Troubleshooting Ride-Along: Why Is My AC Coil Freezing?
Ice is great in a glass.
It’s useful in a cooler.
It’s even acceptable in Florida for approximately the four minutes per year when we pretend it’s winter.
Inside your air conditioner?
Not so much.
If you see ice forming on the copper refrigerant lines or indoor coil, something isn’t operating correctly.
And one of the biggest misconceptions about a frozen AC is that ice automatically means:
“It’s low on refrigerant.”
It can.
It can also be a dirty filter.
A blower problem.
Bad ductwork.
A thermostat.
A refrigerant leak.
A restriction.
A TXV.
Same ice.
Very different repairs.
Customer Complaint
“My AC isn’t cooling right, and the coil keeps freezing.”
This is a symptom we’ve encountered plenty of times around Flagler and Volusia counties, and we’ve found very different causes.
One particularly good example happened in Flagler Beach.
The indoor coil was freezing.
It would have been very easy to look at the ice and immediately start investigating refrigerant.
Except the refrigerant wasn’t what was causing this one.
The thermostat was.
We’ll get to that.
First, we need to understand why an AC freezes in the first place.
Wait. Isn’t My AC Supposed to Be Cold?
Absolutely.
Just not that cold.
Your indoor evaporator coil operates at a temperature cold enough to cool the air passing across it and remove moisture from your home.
That moisture normally collects on the coil as liquid water.
It falls into the drain pan.
Then it leaves through the condensate drain.
That’s normal.
But if the surface of the evaporator coil falls below freezing, that moisture becomes ice instead.
Once a little ice starts forming, things can get worse quickly.
The ice begins blocking airflow through the coil.
Less airflow means less warm household air reaching the coil.
The coil can get even colder.
More ice forms.
Which blocks more airflow.
Eventually you can have a pretty impressive block of ice where your evaporator coil used to be.
Unfortunately, impressive isn’t the same thing as good.
What We Check First
When we find a frozen evaporator coil, we don’t immediately start adding refrigerant.
And we don’t immediately condemn the TXV.
First, we look at airflow.
Is the filter dirty?
Is the indoor blower running?
Is it moving enough air?
Is the evaporator coil dirty?
Is the return restricted?
Is the ductwork badly undersized or damaged?
Are we actually moving enough warm household air across the coil?
Because airflow is a huge part of this equation.
Why Can an Airflow Problem Freeze My AC?
Your air conditioner is constantly moving heat.
Cold refrigerant travels through the indoor coil.
The blower moves warm household air across that coil.
Heat from the house moves into the refrigerant.
The air gets colder and returns to your rooms.
Everybody wins.
Now imagine the refrigeration system continues operating…
but we dramatically reduce the amount of warm air moving across the coil.
We’re still refrigerating the coil.
But we’re not giving it enough heat.
Its temperature can drop below freezing.
Moisture turns into ice.
That’s why a frozen coil doesn’t automatically mean there’s something wrong with the refrigerant system.
Sometimes the refrigeration system is doing exactly what it’s capable of doing.
The air just isn’t there.
The Flagler Beach Call
This particular call was a great example.
We had a frozen evaporator coil in Flagler Beach.
Naturally, airflow was one of the first things we wanted to investigate.
The indoor fan wasn’t operating when it should have been.
That gives us a pretty obvious suspect:
Blower motor.
Except the blower motor wasn’t bad.
It could run.
The problem was farther upstream.
The thermostat wasn’t reliably turning the indoor fan on while allowing the compressor to continue operating.
That’s a problem.
A pretty icy one.
What Was Actually Happening
During a normal call for cooling, the indoor and outdoor portions of your AC have to work together.
The compressor operates the refrigeration cycle.
At the same time, the indoor blower moves warm household air across the evaporator coil.
On this Flagler Beach system, the compressor could continue operating without the thermostat properly bringing on the indoor fan.
So the refrigeration system kept making the evaporator coil cold.
But there was no blower moving warm household air across it.
Cold coil.
No airflow.
Moisture.
Ice.
And eventually the evaporator froze.
What We Found
The important part was figuring out why the fan wasn’t running.
It would have been easy to replace the blower motor.
After all:
Fan isn’t running.
Must need a fan motor.
Except we tested it.
The blower could operate.
The problem was the thermostat controlling it.
It wasn’t properly bringing on the indoor fan during cooling operation.
Once we identified that control problem, we had an explanation that matched what the system was actually doing.
What We Ruled Out
We didn’t diagnose this system based on the appearance of the ice.
We verified the blower itself could operate.
That ruled out simply condemning the blower motor because we happened to find it off.
We also didn’t assume the frozen coil meant the system needed refrigerant.
And we didn’t blame the TXV because:
Frozen coil = TXV.
The problem was a thermostat/control failure.
That’s a very different repair from opening the refrigeration system.
What We Did
We replaced the faulty thermostat and verified that the cooling equipment and indoor blower were now operating together correctly.
That’s the important part.
The compressor shouldn’t be happily refrigerating the indoor coil while the blower sits there taking the afternoon off.
Once the control problem was corrected, the blower came on properly during cooling operation and warm household air was once again moving across the evaporator coil.
No more accidental ice maker.
The Verdict
The frozen coil at this Flagler Beach home wasn’t caused by low refrigerant.
It wasn’t a failed blower motor.
And it wasn’t a bad TXV.
It was the thermostat.
The thermostat was allowing the compressor to operate without properly bringing on the indoor blower.
With no airflow across the evaporator, the coil became cold enough to freeze the moisture collecting on it.
Same symptom:
“My AC coil is freezing.”
Completely different diagnosis.
And that’s particularly important with this symptom because there are a LOT of things that can cause it.
Other Reasons Your AC Coil Can Freeze
This is one of those problems where the ice tells us something is wrong.
It just doesn’t tell us what.
Your air filter is extremely dirty
Let’s start with the easy one.
Your AC needs air.
If the filter becomes severely restricted, the blower can’t move enough warm household air across the evaporator coil.
The coil can become too cold.
Then the moisture on it begins freezing.
Does every slightly dirty filter freeze an air conditioner?
No.
But when we find a frozen coil, checking the filter takes approximately three seconds.
We’re going to do that before discussing a major refrigeration repair.
The evaporator coil may be dirty
Air has to pass through the coil itself too.
Over time, dirt and debris can accumulate on the coil and restrict airflow through it.
So you can have a perfectly reasonable-looking filter today while the coil behind it has years of buildup.
Again:
Less airflow.
Colder coil.
Potential ice.
The blower motor may actually be bad
Unlike our Flagler Beach call, sometimes the blower really is the problem.
The compressor and outdoor equipment may continue operating while the indoor blower isn’t moving air.
Without warm air crossing the evaporator, the coil can freeze quickly.
The motor itself may have failed.
A capacitor may have failed on equipment using one.
An ECM motor or module can have a problem.
Or something else in the electrical circuit may be preventing the blower from operating.
That’s why:
“The blower isn’t running”
is still a symptom.
We have to figure out why.
The blower may run but not move enough air
This one’s less obvious.
You hear the fan.
You feel air coming from the vents.
So the blower must be fine.
Maybe.
A blower can operate at the wrong speed.
A blower wheel can be dirty.
The system can have severe static-pressure or airflow problems.
There can be enough air movement for the homeowner to feel something at the register while still not having the airflow the equipment actually needs.
“Fan is running” and “airflow is correct” aren’t the same diagnosis.
The ductwork may be restricting the system
We’ve seen this plenty of times.
Undersized returns.
Undersized supply ducts.
Crushed flex duct.
Collapsed duct.
Badly routed flex.
Restricted filters.
Poorly designed plenums.
The air conditioner itself can be perfectly capable of moving the required airflow and still be connected to a duct system that won’t allow it.
We’ve inspected five-ton systems connected to ductwork that was clearly restricting what the equipment could move.
Replacing a TXV won’t make an undersized return bigger.
Neither will adding refrigerant.
Sometimes the AC isn’t the problem.
The road it’s trying to move the air through is.
The return-air system may be too restrictive
Your AC doesn’t just need somewhere to send conditioned air.
It needs enough air coming back.
If the return system is severely undersized or restricted, the blower can’t get the airflow it needs.
That affects what happens across the evaporator coil.
This is why airflow diagnosis includes both sides of the duct system.
Supply and return.
The thermostat or controls may shut the blower off
This is what we actually found in Flagler Beach.
And it’s a great reminder not to condemn the component that isn’t operating until we know why it isn’t operating.
The blower motor wasn’t being properly commanded on.
The thermostat was the problem.
A control problem elsewhere in the system can potentially create similar symptoms.
The important question isn’t:
“Is the fan off?”
It’s:
“Why is the fan off while the compressor is still running?”
The system may be low on refrigerant
And now we get to the one everybody knows.
Yes.
A refrigerant problem can cause a coil to freeze.
But there’s something really important homeowners should understand:
Your AC does not normally use up refrigerant.
It isn’t gasoline.
If the refrigerant charge is actually low, there is usually a reason.
Often, that reason is a leak.
We’ve dealt with exactly that on another Ride-Along.
At a seven-story beachfront building, the system was losing refrigerant.
Instead of guessing where the leak was, we isolated the outdoor unit, indoor equipment and refrigerant lines from one another and pressure-tested them.
The equipment held.
The refrigerant line running through the building didn’t.
That system didn’t need somebody to keep “topping it off.”
It needed a leak repaired.
The TXV may be the problem
And yes.
Sometimes it really is the TXV.
TXV stands for thermostatic expansion valve.
You absolutely do not need to remember that.
Think of it as a traffic cop for refrigerant entering the indoor coil.
It controls how much refrigerant is fed into the evaporator based on operating conditions.
If it isn’t feeding refrigerant correctly, the refrigeration system can behave abnormally and coil temperatures can be affected.
But here’s the important part:
Ice is not a TXV diagnosis.
How Do We Know If It’s Actually the TXV?
We measure.
This is where refrigeration diagnostics become important.
We look at system pressures.
We measure temperatures.
We calculate and evaluate superheat and subcooling.
We verify airflow first because bad airflow can affect the refrigeration readings we’re looking at.
We consider the refrigerant charge.
We look for evidence of restrictions.
And we evaluate whether the metering device is actually feeding the evaporator correctly.
Only then do we start becoming comfortable saying:
“The TXV is the problem.”
Because replacing one is not a particularly fun way to discover that the actual problem was a dirty blower wheel.
Another refrigerant restriction may be present
Not every refrigerant-flow problem is the TXV.
Not every system even uses one.
Some equipment uses a fixed metering device such as a piston.
Restrictions elsewhere in the refrigerant circuit can also affect how refrigerant moves through the system.
Again, the homeowner doesn’t need to diagnose any of this.
That’s why you called us.
“My AC is frozen” is plenty.
Why Does My AC Start Leaking Water After It Thaws?
Remember what all that ice is made of.
Water.
The same moisture your AC normally removes from the air and sends down the condensate drain has frozen onto the evaporator instead.
Eventually, the coil warms up.
And all that ice becomes water again.
Hopefully, the water falls into the primary drain pan and leaves normally.
But a heavily frozen coil can create a lot of water as it thaws.
Sometimes it overwhelms the pan.
Sometimes ice has formed somewhere water wasn’t designed to be.
And suddenly:
“My AC froze yesterday”
becomes:
“Why is there water around my AC today?”
Those may not be two separate problems.
They’re often two chapters of the same story.
What Should I Do If My AC Is Frozen?
If you see ice on the refrigerant lines or indoor coil, continuing to operate the system in cooling mode isn’t going to solve the underlying problem.
Turn cooling off.
If the indoor blower is operating normally, running the fan can help move warmer household air across the coil and speed up thawing.
And take pictures.
Seriously.
Take pictures before everything melts.
Show us where the ice was.
Was it only on the copper line?
Was the entire coil frozen?
Was there very little air coming from the vents?
Was the indoor fan running?
Those clues can be extremely useful.
Because by the time we arrive, your giant block of evidence may have turned into a puddle.
What you shouldn’t do is start attacking the ice with tools or trying to physically remove it from the coil.
The coil is considerably more expensive than the ice.
Let it thaw.
Then figure out why it froze.
Don’t Just Add Refrigerant
This is probably the biggest takeaway from this article.
If someone sees a frozen coil and immediately tells you:
“You’re low on Freon.”
there should be some testing between those two statements.
Maybe the system is low on refrigerant.
But our Flagler Beach system wasn’t.
It had a thermostat problem.
Another system could have a failed blower motor.
Another could have a severely dirty filter.
Another could have terrible ductwork.
Another could have a refrigerant leak.
Another really could have a bad TXV.
And the evaporator coil can look remarkably similar after all of them:
Covered in ice.
That’s why troubleshooting matters.
What This Means for You
If your AC coil is freezing, you don’t need to know what a TXV is.
You don’t need refrigerant gauges.
You don’t need to diagnose static pressure.
And you definitely don’t need to decide which expensive component should be replaced before somebody tests the system.
You already gave us the useful information:
“My AC is freezing.”
From there, we work through the possibilities.
We’ve diagnosed frozen coils caused by airflow problems, refrigeration problems and even a thermostat in Flagler Beach that allowed the compressor to operate without properly turning on the indoor blower.
The ice looked the same.
The repairs weren’t remotely the same.
So yes:
Ice tells us there’s a problem.
It just doesn’t tell us which one.
Because the symptom isn’t the diagnosis.