This is a problem – you want to stay cool inside your home but your air conditioner seems to have the same idea and keeps freezing up. So why does an AC freeze up and what can you do to fix it?
The Science Behind ‘The Freeze’
Keith Hill, manager of technical support at Minnesota Air says that the indoor coil on the AC will freeze up when the refrigerant within the coil has a boiling point (or evaporating point) below 32 degrees. Remember that refrigerant is the substance that actually cools the air that gets pushed into your home. Normal operating range with proper refrigerant charge is 38-45 degrees. If the charge is low, the temperature drops significantly.
“It sounds counterintuitive,” Keith admits. “You would think that low coolant level would cause it to operate at a warmer temperature, but in an AC system, it’s the opposite. A low refrigerant charge causes the pressure in the system to drop, which lowers the evaporating temperature … and then when this boiling point of the refrigerant drops below 32 degrees the coil begins to frost.”
Keith says it’s just like when water has a lower boiling point at a higher altitude or with lower atmospheric pressure.
Here’s a link to a This Old House episode that talks about frozen air conditioners if you wanted to learn even more.
Other Freezing Causes
Besides low refrigerant charge, there are three other reasons that can cause the coil to freeze, says Keith.
- Low Airflow
This can be caused by having a dirty filter, a dirty coil, a blockage in the duct system, or blocked registers and grilles. “Low airflow causes the same effect with low refrigerant pressures since it’s the heat in the air that causes the refrigerant to boil, generating the refrigerant pressure,” says Keith.
- Low Indoor Air Temperature
If the temperature is too low inside, the same thing can happen. Because there won’t be enough heat in the airstream to generate pressure in the coil, it causes a low evaporating pressure. “Usually, this only happens if you turn your thermostat to a low set point – below 68 degrees,” says Keith.
- Operating The AC With A Low Outdoor Temperature
Lastly, and a big no-no, forgetting to turn your AC off when it gets colder outside. Even on a cooler summer evening, remember to turn it off. “When the outside temp is below 60 degrees, there is not enough pressure in the system to function properly, and again the indoor evaporating temperature drops below 32 degrees. Only with specialized accessories should you operate your AC below 60 F,” says Keith.
What Can You Do To Fix It?
The fix depends on which of the reasons above is causing the freeze. Some corrections may be as easy as getting a new filter, but others may need a leak repair or possibly a new unit if the refrigerant charge has been lost. A professional can help you know for sure, but first you need to thaw it out.
“When a freeze-up does occur, turn it off and give it plenty of time to defrost,” says Keith. “It will not defrost while operating, since the airflow is blocked from all of the frost. Once it defrosts and the reason for it is corrected, you can fire it back up and enjoy the cool.”