AC Freezing Up: What the Ice Means and What to Check First

How to tell the AC is frozen, and where to look

Lennox says the refrigerant absorbs heat from the air at the indoor evaporator coil, and Trane says that coil is paired with your furnace or inside your air handler. Refrigerant is the fluid that moves heat around the system, and the air handler is the indoor cabinet that holds the blower fan and often the coil. The condenser coil is in the outdoor unit. Trane lists two signs of a frozen evaporator coil: visible icing of the coil or its cabinet, and frozen refrigerant lines at the outdoor unit. It adds that puddles of water on the floor can appear as a frozen coil thaws. Lennox adds weak or warm airflow, and gurgling, hissing or buzzing sounds. Trane warns that apart from visible ice, signs such as warm air or water near the drain pan may come from another problem with the system. Our reading: ice is the only sign on these lists that points straight to a freeze, so look for it on the outside of the indoor unit and on the copper lines first. Do not open the indoor cabinet to look, because that is work for a licensed professional.

Trane says outdoor coils on an AC unit are unlikely to freeze in the summer. Our reading: in summer, treat ice on the outdoor copper lines as part of the same freeze, and take the same first step. A heat pump in heating mode is different, because Carrier says a light frost, generally less than 1/4 inch, is normal and should clear during the defrost cycle. In the defrost cycle, a heat pump reverses for a few minutes to melt that frost, according to Lennox. Heater not working in Florida covers normal defrost and what is not normal. Trane warns heat pump owners not to switch to defrost mode when the indoor coil is frozen, because that mode only removes ice from the outdoor coil.

Why ice forms: the 32°F line

Bryant says moisture on the evaporator coil can freeze when the coil operates below 32°F. Lennox says that when airflow drops, humidity in the air condenses on the coil and freezes. Ice then makes the problem worse, because Bryant says it can further restrict airflow. The Department of Energy guideline describes the same loop for low refrigerant, where ice on the evaporator restricts heat transfer, increases airflow resistance and reduces airflow. Our reading: a freeze can start small and grow while the system keeps running, which is why every maker above says to turn the cooling off. Bryant says running a frozen unit puts extreme strain on the compressor, which can lead to complete and costly system failure. The compressor is the pump in the outdoor unit that moves refrigerant around the system.

Airflow or refrigerant: what the sources list

Trane calls restricted airflow the biggest reason for an evaporator coil to freeze. Its list is a clogged or dirty filter, blocked return air vents, a dirty evaporator coil, low refrigerant and thermostat issues. UF/IFAS Extension, the University of Florida's extension service, says supply registers bring conditioned air in, and return registers draw air out to be reconditioned. Bryant adds closed or blocked supply registers, and a blower motor that has failed or runs at reduced speed. Bryant also says closing vents in unused rooms might seem like a way to save energy, but it can disrupt the system's airflow balance. Airflow is measured in cfm, cubic feet of air per minute, and cooling capacity in tons. The Department of Energy guideline says very low airflow, below 200 cfm per ton, can lead to system failure due to icing of the indoor coil.

Lennox says low refrigerant, often due to a leak, drops the pressure inside the coil so that it can get too cold and freeze. Trane says the only way to have low refrigerant is a leak, or a charge that was set wrong when the unit was installed. Bryant says there is no single refrigerant-pressure reading that applies to every AC system. It adds that pressures vary by refrigerant, outdoor temperature, indoor conditions, equipment design and operating state. Our reading: a pressure number from a web page cannot tell you whether your system is low. AC refrigerant leak repair covers how leaks are found and why adding refrigerant alone is a bandage.

The Department of Energy guideline adds that airflows below 350 cfm per ton render standard tests for determining refrigerant charge invalid and can lead to improper charging. Our reading: that is why airflow comes before refrigerant in the order of checks, and why ice alone is not proof that the charge is low.

Lennox says running the AC when it is below 60°F outside can cause it to freeze up. Carrier says to avoid setting the thermostat too low, because that can cause the coils to drop below freezing.

What to check safely, in order

These steps use only the thermostat, the filter and the registers. Everything else is for a licensed professional.

1. Switch the thermostat from cool to off. Trane says running the system with a frozen coil can cause major damage.
2. If your equipment maker's instructions allow it, set the fan to On, which circulates air across the coil while it thaws.
3. Do not chip or scrape the ice, because that can damage the coil or the refrigerant tubing.
4. Check the air filter and replace it if it is dirty. Open any closed supply registers, and clear anything covering a return, such as furniture.
5. Place towels around the indoor unit, because melting ice can create enough water to overwhelm the condensate drain pan. The condensate drain pan catches the water that forms on the coil.
6. Do not restart the AC while significant ice remains on the coil. When the ice has melted completely, you can turn the cooling back on.

Bryant says thawing can take several hours, and longer with heavy ice. Carrier gives about 1 to 3 hours for a light frost and 4 to 8 hours or longer for heavy ice. Air conditioner leaking water covers where thaw water goes and what a full pan means.

Carrier's page says never to use heat sources like hair dryers or space heaters, because this can cause thermal shock and crack components. Our reading: skip added heat, and use the fan setting and time instead.

Fan On is a thaw setting, and why your house feels humid with the AC running covers the On versus Auto choice for everyday use. Opening the cabinet, cleaning the coil, checking the blower, finding a leak, and anything that touches refrigerant or wiring are work for a licensed professional. The evaporator coil covers who should clean the indoor coil.

If it freezes again, and what a technician checks

Lennox says to call a technician if the AC keeps freezing after you replace the filter and restore airflow. Bryant says a freezing AC will not fix itself, because the unit will freeze again once turned back on unless the underlying issue is resolved. Bryant says a dealer can check refrigerant conditions, test electrical components, measure airflow and static pressure, and inspect the evaporator coil and blower. Static pressure is the resistance the blower pushes against as it moves air through the filter, the coil and the ducts. Bryant says adding refrigerant without fixing the underlying leak is a temporary and costly band-aid. Trane says an oily residue around the indoor coil, or a hissing sound near the coils, is a reason to call a professional right away. Bryant says a frozen AC is not always an emergency, but it can become one if it leads to a complete system shutdown or occurs during extreme heat. Why your AC is not blowing cold air covers the wider list of reasons for weak cooling.

What Tampa Bay changes

The National Weather Service Tampa Bay Area office ties the rainy season to dew points that stay above 70°F, and says the higher the dew point, the more moisture is in the air. Our reading: in a Tampa Bay summer the air crossing the coil carries a lot of moisture, so a coil that falls below 32°F has plenty of water to turn into ice. UF/IFAS Extension says that when a system does not remove enough moisture, the typical owner response is to lower the thermostat setting. Our reading: that habit, set beside Carrier's warning about a very low setting, makes the thermostat setting worth a look when ice appears.

Keeping it from freezing again

Filter intervals differ by maker: Bryant gives a common range of every 30 to 90 days, and Carrier recommends every 60 to 90 days. UF/IFAS Extension says to check filters monthly during peak season, or as the manufacturer recommends. AC filter replacement covers sizes, MERV ratings and how often to change a filter. ENERGY STAR says a contractor's annual pre-season check-up should include cleaning the evaporator and condenser coils and checking the refrigerant level. AC maintenance checklist for Florida homes covers the seasonal list.

What to ask the technician, in writing

These questions come from the documents above, and they say nothing about how any one company works.

1. What did you find was the cause of the ice, and how did you confirm it?
2. Was airflow measured before the refrigerant charge was judged? The Department of Energy guideline says low airflow makes standard charge tests invalid.
3. If the charge was low, where is the leak, and was it repaired or only topped up?
4. Was the evaporator coil inspected, and was it clean?
5. Were the filter, the supply registers, the returns and the blower checked, and what was found?
6. What readings did you record, and can I have a copy?
7. What should I do if ice returns before the next visit?

Our reading: written answers make it easier to compare against a second opinion. When to get a second opinion on an AC repair diagnosis covers when that makes sense.