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No one wants to shell out hundreds of bucks for an AC repair, especially when it's something minor. One of the most common reasons I get called is a frozen AC.
I've compiled this list of fixes so anyone can quickly troubleshoot and repair their air conditioning system, and I have noted when to call a professional.
Solutions I use to fix an AC when it freezes
ACLAB Note:
When it comes to unfreezing your AC, let the coil defrost naturally. Do not use a screwdriver, pick or any other pointed instrument to remove ice. If you pierce the coil the only repair is to replace it.
Before getting started, I note where ice is building up and whether or not air is blowing from the return vents.
It is common to find ice forming on the refrigerant lines both inside and out. Now, let's get started with common solutions!
Dirty filter and poor air flow
The first thing I always check for when an air conditioner freezes up is a dirty air filter.
A dirty air filter leads to poor airflow over the evaporator coil, which causes frozen coils.
Solutions
Quick Tip:
Turn on the heat for a few minutes to quickly melt the ice on the coil.
Too many vents closed or blocked (for central ACs)
Homeowners often believe that closing vents in unoccupied rooms will help cool the rest of their house or even save energy.
Unfortunately, that is not how a central AC works, and closed vents can lead to significant problems like AC freezing.
Ductwork and vents are sized based on the tonnage of a central air conditioner. Closing supply vents in a spare bedroom limits the total air flow through the evaporator coil.
Blocked vents mean inadequate airflow, eventually leading to a damaged blower motor and a frozen AC unit.
Solutions
Related article: How to extend the life of your central AC
Blocked/collapsed duct
Similar to closed vents, another common problem is blocked air ducts. Most houses use flex ducts, which are easy to install but are easily damaged.
Boxes of old toys and Christmas decorations in the attic can easily fall and “squish” flex ducts. Similarly, a suspended flex duct may eventually get pinched or crease.
Solutions
ACLAB Note:
Many HVAC companies offer duct cleaning services that tout improved airflow. However, I would not waste money on these services unless you have respiratory problems, dust sensitivity or you smell something from the ductwork. Otherwise, these services offer minimal benefit, and removal of the dirt will not noticeably improve airflow.
Improperly sized ductwork (for central ACs)
Improperly sized ductwork is becoming a major problem. Older systems could often operate with undersized ductwork without a problem, but newer systems are not as forgiving, and this could lead to AC freezing.
Just like dirty air filters and blocked ducts cause insufficient airflow, so do too-small ducts. An AC system manufacturer dictates proper airflow.
Airflow in an HVAC system is very similar to blood pressure in a person. When the pressure gets too high, the air conditioner has to work hard to keep up.
Just like the heart in a person, an AC will give out if pushed to its limits.
Solutions
ACLAB Note:
If you are installing a larger system then your supply and return ductwork may need to be enlarged, reinforced or more ductwork may need to be added. Larger fans create more suction/discharge and static pressure that can blow your ductwork apart. An HVAC professional should calculate this into the price of the new system.
Dirty evaporator coil
Just because an air filter is clean doesn't mean the evaporator coil is debris-free.
Homeowners often change their air filter before the AC repair person arrives, but that doesn't change the fact they haven't replaced the filter for the last six months.
Like a dirty filter, dirt in the evaporator coils also restricts airflow. I try to inspect coils whenever possible and clean them as needed.
Still, many HVAC technicians don't even bother to check them as part of their annual maintenance.
Once clogged up, the dirt turns into a sticky mess once it mixes with condensate, which eventually causes a clogged condensate drain line.
Solutions
Running AC when it is cold outside
Air conditioners cannot run below their designed operating ranges, which the manufacturer determines.
I advise not running an air conditioner when the outdoor temperature is below 60 degrees Fahrenheit, but some units may work in temperatures as low as 45 degrees Fahrenheit.
The refrigerant inside an AC unit undergoes complex chemical changes, and its properties change depending on the ambient outdoor temperature.
When it gets too cold, the pressures drop, and the saturation temperature goes below freezing.
Solutions
Bad blower fan
While trying to rule out airflow problems, it is also a good idea to ensure the blower motor is running.
The blower motor is located inside the air handler, next to the evaporator coils. It is responsible for moving air throughout the house.
Electric motors wear out over time, so it's common to find dead or malfunctioning blower motors on older systems.
With that in mind, newer variable speed fans are also dying much sooner than those from old systems. Power surges and lightning strikes can prematurely lead to a damaged blower fan outside.
Once the fan goes out, the air stops moving over the coils, and they begin freezing up.
Solutions
Related article: Air conditioner fan not spinning or turning on (causes)
When to call HVAC professionals for repairs
It’s always great to save a few bucks and avoid the headaches of dealing with an HVAC repair person, but some jobs require a professional.
These are the repairs that homeowners should put their toolbelts down for and call in reinforcements.
An oversized AC
Proper AC sizing is just as important as proper duct sizing. Many homeowners choose to upsize their units, buying equipment that is too large for their homes.
In rare situations, a severely oversized air conditioner will freeze up due to short cycling. Moisture in the air builds up quickly and accumulates on the coils, where it freezes.
An HVAC professional can perform a Manual J load calculation to determine the home's heat load. The result of the calculation is the correct air-conditioning tonnage.
From there, it is possible to get a slightly larger unit (about a half-ton larger) to ensure the house stays cool on the hottest days.
I've begun recommending variable-speed air conditioning systems to homeowners who want oversized units.
These systems ramp up and down depending on demand, which is excellent for energy efficiency and comfort.
Malfunctioning equipment
HVAC systems have many parts that can fail. An experienced HVAC professional can easily diagnose most problems.
A TXV valve can unnecessarily restrict flow in the refrigerant lines and cause the condenser coils to freeze.
Similarly, a stuck reversing valve may prevent refrigerant from flowing and starve the evaporator.
Homeowners cannot diagnose these issues as they require training, gauges, and advanced HVAC knowledge.
Related article: Guide to fixing AC not turning on
Low refrigerant
Refrigerant is the main ingredient in an air conditioning system. Unlike the oil in a car, it should never need replacing or topping off.
An air conditioner is a closed system that refrigerant should remain in for its entire life.
However, joints break, and refrigerant can leak out. At low refrigerant levels, the coils begin freezing.
Unfortunately, a homeowner cannot do anything because an HVAC technician must diagnose, repair, and recharge a leaking system.
A normal low-pressure reading for an R410A condenser unit is around 125 PSI, and the high should be in the 300 PSI range.
Bear in mind that these are rough estimates and that are based on a 70-degree Fahrenheit ambient temperature.
There should be a pressure chart on the access panel of the outdoor unit with exact specifications.
It is also important to check superheat and subcooling levels on modern systems and adjust freon based on all readings.
