5 Common Mini Split Problems: Not Cooling, Leaking Water, Not Heating…

Common mini split problems are not cooling, not heating, not dehumidifying leaking water, and having power but not turning on.

Each common mini split issue is explained along with solutions for fixing the problem.

Mini Split Not Cooling

Let’s start simple and then explore more challenging reasons a mini split isn’t cooling.

1). First things first.

Is the “Standby” light on the indoor unit lit? That means it is getting power. Good. If not, check the electrical panel to make sure the circuit for the system is in the On position. Turn the breaker to Off and back to On.

If the circuit is on but the Standby light isn’t lit, the unit’s wiring has a short or disconnect.

Is the system set to AC or Cool mode? If so, turn down the thermostat setting to at least 3 degrees below the indoor temperature.

Is the power on outside? There’s a disconnect box next to the outdoor unit. Make sure it is in the On position too.

Are the remote batteries fresh?

If you’re using the remote to attempt to control the mini split, the batteries should be good. Changing them can do the trick.

OK, just needed to check those potential easy fixes.

2). Next, is your mini split really dirty? Change or clean a dirty filter. Use a shop vacuum to remove caked-on dirt from the outdoor unit (remove the cover and clean the coils, if possible). Do the same for the indoor unit.

3). Is there a sign of a refrigerant leak? Sometimes refrigerant leaks don’t leave any clues. But occasionally you can see where a substance has leaked around the refrigerant line connections at either the indoor or outdoor unit.

A recent or ongoing leak might produce a strange odor too.

Refrigerant leaks require professional service. Only technicians with a refrigerant card can purchase and handle it. Plus, you’ll want to be sure the cause of the leak is repaired and the system is charged with exactly the right amount of refrigerant.

4). Is the compressor starting? The compressor is in the condensing unit outside. Ask someone to turn down the thermostat while you stand by the outdoor unit. You should first hear the system get power, and then the compressor should start with a slight buzz a few seconds later.

If the unit starts but the compressor doesn’t, that might be your issue. A technician can diagnose a bad compressor and give you an estimate on repair or replacement of the unit or system.

5). Is the wiring faulty? Is the issue intermittent? Is the system old?

Over time, connections can loosen. Wiring can fray and short. A mini split not cooling can be the result.

The best solution is to call an HVAC technician to diagnose potential wiring problems. However, if you are experienced using a multimeter, you can test connections on the terminal board. A common setup is:

Terminal 1 and N – Compressor

Terminal 2 and N – Compressor Fan

Terminal 3 and N – 4-way Valve

Check power between each set of terminals. If there’s no power in any of the connections, you have diagnosed a mini split wiring problem.

Depending on your experience and DIY confidence, you can proceed with attempting to repair the wiring problem. But most homeowners will want to call for pro service at this point.

Mini Split Leaking Water

Mini split units remove humidity, and the condensate is drained to the outdoors.

1). Does your indoor unit have a condensate pump? If so, it needs to be checked. If it isn’t working, i.e. it is bad, then it must be replaced.

If it is running, but you still have a leak, you have a clog.

-The pump should be cleaned

-The condensate pan and condensate line should be cleaned out and flushed

2). No pump? Then cleaning and flushing the condensate drain line should remove the blockage that caused the backup and leak.

Tip: If the leak starts immediately after installation of the unit, then the line is either kinked or it runs uphill at some point. Ask your installer to recheck the drain line to ensure poor installation isn’t the issue.

3). What are other causes of mini split leaking?

-Check the drain pan inside the indoor unit, especially if it is old. It might be rusted through, and a leak is inevitable. Replacement is necessary.  

-Check the filter. A dirty filter is the root of all kinds of mini split evil. A filthy filter reduces airflow needed to prevent the indoor coil from freezing. No airflow = a coil frozen over with condensate. When it thaws, it might leak.

Check your filter monthly, and clean it or change it as needed (minimum 3-4 months when the unit is in active use).

Mini Split Has Power But Won’t Turn On

Some easy issues have been covered here.

-Make sure it is set to the mode you want: Cool or Heat.

-Make sure the temperature setting on the thermostat is lower than the air temperature if you want Cool and above the air temp if you want Heat.

The rest of the potential causes of mini split power on but not turning on have to do with more significant issues.

1). Does the unit start – but keeps shutting off?

If this is the case, a couple issues are possible.

-The compressor isn’t coming on. The outdoor unit might start, and the fan might come on, but when the compressor doesn’t start, the unit keeps shutting off, trying again, and shutting off.

-The refrigerant is low. The refrigerant pressure sensor prevents the compressor from starting or continuing to run when there’s not enough refrigerant. This indicates a refrigerant leak.  

Both of these mini split common problems require troubleshooting by a technician with the proper testing equipment.

Mini Split Not Heating

OK, this is similar to the first of the common mini split problems in the list – Not Cooling.

First, is the system in Heat Mode? Does it have power (check the electrical panel circuit and the outdoor shutoff switch)? Is the filter clean? Are the remote batteries fresh?

When those simple solutions don’t work, here are possible reasons for the mini split not heating. They are fully explained above in the Not Cooling section:

Dirt: The system needs a thorough cleaning, as dirty coils will prevent heat transfer.

You can DIY clean your mini split outdoor unit and the indoor unit with the right tools, supplies and techniques.

Refrigerant/freon: The refrigerant is low or gone due to a leak.

Compressor trouble: The compressor won’t start if the refrigerant is low. If there is enough refrigerant and it won’t start, the compressor will need to be serviced (wiring checked too) or replaced.

Bad wiring: The wiring has a disconnect or short – or it was mis-wired at installation or repair.

Mini Split Not Dehumidifying

Mini split systems remove excess humidity when in Cool Mode and in Dry Mode aka Dehumidify Mode.

If your indoor humidity is still high – and the air feels cold and clammy or just clammy – here are common causes of a mini split not dehumidifying.

1). It does a poor job of dehumidifying, and there’s not much you can do about it. You might not like this answer. If the system is new, and it doesn’t remove as much humidity “as the old one,” then maybe it never will. Mini split systems don’t remove as much moisture as standard split systems.

2). The fan speed is too high. Water in moist air moving too quickly through the indoor unit won’t condense as readily onto the coil as moisture in air moving slowly.

You or your installer might be able to adjust the fan speed to improve dehumidification.

3). Is the fan on continuously?Is the system set to On or Fan? You’ll have the same issue. With the fan running all the time, moisture that is condensing on the coil just gets blown back into the room before it can collect and be drained away.

Only run the unit on AC or Dry (Dehumidify) if you want to remove moisture from the air.

4). The indoor unit is dirty. Moisture won’t collect on dirty coils. So, it remains in the air. You or a technician can thoroughly clean the indoor unit following the tutorial linked to above.

5). The system or indoor unit is too big for the space. A unit that cools the space too quickly because it is too big doesn’t have time to remove sufficient moisture from the air. So, if the system turns on but shuts off within a few minutes, a problem called short-cycling, it might be too large.

6). There’s a refrigerant leak. If the system is in Cool mode, but it isn’t cooling or dehumidifying, this could be an issue too.

Have a technician go through this 1-6 checklist to find which of these issues is causing the “mini split not dehumidifying” problem. 

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.
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