Troubleshooting Ride-Along: There’s Water on the Floor Under My Mini Split
SomTroubleshooting Ride-Along: Why Is My Mini Split Leaking Water?
Sometimes the hardest HVAC problems to diagnose are the ones that completely disappear the second the HVAC technician walks through the door.
You know when your car has been making a horrible noise for three weeks?
You finally take it to the mechanic.
He starts the car.
Silence.
“I swear it was doing it.”
We know.
We believe you.
HVAC equipment apparently went to the same school.
Customer Complaint
“There was water on the tile in front of the mini split.”
We installed this mini split in June, so when the customer called us on a Saturday because they found water on the floor, we wanted to get over there and figure out what was happening.
And this is important:
We believe the customer.
People generally don’t call their HVAC company out on a Saturday because everything is wonderful. They saw water where water wasn’t supposed to be, and they had every reason to think their new mini split might be leaking.
By the time we arrived, however, the floor was dry.
The mini split wasn’t dripping.
Nothing was obviously wrong.
Of course.
That doesn’t mean the water wasn’t there.
It means we now have to figure out what happened after the evidence has inconveniently disappeared.
What We Knew Going In
There was another important piece of information with this room.
The apartment directly above it had previously experienced a water leak that went on for some time.
We could also smell dampness around the area behind our mini split, where the refrigerant lines and drain go through the wall.
So we had several possibilities.
Maybe our mini split was leaking.
Maybe the drain wasn’t working correctly.
Maybe water was being blown out of the unit in tiny droplets.
Maybe the cold plastic louvers on the front were sweating.
Maybe water from the previous leak was still finding its way through the building.
Or maybe it was something we hadn’t thought of yet.
The important thing was not to pick our favorite answer and spend the rest of the afternoon trying to prove ourselves right.
So we started testing.
What We Checked First
First, the simple stuff.
Was the mini split level?
Yes.
Was the drain clogged?
No.
Was water actually flowing through the drain?
Yes.
Was the indoor coil frozen?
No.
The coil was evenly cold and wet, and the water it was producing was draining normally.
If you’re new to Florida, here’s a quick AC lesson:
Your air conditioner is supposed to make water.
It doesn’t just cool the air. It also removes moisture from it.
Think about a glass of iced tea sitting outside in Florida. Before long, the outside of the glass is covered in water.
Your AC is doing something similar.
The difference is that an air conditioner has a pan underneath the cold coil and a drain to carry that water away.
And ours was doing exactly that.
Which still left us with one small problem:
Where did the water on the floor come from?
Trying to Make It Happen
At this point, the mini split was doing that incredibly helpful thing HVAC equipment likes to do when we’re trying to diagnose it:
Working perfectly.
But we weren’t going to run it for five minutes, shrug our shoulders and leave.
If the customer saw water, something happened.
So we tried to make it happen again.
We lowered the temperature setting all the way to 67°F and let the mini split run.
The goal wasn’t to turn the room into a meat locker.
We wanted the system to run hard and stay running long enough to produce plenty of water.
If it was going to leak, freeze, sweat or spit water across the room, now would be an excellent time.
The mini split accepted our challenge.
The fan ramped up.
The air coming out got colder.
The indoor coil became thoroughly wet.
The drain started carrying away plenty of water.
And…
Nothing.
The floor stayed dry.
What We Measured
Before we started intentionally pushing the system, the room was approximately 74–75°F with humidity around 53–54%.
Those are perfectly reasonable indoor conditions.
As we continued running the mini split at 67°F, the air coming from the unit got progressively colder.
Eventually we measured:
Air entering the mini split: 71.2°F
Air leaving the mini split: 41.4°F
Temperature difference: 29.8°F
Indoor relative humidity: 56.7%
Yes.
41-degree air.
For a moment we wondered whether we’d accidentally installed a walk-in freezer.
But the number by itself wasn’t what mattered.
The coil wasn’t freezing.
There was no frost.
The coil was evenly wet.
Water was forming normally.
And that water was going down the drain exactly where it belonged.
Then We Broke Out the Really Expensive Diagnostic Equipment
We still had another theory.
What if the mini split wasn’t technically “leaking”?
What if tiny droplets of water were getting picked up inside the unit and blown into the room?
That could potentially leave water on the tile without anyone ever seeing the unit dripping.
So we broke out one of the most sophisticated diagnostic instruments available to the modern HVAC technician:
A piece of tissue paper.
Seriously.
We held it directly in front of the air coming from the mini split.
If tiny water droplets were being blown into the room, they should start showing up on the tissue.
It stayed completely dry.
Thousands of dollars worth of HVAC tools in the truck.
Tissue paper gets the call.
What We Ruled Out
By this point, we’d deliberately made the mini split work pretty hard.
And it gave us a lot of information.
The drain wasn’t clogged.
The indoor unit wasn’t out of level.
The indoor coil wasn’t freezing.
The drain pan wasn’t overflowing.
We couldn’t find water being blown from the unit.
The tissue stayed dry.
The plastic cabinet stayed dry.
The louvers stayed dry.
And the floor stayed dry.
We had even gotten the air coming out of the unit down around 41°F. If those plastic louvers were going to get cold enough to sweat under the conditions we had that day, we’d given them a pretty good opportunity.
They didn’t.
Our mini split was being annoyingly well behaved.
What We Found
Then we found something else.
Directly underneath the mini split is an old return-air grille from the central AC system that used to serve this room.
That system is gone.
The grille isn’t.
Behind that grille is an opening into an area that isn’t air conditioned.
And we could actually smell the dampness coming through it.
Regardless of whether that opening had anything to do with the water on the floor, we don’t want warm, humid air from an unconditioned area freely entering the room.
Especially directly underneath an air conditioner.
There was also still the previous leak from the apartment above to consider, along with the damp smell we noticed around the wall behind the mini split.
Could moisture remaining from that leak have contributed?
Maybe.
Could humid air entering through the old return opening have contributed?
Maybe.
Could the conditions have been different the night before because there were more people in the room and doors were opening and closing?
Absolutely.
But none of those answers had earned the title of “cause.”
Possible isn’t the same as proven.
What We Did
There wasn’t anything on the mini split that needed to be repaired during this visit.
Instead, we kept testing.
Eventually, we raised the temperature setting back to a normal 72°F and left the fan on Auto.
The mini split responded almost immediately.
As the room got closer to the temperature we were asking for, the system backed itself down and the temperature of the air coming from it began rising.
That’s exactly what we want an inverter mini split to do.
We also made a plan to address the old return opening underneath it.
On our next visit, we’ll install insulated ductboard behind the existing grille and seal the opening from the unconditioned area.
The grille can stay right where it is.
Whether that opening caused Saturday’s water or had absolutely nothing to do with it, it shouldn’t be open.
So we’re fixing it.
Before We Left
With the mini split returned to its normal 72°F setting, we measured:
Air entering the unit: 71.6°F
Air leaving the unit: 45.9°F
Indoor relative humidity: 58.7%
The system continued cooling normally.
The drain continued draining normally.
No ice.
No water from the discharge.
No sweating on the louvers.
No water on the cabinet.
No water on the floor.
The mini split had officially refused to cooperate with our investigation.
The Verdict
We could not reproduce the water problem.
But that does not mean we’re saying there wasn’t water.
There was.
We believe the customer.
What we’re saying is that during our visit, we couldn’t make the mini split produce it.
And that’s an important difference.
We ran the system harder and colder than it would normally need to run. It produced plenty of water internally. That water drained normally. Nothing froze. Nothing overflowed. The louvers stayed dry. The tissue stayed dry. The floor stayed dry.
We also found some other conditions around the area worth addressing: a known previous water leak from the apartment above, a noticeable damp smell and an abandoned return opening communicating with an unconditioned space.
Any of those may become relevant.
But we don’t have enough evidence to point at one of them and say:
“That’s where the water came from.”
So we’re not going to.
Sometimes the honest diagnosis is:
We don’t know yet.
Other Reasons Your Mini Split Can Leak Water
This particular mini split wouldn’t leak for us.
That doesn’t mean mini splits don’t leak. They absolutely do.
And here’s why it’s dangerous to assume every water problem is “just a clogged drain.”
We’ve encountered some strange ones.
The drain really is clogged
This is probably one of the first things most technicians will check.
Mini splits remove moisture from the air, that water collects inside the indoor unit, and a small drain carries it away.
Dirt, buildup or other debris can restrict that drain. Eventually the water has nowhere to go.
And gravity begins exploring alternative options.
Usually your wall and floor.
The drain exists…but somebody accidentally plugged it
We’ve actually encountered this one.
On another mini split installation, the installer had used silicone around the connection between the drain tube and the drain pan.
Sounds reasonable.
Except some of the silicone ended up inside the drain opening.
It hardened.
The drain wasn’t completely blocked, but the opening had become restricted enough that water couldn’t leave as quickly as it needed to.
The system had a drain.
The drain tube was connected.
Everything looked pretty normal.
Until you looked closely at the actual opening.
Sometimes the problem isn’t the part.
It’s what somebody did to the part.
The cold refrigerant lines are sweating
We’ve found this one too.
The copper refrigerant lines connected to a mini split get cold. That’s why they’re supposed to be properly insulated.
On another leaking mini split we investigated, sections of the refrigerant lines behind the indoor unit weren’t properly insulated.
Cold copper + humid Florida air = water.
Same iced-tea-glass principle we talked about earlier.
Water formed on the exposed cold lines behind the mini split and eventually made its way down the wall.
To the homeowner, it looked like:
“My mini split is leaking.”
And technically?
There was water coming from behind the mini split.
But the drain wasn’t the problem at all.
The indoor unit isn’t draining properly
Water depends on gravity.
If the indoor unit isn’t installed correctly, or the drain tubing isn’t routed properly, water may not travel where it’s supposed to go.
A drain tube can also become kinked, sagged, disconnected or otherwise restricted.
The water doesn’t care that the installation looked beautiful when everyone packed up and went home.
It still has to get downhill.
The coil freezes and then thaws
Sometimes the water you’re seeing isn’t the original problem.
If an indoor coil gets cold enough to freeze, ice can build up inside the unit.
Eventually that ice melts.
Now you’ve suddenly got considerably more water than the unit normally deals with at one time.
In that situation, the water is a symptom of another problem that needs to be diagnosed.
Cold surfaces can sweat
Florida air contains a lot of moisture.
If part of the mini split gets cold enough while the surrounding air is warm and humid enough, moisture can form on that cold surface.
That’s why we paid particular attention to the plastic cabinet and louvers during this call.
We got the discharge air extremely cold and watched them.
They stayed dry.
Different room conditions, different humidity levels or a different installation could produce a different result.
And sometimes the water isn’t from the mini split
This call is a pretty good reminder of that.
Plumbing leaks happen.
Roofs leak.
Condensation forms on other cold surfaces.
Water travels through walls.
And in multi-story buildings, water has the particularly charming ability to start somewhere upstairs and introduce itself downstairs.
Finding water underneath your mini split gives us a very good reason to investigate the mini split.
It doesn’t automatically prove where the water originated.
Same symptom.
A whole bunch of possible causes.
Which is exactly why:
“My mini split is leaking water”
tells us where to start.
It doesn’t tell us what part to replace.
What Happens Next?
First, we’re going to seal and insulate the abandoned return opening.
Then we’re going to monitor the situation.
If water appears again, we’ve asked the customer to do something extremely helpful:
Don’t wipe it up immediately.
Take pictures first.
Take a picture of the front of the mini split.
Take a picture of the louvers.
Take a picture of the wall underneath and around it.
And take a picture showing exactly where the water first appeared on the floor.
Sometimes catching a problem in the act gives us the one piece of information we couldn’t get during hours of testing.
Just like that car that suddenly remembers how to make the noise approximately three minutes after you leave the mechanic.
What This Means for You
Finding water near your air conditioner doesn’t automatically mean you have a clogged drain.
It doesn’t even automatically mean the water came from the air conditioner.
And as you can see from the examples above, two mini splits can produce what looks like exactly the same problem for completely different reasons.
That’s why we don’t diagnose HVAC systems from symptoms alone.
“There is water on my floor” is a symptom.
It isn’t a diagnosis.
A good HVAC technician shouldn’t arrive already knowing what they’re going to find.
We listen to what you experienced.
We believe you.
Then we start eliminating possibilities until the evidence takes us somewhere.
Sometimes we find the answer in five minutes.
Sometimes our first theory gets completely destroyed by the measurements.
And sometimes we spend part of a Saturday doing everything we can think of to make a perfectly functioning mini split leak…
…and it absolutely refuses.
That’s troubleshooting.
And we’ll take that over guessing every time.