a graphic of a web of mini split air conditioners with a man and woman standing in front of a big question mark

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.

In the US, central air systems have traditionally been favored, which blow conditioned air around the building using ductwork. However, these systems are less efficient than mini-splits due mainly to the leaks, constrictions, and insulation issues common with ducts.

Mini-splits also make use of highly efficient inverter-driven compressor technology that allows them to precisely match their heating and cooling output to the requirements of the building, lowering their running costs and further improving their appeal.

Americans are keen to take advantage of these more efficient systems, and mini-splits are increasing their market share in the states, becoming the HVAC system of choice in new homes and retrofit projects alike. According to data published by Google Trends, the number of searches in the US for “mini-split” has shown a marked rise since 2015.

To see if a mini-split is suitable for your home, why not read our article on the pros and cons?

Mini-splits are very reliable, but now and again, they can have issues like any mechanical device. 

This article will address some common problems that mini-splits can suffer and what you can do about them. So, read on to find out more.

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.

A Refrigerant Leak

A common misconception is that smells can be indicative of a refrigerant leak.

Refrigerant R22, which the EPA has banned in the US, does have a slight aromatic odor. However, this refrigerant is only used in older units and is being replaced by other, odorless refrigerants such as R410A (also soon to be phased out) or R32.

In any case, refrigerants used in mini-splits would need to be present in very high concentrations for their presence to be detectable by smell. If that were the case, the leak would have to be so severe that all the refrigerant gas would be gone from your unit, and it would cease to operate.

Small refrigerant leaks cannot be detected by smell and would instead need to be found by specialized monitoring equipment or bubble solution.

Electrical Issues

If you can smell burning or a fishy smell, you might have some sort of electrical fault on your hands. For example, it could result from damaged wiring or a motor or other component overheating.

If that’s the case, it’s time to call a professional to investigate the problem.

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 specialized cleaning solutions.

The best way to prevent this is to clean the air filters on your unit regularly. In addition, you should periodically clean the coils of your air handler units, which is best done by a professional as part of an annual service.

A technician adjusts the indoor component of a mini-split

The fins on the coils are delicate and could quickly get damaged or bent during cleaning. Acidic cleaning products can also cause damage to the metallic alloys used in the coil.

An HVAC technician will ensure that your mini-split is cleaned per the manufacturer’s instructions, which is very important.

They will typically also use specialized equipment such as a purpose-made fin cleaning brush and a compressed air tool. They will often partially dismantle your unit to get right into where the problem is and clean it out thoroughly.

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?

There are a few reasons your mini-split could blow cold air when in heating mode.

Mini-Splits Operate at Lower Temperatures Than Furnaces

The first thing to consider is whether your expectations are too high. Heat pumps operate at lower temperatures than traditional fossil-fuel-powered furnaces, which typically blast hot air through your home’s vents at temperatures of over 110°F.

By contrast, a heat pump might blow air at 90°F to heat your living spaces. This temperature is more than adequate to do the job but might not give you that satisfying hot blast when you place your hand next to the vent. 

So, your impression that your heat pump is not blowing hot air might just result from you expecting it to be hotter than it needs to be to do its job.

Slightly less warm air is normal and just how heat pumps are designed. So it does not necessarily mean there is a problem.

The true test is whether your home is at a comfortable temperature overall, not the temperature of the air being blown into the room.

A couple of homeowners enjoying each other's company on the couch with a mini-split air handler prominently shown on the wall behind them

Mechanical Failure

If your mini-split is blowing cold air while in heating mode, it may have suffered a mechanical failure, such as a broken reversing valve, which has effectively caused it to remain in cooling mode.

The Coils Are Clogged

If you are not getting sufficient heating effect from your mini-split, it could be due to a buildup of leaves or other debris on or around the outdoor unit.

Another cause can arise during winter when the outdoor unit can freeze, or snow can prevent the proper circulation of air around it.

Cleaning debris from around your unit or digging away any snow can fix problems caused by improper air circulation.

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.

A mini-split compressor unit outside a home in wintery conditions

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.

Problem solved!

Why Does My Mini-Split Drip Water?

Sometimes, the indoor air handler units can leak, allowing water to drip down your interior wall. This creates a mess and can cause further problems such as mold and even structural issues if left unrepaired.

Blocked Condensate Drain

The most common cause of this problem is a blockage in the condensation drain.

Over time, dust 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.

A ductless mini-split condensate drain at the bottom of line near the compressor
Courtesy of Quality Heating and Sheet Metal Company, Inc.

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 deal possible.

Allow the vacuum to run for a minute or so—you may be surprised by the amount of material it pulls out. Once the blockage has been cleared, you should have no more leaks.

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.

Closeup on a mini-split compressor with a homeowner cleaning it by spraying water into the grille.

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.

Check Your Levels

The condensate drain usually drains the water away under gravity. Make sure it has a consistent fall along its length; otherwise, you’ll get water stuck in the pipe, which will cause blockages as debris can’t drain away.

A sagging condensate line can allow water to sit in the low points, where debris can accumulate and cause blockages.

If insufficient care has been taken during installation, the drain immediately behind the condensate pan might slope upwards slightly, preventing condensate water from draining away properly.

In some situations, it is impossible to angle the drain consistently downwards to allow the condensate to drain under gravity. When this is the case, a condensate pump is required to pump condensate up to a point from which it can drain under gravity.

If you have a condensate pump that stops working, this is another potential cause of leaks, which is why it’s better to rely on gravity wherever possible.

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.

Is your indoor unit level in all planes? This is a requirement for most models’ installation and could cause leaks if it is out of plumb.

A damaged condensate pan could also allow water to escape, which, although rare, is something to watch out for.

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. Let’s look at a couple of these now.

Poor Thermostat Location

If your thermostat is too close to your mini-split’s air outlet, it will receive a blast of conditioned air directly from the unit.

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.

In many mini-splits, the thermostat is in the unit itself, so it’s imperative to ensure sufficient space for air to circulate around the system and avoid “bounce back” from nearby walls. Unfortunately, this effect can make the unit prematurely think the room has been sufficiently cooled.

Always follow the manufacturer’s instructions to ensure you have at least the minimum distance between the unit and the nearest wall, which is typically around 15 feet.

This issue is demonstrated in the video below:

Screenshot from a YouTube video with an expert demonstration what mini-split bounce back is
Courtesy of Taddy Digest

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.

Why Does My Mini-Split Run All the Time?

Mini-splits are designed to run constantly 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 flat out all the time, this is likely less efficient and could point to the unit being undersized or suffering from one of the other problems in this troubleshooting guide.

However, this is normal and desirable if the unit can run efficiently and adjust its output to provide the right amount of heating and cooling by running at lower speeds.

Final Thoughts on Mini-Split Troubleshooting

Mini-split heat pumps are very reliable and should give you many years of trouble-free service. However, occasionally they do go wrong.

Most brands offer five, seven, or even 12-year warranties, which provide peace of mind and protect you from a large, unexpected bill should the worst happen.

You can fix some problems yourself with common sense and tools most people have access to, for example, vacuuming a clogged condensate drain. However, some issues can be due to poor design or installation, such as an oversized system.

Some problems are not problems at all. For example, when your system runs at its most efficient capacity 24/7, it is designed to do this.

If you are unsure how to fix your system, or if the fault is electrical or refrigerant-related, always call a professional who is adequately trained and will fix the issue safely.

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