Founder | BPI® Energy Auditor Certified Professional
Technical Editor | HVAC Tech at Kearns Heating and Air
Mini-split heat pumps are an efficient, convenient, and highly effective way to heat and cool your home or office.
They have long been a popular HVAC choice worldwide, accounting for more than 75% of air conditioning stock globally. They are becoming more and more popular in residential homes throughout the U.S. due to their eco-friendly technology and their ability to both heat and cool with a single system.
To see if a mini-split is suitable for your home, why not read our article on the pros and cons?
Mini-splits are typically reliable systems, but now and again, they can have issues like any mechanical device. Let’s take a loot at some common problems and what you can expect for repairs.
Why Does My Mini-Split Smell?
There are several reasons your mini-split could be giving off a smell, and the type of odor offers clues about the cause of the problem.
Electrical Issues
If you can smell burning or a fishy smell, you might have some sort of electrical fault on your hands.
Failing Motor
A burning smell could also indicate a fan motor that is on its way out. There are fans located inside the indoor air handler to help distribute warm and cold air from the unit. There is also a fan in the condenser unit outside.
Mold Buildup
By far the most common cause of odor in mini-split heat pumps, the buildup of mold on the coils in the system can give rise to an unpleasant “dirty sock” or musty smell.
This odor is caused by microorganisms growing in your system and can be cleaned with a type of coil cleaner.
Note: This cleaner is highly caustic and should only be handled by trained professionals.
The best way to prevent this is to clean the air filters on your unit regularly. You can reach these filters in the access compartment. They should be removed for cleaning roughly every 6 weeks. The easies way to clean them is to simply rinse them off in the sink.
For more tips on keeping your filters clean and the benefits of doing so, please read our guide here.
Why Does My Mini-Split Blow Cool Air in Heating Mode?
The Outdoor Temperature is Below the Unit’s Operating Range
Only certain models of mini splits are equipped with technology to provide heat in subzero weather. All mini splits will have a temperature range they are manufactured to operate in.
If you choose a system for a mild to moderate climate, it will not be able to adequately heat in cold weather.
It is crucial for homeowners who live in harsh winter climates to spend the extra money and buy a unit rated for cold weather operation or have some sort of backup heating available.
Mechanical Failure
Blowing cold air could also be a sign of a mechanical failure, such as a broken reversing valve, which has effectively caused it to remain in cooling mode.
The Coils Are Clogged
Leaves, grass clipping, cottonwood seeds and other debris can build up on the coils of the outdoor unit. You can clean the condenser yourself with a garden hose.
Use a sprayer and angle the hose down at a 45° angle and wash the debris away until the condenser is clean.
Similarly, snow can build up on the unit during the winter and block the coils.
However, a frozen coil would require a defrost cycle, which brings us to the next reason you might have cool air coming out of your indoor unit while operating in heating mode.
It’s Running in Defrost Mode
During frigid weather, sometimes ice can build up on the outdoor coil, which insulates the coil from the outside air being blown across it, thereby preventing heat transfer to the refrigerant.
This occurrence can cause the heat pump to struggle to provide sufficient heat.
The heat pump operates a defrost mode, either periodically or in response to the detection of ice on the coil to prevent ice buildup.
The defrost mode puts the system into reverse for a few minutes, so it operates in cooling mode and heats the coil on the outdoor unit to melt the ice.
The only difference between defrost and cooling modes is that the fan that normally blows air across the outdoor coil will not be on during a defrost cycle.
After a short time, the ice will have melted from the outdoor coil, and the heat pump will return to heating mode once again, blowing warm air into the building instead of cold.
So, if your heat pump is blowing cold air into the room while in heating mode, check whether the defrost cycle is running.
If it is, wait a few minutes until it finishes defrosting, and it should once again blow warm air into your living space.
Why Does My Mini-Split Drip Water?
Blocked Condensate Drain
The most common cause of water leaking is a blockage in the condensation drain.
Over time, dirt and other debris can get trapped by moisture in your mini-split. Under regular operation, these particles should be carried away with the condensate in the drain line.
However, in certain circumstances, they can accumulate and, without regular cleaning, may cause the drain to become blocked.
Luckily, a straightforward fix for this is worth trying before you incur the cost of bringing in an HVAC professional—you’ll need a wet vacuum cleaner, which many people already have in their home, or can be bought relatively cheaply.
Simply locate the end of the condensate drain outside your home and place the vacuum hose over it to suck out any debris responsible for clogging the drain.
If the drain hose’s diameter differs from the vacuum hose, cup your hands around the joint or use a damp cloth to ensure the best seal possible.
If this doesn’t clear the drain line, you’ll have to call an HVAC technician.
Poor Air Circulation
Dirty filters could cause poor air circulation. A buildup of dust and other airborne particles on the filters reduces their porosity and allows less air to pass through the unit.
Reduced airflow means that the cold air around the coil is not displaced by warmer air entering the unit, resulting in the moisture in the air freezing onto the coil.
When the unit turns off, the ice melts and can overwhelm the condensate drain pan, allowing water to overflow and drip down the wall.
If cleaning the filters doesn’t solve your problem, ice on the coils could also be indicative of a refrigerant leak.
Drain Trap
Most mini splits use gravity to drain water from the air handlers. In some circumstances, condensate pumps are used.
Before you call an HVAC professional, inspect your drain lines and see if the tubing somehow got moved (sometimes this happens when homes are being renovated) and is causing a trap in the drain.
If you find a trap, simply straighten the drain out again and you’ll be good to go.
If your system uses a condensate pump, check that you have power to the pump. If the pump fails, you will have waste water back up in the drain line and leak out of the unit.
For more information about mini-split condensate drains, check out our article that covers the things to bear in mind during installation, including whether you should use a condensate pump.
If your indoor unit is not level on the wall, it could cause a drain issue. However, if the bracket was properly installed, this should not be an issue.
Why Does My Mini-Split Keep Shutting Off?
There are several reasons why your mini-split might keep shutting off.
First, you may have a mechanical issue or a problem with the electronic control systems, but these are difficult to diagnose without professional help.
Other problems are easier to identify. Check these first and then call a professional
Poor Thermostat Location
If your thermostat is too close to the air handler, it will read the air temperature coming straight out of the unit, not the ambient temperature of the room.
When this happens, the thermostat will almost immediately register the temperature at the correct level and tell the unit to switch off.
Once the air dissipates, the thermostat tells the system to start up again, and the cycle continues.
For the majority of ductless mini splits, the remote also functions as the thermostat. Make sure you replace the remote back on the wall mount after you use it. If you leave it too close to the air handler, it won’t receive an accurate reading.
Oversized Unit
Modern mini-splits use inverter-driven compressor technology, which enables them to modulate their output precisely to the requirements of the room they are heating or cooling.
However, if the unit’s capacity is too large, it might not be able to modulate its output to a low enough level. When the heating or cooling load is less than the mini-split’s minimum output, it must turn itself off.
If that happens quickly because the load is much lower than the unit’s output, it will have to turn off frequently, known as “short cycling.”
Short cycling is undesirable because it dramatically reduces the system’s efficiency as the compressor and blowers must spin up every time. It also increases the wear on the components, reducing their lifespan.
If your mini split was installed by a reputable company, this should not be a problem.
However, we see this situation fairly often when homeowners decide to do DIY installs. Installing a bigger unit than you need will create significant problems down the line.
Why Does My Mini-Split Run All the Time?
Mini-splits are designed to run continuously and modulate their output according to the heating or cooling load of the room.
As explained above, running continuously at a low output is much more efficient and prolongs the lifespan of your system.
If your mini-split runs constantly and can’t keep up with your indoor temperature settings, it could point to the unit being undersized or suffering from a mechanical problem.
Final Thoughts on Mini-Split Troubleshooting
Most mini split repairs need to be done by a HVAC professional, but there are a few things you can do yourself.
As a homeowner, the best thing you can do for your mini split system is remember to clean the filters regularly and inspect the outdoor condenser for debris. Beyond that, have your mini split serviced by a professional every 2-3 years.
Most people only call for service when their equipment breaks but you might be able to catch potential problems before they arise with regular maintenance.