It is not unusual for an air conditioner to freeze up during summer. A frozen AC unit is inconvenient since it does not cool properly and it can also cause water to leak inside the house.
Air conditioners freeze when the temperature of the refrigerant inside the evaporator coil drops below freezing point. That causes the condensate/moisture on the surface of the coil to ice up, turning the entire coil into a block of ice.
The temperature of the refrigerant drops below freezing point if enough warm air is not getting to the evaporator coil (restricted air flow) or if the level of refrigerant in the system is too low.
If the problem is not fixed immediately, freezing continues towards the outside AC pipe. Eventually the liquid refrigerant will flow back inside the compressor and damage it. The compressor should only handle the refrigerant in its gas state.
So, how can you prevent your air conditioner from freezing? The following are the different ways of preventing a frozen AC unit:
- Change the air filter regularly
- Inspect air ducts for clogs and leaks
- Check the refrigerant level
- Clean the AC coils regularly
- Keep the AC drain line clean
- Make sure the blower/fan is working properly
- Keep your vents open
- Monitor night temperatures
- Schedule regular maintenance by an AC professional
Air conditioners can sometimes freeze only during the night. That is because night temperatures can fall drastically even during the summer, which makes the refrigerant temperature also to drop and hence a frozen AC unit.
What Causes Air Conditioners to Ice Up?
Air conditioners have 2 functions. The first and primary function is cooling the indoor air and the second one is dehumidification (remove humidity from the indoor air). Both cooling and dehumidification happen inside the house on the surface of the evaporator coil.
Air conditioners have an inside and outside unit. The refrigerant/coolant/Freon absorbs heat from the indoor air inside the evaporator coil and releases it to the outdoors inside the outside unit. It is also able to turn from gas to liquid and back to gas easily.
The refrigerant enters the evaporator coil as a very cold liquid (32 degrees, which is the freezing point of water). At the same time, the evaporator fan/blower will be pulling the hot and humid air from the house through the supply air ducts.
The warm air is forced across the evaporator coil where the refrigerant absorbs heat from it and that is how cooling happens. The cooled air is then sent back to the living spaces through the supply air ducts.
When the moisture/humidity in the air comes into contact with the cold evaporator coil, condensation happens, similar to the condensation that happens on windows.
The vapor turns into moisture and it drips on the condensate drain pan below the evaporator coil. From there it is removed to the outside through the condensate drain line.
Now, when the temperature of the refrigerant drops below freezing point, the condensation does not drip off the evaporator coil. Instead, it ices over the coil and that is how air conditioner evaporator coils freeze.
How to Keep Your Air Conditioner from Freezing Up
There are certain things you can do to prevent your air conditioner from freezing. Let us look at each one of them in more details.
1. Inspect the Ductwork for Leaks and Clogs
If you have a central air conditioner, you need ductwork to bring warm air to the evaporator coil and also carry cooled air from the coil back to the living spaces.
For an air conditioner to work properly, the ductwork has to work seamlessly. That however does not always happen.
Air ducts leak a lot. If the return air duct (supplying warm air to the evaporator coil) is leaking, less warm air will get to the evaporator coil. That will cause the refrigerant to keep expanding, pressure dropping and therefore temperature will fall below freezing point resulting in a frozen AC coil.
That is also the same thing that will happen if the duct is clogged. And that is why it is important to make sure that the ductwork is not leaking and that it is not clogged.
Not many homeowners have the expertise to inspect their AC ductwork. This job should be done by a trained HVAC technician.
2. Make Sure the Air Vents are Open
Again, central air conditioners will have supply air vents and return air vents. These are the grills/grates on the ceiling or wall where warm air is pulled out of the house and supplied to the house.
The return air vents are connected to the return air ducts while the supply air vents are connected to the supply air ducts. These vents should remain open.
Some folks try to close the vents in a bid to save money by turning off cold air supply to areas where it is not needed. However, what that does is to restrict air flow in the system and may cause the AC to freeze up.
Also, make sure that the vents are not obstructed by furniture and other objects in the house. The AC needs to effortlessly pull air from the house and also deliver it.
3. Regularly Change the Air Filters
Air conditioners have an air filter usually installed in the return air ducts to remove dust, pollen, dander, lint, far, hair and other impurities from the air. That ensures that the air being supplied to the house is not only cool but clean as well.
Ideally, air conditioner filters should be replaced after every 3 months. You can even replace yours sooner than that if you have lots of people in the house, pets and if your house attracts dirt from outside.
Failure to change the AC filter restricts the flow of warm air to the evaporator coil. That causes the refrigerant temperature to keep on dropping inside the evaporator coil and once it falls below freezing point the coil will start to freeze.
Luckily, an air conditioner (sometimes called a furnace filter) is easy to change. I have written an easy guide on how to do that in this post.
You however need to be careful and make sure that you buy the correct size of air filter. To know what size your AC filter is, check out this post.
4. Ensure the Fan is Working Properly
The evaporator blower/fan is responsible for pulling warm air from the house and delivering it back after cooling. When the fan fails to work as intended, the evaporator coil will surely freeze.
While it is hard for the average homeowner to inspect a blower fan, you can ask an HVAC technician to have a look at it during their regular/routine checks.
Apart from the fan itself, the technician will also inspect the motor and ensures that it is powered sufficiently.
5. Clean the Condensate Drain Line
The condensate drain line or AC drain line as it is commonly known is the pipe which drains the condensate/water from the AC drip pan to outside of the house.
Due to the humid nature inside the pipe, it is a great breeding ground for algae which rapidly multiply and may very easily clog the drain line. A clogged drain line can also cause the evaporator coil to freeze.
Cleaning the AC drain line is however a simple task that will also prevent your air conditioner from leaking water. Here is how to do it:
- Turn off the AC to avoid risk of electrical shock.
- Remove the cap on the vent tee (usually a white PVC piece near the indoor AC unit).
- Pour 1 cup of distilled white vinegar down the drain line
- Wait for 30 minutes then flush the line with water.
- Turn the AC back on.
The vinegar will kill the algae and prevent them from multiplying and clogging the line. Alternatively, you can connect a shop vac to the outside end of the drain line and suck out any gunk that maybe inside it.
6. Check Your Refrigerant Levels
Air conditioning is a close-looped system. That means that the refrigerant level should not drop unless there is a leak somewhere.
When the levels of the refrigerant inside the AC system drops, its pressure reduces as well. A reduction in refrigerant pressure translates in a drop in temperature. And when the temperature of the refrigerant drops below 32 degrees, the evaporator coil will freeze.
Checking the refrigerant level in an air conditioner and fixing leaks needs to be done by a licensed technician. After fixing the leak, they will recharge the system with enough of the refrigerant.
You however don’t need to wait until the coil is frozen for you to call a technician. You can have them do it during their routine checks to prevent the unit from freezing in the first place.
7. Monitor Night Temperatures
Does you air conditioner freeze at night? It is not unusual for summer night temperatures to fall below the set AC working temperature range.
When that happens, the pressure of the refrigerant will drop, causing the temperature to also drop below freezing point. Needless to say, the evaporator coil will freeze.
Keeping an eye on the weather forecast can help you determine if the night temperatures will be cool or hot. If the night temperatures will be cool, turning off the AC will not only prevent it from freezing but it will also save you some money.
8. Clean Evaporator Coil
A dirty evaporator coil fins will also restrict the flow of warm air to the actual coil. That is usually another reason why your coil is constantly freezing.
Cleaning the coil allows the air to flow freely to the coil for maximum heat exchange and dehumidification. You can clean the evaporator coil yourself or you can have an HVAC technician do it for you.
9. Schedule Regular Maintenance
It is important to have an HVAC technician come over and maintain your HVAC system regularly. Technician can think of and troubleshoot problems that homeowners are not aware of.
A properly maintained AC unit is less likely to freeze than one that is hardly maintained. I would recommend having an HVAC technician come over once a year to maintain the HVAC system in your house.
And basically that is how to keep you air conditioner from freezing. I hope that this guide was helpful.