in the air freezes onto it. The ice insulates the coil further, blocking more airflow, and the problem compounds.
Causes you can check
Dirty air filter. This is the most common cause. A clogged filter chokes airflow across the coil. Pull the filter and check it. In Florida, filters should be replaced every 30 to 60 days during the cooling season. The 90-day interval printed on the packaging assumes lower humidity and less system run time than you get here.
Blocked return vents. If furniture, curtains, or rugs are covering the return air grilles, the system is starved for air even with a clean filter. Walk the house and confirm every return is clear.
Closed supply vents. Closing vents in unused rooms is a common misconception. It does not save energy. It changes the pressure balance in the duct system and can restrict enough airflow to freeze the coil.
Causes that need a technician
Dirty evaporator coil. Over time, dust and debris get past the filter and coat the coil surface. The layer of grime acts as insulation, reducing heat transfer. The coil runs colder than it should. Cleaning an evaporator coil requires removing panels and using a specialized coil cleaner — not something you can do with a garden hose.
Low refrigerant. A refrigerant leak reduces system pressure, which lowers the boiling point of the refrigerant inside the coil. The coil gets too cold and freezes. Signs include an oily residue around the indoor coil or a hissing sound near the refrigerant lines. A technician needs to find and repair the leak, then recharge the system.
Faulty thermostat or control board. If the thermostat fails to signal the system to cycle off, the compressor runs continuously and the coil overcools. This is especially common overnight — if you find the coil frozen every morning but it seems fine during the day, the thermostat may not be cycling correctly.
Blower motor failing. A blower motor running at reduced speed moves less air across the coil. The motor may sound normal but deliver far less airflow than it should, particularly if the capacitor that powers it is weakening.
How to thaw it
- Turn the thermostat to "off."
- Set the fan to "on" (not "auto"). This keeps the blower running and pushes room-temperature air across the frozen coil.
- Wait. A fully frozen coil can take two to four hours to thaw. Do not chip the ice off — you will damage the aluminum fins.
- Place towels or a pan under the air handler. As the ice melts, more water than usual will flow through the drain pan.
- While it thaws, replace the air filter and check return vents.
- After the coil is fully thawed, switch back to "cool" and monitor. If it freezes again within a day, the problem is not the filter.
Call (727) 477-3256 if the coil refreezes or if you see signs of a refrigerant leak.