Mini-Split Heat Pump FAQ: Your Questions Answered

Do you have questions about mini-split heat pumps?

If so, you’ve come to the right place. This article is jam-packed with helpful information for anyone looking to learn more about these heating systems.

Perhaps you’re wondering where would be the best place to install your new system or whether you can mix and match an air handler you found on eBay with your existing outdoor unit.

Whatever your question, chances are you’ll find an answer either here in this article or in our archive of mini-split articles.

So, without further ado, let’s get onto the first question.

Can a Generator Run a Mini-Split?

Mini-splits are a very convenient way of heating and cooling your home, but what can you do if the power goes off?

The obvious thing to do is hook up your mini-split to a generator, but you must be careful if you’re considering doing this because you risk damaging your heat pump if you use the wrong type of generator.

Many generators produce “dirty power,” a generalized term describing electricity that exhibits characteristics such as frequency variations, voltage fluctuations, and surges.

If you are running a standard lighting circuit, dirty power will cause lights to flicker, and it’s no big deal. However, dirty power can damage a mini-split, which contains sensitive electronics that don’t fare well when voltages spike. That’s why it’s essential to use the correct type of generator.

If your generator doesn’t produce clean power, it might not immediately fry your mini-split’s inverter board, but it increases the risk of failure a few months down the line or the following season.

The right type of generator to use is inverter style, which produces much cleaner power.

Your options range from a large generator that kicks in seamlessly when the power goes off so that you don’t even notice, to models where you need to go out to the shed and connect things up. Prices and lifestyle requirements vary, and you need to choose a solution that’s right for you.

Can You Install a Mini-Split Under a Window?

There are many options for installing mini-splits, which offer great flexibility when choosing the location for your equipment.

Mini-splits comprise an outdoor unit, which houses the compressor, and an indoor component called the “air handler” or “head unit” that blows conditioned air into the room.

The two units are connected by copper refrigerant lines and electrical cables, normally bundled together with the condensate drain. These “line sets” are often enclosed in conduits to protect them and make them look tidier.

The compressor can be installed on the ground outside or the wall. It makes sense to mount it on the window ledge in some circumstances, such as in an apartment building.

Mounting on the window ledge obscures the view through the window, but the sacrifice can be worth it to keep an apartment cool.

One thing to bear in mind if mounting the outdoor unit next to a window is that you might be able to hear it running.

Sound will generally not be an issue outside living areas, particularly in urban settings with a lot of background noise. Still, the noise outside a bedroom window can disturb some people’s sleep.

Weighed against the difficulty in sleeping caused by sky-high temperatures, this is probably a good trade-off. Also, modern units are relatively quiet, so you need to be especially susceptible to noise for this to be a concern.

Indoor units come in several options and can be installed high up on the wall, recessed into the ceiling with a cassette unit, or mounted on the floor.

There is a common misconception that you must mount the indoor unit on an external wall, on the other side of the wall to the outdoor unit—this is a myth.

While it makes the installation process more manageable, it is not essential; you just need to run the line set a bit further to connect the two units.

Can a Mini-Split Be Mounted Vertically?

Mini-splits are designed to be installed horizontally and must not be installed vertically.

When operating in cooling mode, water condenses onto the evaporator coil from the warm, humid air in the room. Condensation drips down from the coil into a condensate pan, then passes into a condensate drain and out of the building.

Because indoor units are designed to be installed horizontally, if it installed vertically, the condensate will drip from the evaporator coil with nothing to catch it. As a result, the water will run down your wall causing damage to your house.

Even if there is not much condensate, there will likely be enough to cause dampness, which could lead to mold growth, which is bad for your family’s health.

Can a Mini-Split Be Plugged into an Outlet?

Most heat pumps run on a 220-volt supply, although some smaller models are available that run on 110 volts. Standard household electrical outlets in the US provide 110V, which would seem just right for one of the more modest systems that only require 110V.

But not so fast! This would be a code violation and, therefore, not something you should do.

The National Electrical Code (NEC) is the reference standard that homeowners must follow across all 50 states. It’s important to follow its requirements for safe design, installation, and inspection.

Failure to comply with the NEC will not only get you in hot water with the inspector but could mean your system is unsafe or otherwise not suitable for use.

One of the requirements of NEC is that all significant fastened-in-place electrical appliances, including mini-splits, must be on their own dedicated circuit. This stipulation means you can’t just plug your system into an outlet, even if the voltage is correct, because it would not comply with the NEC.

The main reasons for this requirement are twofold:

• First, the dedicated circuit for your mini-split must have a disconnect box to allow power to be cut off during repair and servicing work.
• Second, if you put a heat pump on the same circuit as other household appliances, you risk overloading the circuit.
  Mini-splits draw a lot of amps, particularly when they start up, although this settles down under steady-state operation. During the start-up phase, you might find your circuit breakers frequently tripping if someone is drying their hair with an electric hair dryer on the same circuit.

That is why you should not plug a mini-split into an outlet.

Can a Mini-Split Cause Mold?

Mold spores exist in the air we breathe every day, whether indoors or outside. The spores are present at relatively low concentrations and don’t cause a problem unless you are particularly sensitive to them.

However, mold can grow into a problem in the right conditions. It needs a dark, damp place to really take hold.

Mini-splits dehumidify the air, and some units have a special “dry” setting to optimize dehumidification. So, they will help avoid mold growth in your home by preventing the conditions that can lead to this problem.

Sometimes, it’s possible mold will grow in the unit itself, which can happen if you don’t clean your unit. Dust and other debris can build up on the coil fins, providing an ideal substrate for moisture to gather and mold to grow.

A well-maintained mini-split will not become a breeding ground for mold, but if you fail to keep it clean and don’t have it regularly serviced, mold can become an issue, especially if you live in a humid part of the country.

Mini-splits should occasionally blow air without any cooling, which will dry the evaporator coil and remove the moisture that can provide the conditions for mold growth. However, if you routinely turn your unit off before the drying cycle has had time to run, this can increase the risk of mold.

If you get a mold problem in your system, you can fix it simply by cleaning it with warm water spray and special cleaning products. Once clean, it’s worth adding a mold-growth inhibitor to prevent recurring problems.

A well-constructed vapor barrier is one of the most crucial protective measures against mold and rot. To read more about vapor barriers and how to install them correctly, read our article on the subject, here.

Can I Leave My Mini-Split on All the Time?

Mini-splits perform better if they constantly run, so it’s a good idea to leave them running around the clock.

Modern units use inverter-driven compressor technology, which allows them to precisely modulate their output to the required level of heating and cooling.

This feature is excellent from a comfort point of view because it keeps the temperature closer to the setpoint for more of the time.

It also improves efficiency because the alternative on/off control mechanism used in older units meant the compressor would be switched on or off to keep the temperature at the right level.

Every time the compressor would start up, it would need to overcome the inertia of the heavy moving parts, which is incredibly inefficient. 

An inverter-driven model slows down in a controlled manner to provide just the right amount of heating or cooling. Operating at these low speeds is very efficient and allows the system to maintain your home at the optimal temperature with minimal electricity, keeping running costs down.

Reduced wear and tear on the parts is the other benefit to eliminating the on/off control mechanism. Mini-splits, therefore, have a longer life expectancy than on/off HVAC systems.

Can You Cover a Mini-Split?

As mentioned, heat pumps have an outdoor unit, an indoor unit, and a line set connecting the two.

The line set is the copper piping used for passing the refrigerant around the system. It is typically bundled together with the electrical power and communications cabling and the condensate drain.

These lines are usually covered by a conduit that is referred to by different names, such as “line hide” or “line set cover.” The conduit conceals the line set to make it look more presentable and helps to protect it from rodents and the weather.

When it comes to the units themselves, they must have sufficient room to allow air to circulate freely around them. So, if you want to cover them, you must bear this in mind.

In most settings, the outdoor unit doesn’t need to be covered, but some experts recommend having a cover in frigid climates to protect it from water dripping off the roof.

The fan can draw this water into the unit, where it will freeze onto the coil, causing the system to go into a defrost cycle more frequently, thereby reducing the efficiency of its operation and costing you more money to heat your home.

Newer indoor units are more stylish and sleeker in their design, but some people still want to conceal them from view. If you intend to hide them, you could consider using a ceiling cassette, which allows the unit to be recessed into the ceiling so that only a vent is visible in the room.

Other alternatives could include using false beams or even creating a custom cover. Still, you must follow the manufacturer’s installation instructions to ensure sufficient room for air to circulate correctly. Otherwise, your unit won’t work well.

Can You Install a Mini-Split Without a Vacuum Pump?

Purging the refrigerant lines on a heat pump system is vital to ensure there is no air or moisture in the unit, which can reduce the efficiency of your system and cause the compressor to overheat and fail prematurely.

A vacuum pump allows you to remove almost all these unwanted substances from the line set before charging with refrigerant. 

This addition is necessary during the installation process unless you are happy to live with lower efficiency and increased running and repair costs.

These days, many mini-splits come with precharged line sets, which have been purged with dry gas at the factory and contain the right amount of refrigerant for the system to operate optimally.

In this case, you don’t need a vacuum pump to install the unit. In addition, some manufacturers have started offering DIY mini-splits that don’t require any specialist tools to install.

Can You Mix and Match Mini-Split Components?

If you’ve had your system for a while and one of the indoor units breaks down, it could be tempting to use a cheap unit from another brand your friend is offering to sell you for a low price.

Don’t be tempted to try this. The outdoor and indoor units must be able to communicate with each other. If you have units manufactured by two different brands, the chance they will use the same communication protocol is very low.

Even non-matching pairs from the same brand can’t be relied on to communicate with each other effectively and will very often give you a communication mismatch error. Some systems will also be designed to operate with different refrigerants, which will cause all sorts of problems if you connect them.

If you are not talking about non-matching pairs and instead are thinking of installing wall-mounted units in some rooms, ceiling cassettes, and floor-mounted units in other parts of the house, that is possible.

Just make sure that the indoor units can communicate with the outdoor, they use the same refrigerant and are otherwise compatible.

A competent HVAC technician will be able to advise you on this. 

How To Mix and Match a Mini-Split

The difficulties of mixing and matching components of a mini-split system have been covered above, but there is a way that you can mix and match system parts in your home.

For example, if you have recently built an add-on room in your home, your existing HVAC system might not have the capacity to add heating and cooling for this additional room. In this case, you could install an additional outdoor unit matched with the recommended indoor one to heat and cool the new living space.

These units could be a different brand from the model installed in the rest of your home and would be a separate system.

You might even want to take the DIY route and install a Mr Cool heat pump, which you can install yourself, for the add-on room.

Always ensure that the indoor and outdoor units are designed to work together. They must be able to communicate and be compatible in other ways, such as using the same refrigerant.

Can You Oversize a Mini-Split?

Ductless heat pumps are very efficient, provided they are designed and installed correctly.

Many people think it would be better to err on the side of making their mini-split system too large, but they don’t realize that it is possible to have too much capacity, which can cause all sorts of problems for the homeowner.

The critical thing to note is that while mini-splits can modulate their output and turn down their capacity to match the load that needs to be met, there is a minimum below which they cannot turn down further.

The result of installing a sizeable system is that, rather than turning down and operating at a low-speed steady-state, which is very efficient, it must turn off completely to avoid overshooting the temperature setpoint.

Turning on and off in this way fails to make the best use of the variable-speed inverter-driven compressor technology that makes mini-splits so efficient. Instead, you effectively have a system that operates similarly to the less efficient on/off systems.

The continual switching on and off is much less efficient and causes increased wear on the system’s components, both of which will cost you money over time.

Can You Paint a Mini-Split?

There’s no reason you can’t paint an air handler—the design-conscious amongst you might want the unit to match the color scheme of your room, and this is fine.

Just make sure you don’t get paint on the inside mechanisms of the unit.

For example, if you got paint on the coil, that would impact its ability to transfer heat and reduce the unit’s efficiency. Likewise, if you got it on the filters, it would block some pores and impede airflow through the filter medium.

Both issues would decrease the unit’s efficiency or even prevent it from working altogether.

If you intend to paint your air handler, removing the front cover would be a good idea to avoid painting over any of the mechanisms inside.Another helpful tip is to use masking tape to cover labels and sensors to ensure they are still visible and operate as they should after painting.

Make sure you clean the unit and wipe it down thoroughly before starting. Then use a surface primer as a first coat and ensure you use non-yellowing paint for a long-lasting, good-looking finish.

Here’s a great “how-to” video showing how to paint an air handler unit:

Do I Need an Air Handler in the Bathroom?

In many cases, there is no need to install a mini-split in a bathroom or other small spaces in the home, such as hallways. The heating and cooling effect by units serving larger rooms in the same zone should be sufficient if the air circulation is adequate.

We have even seen homes served by a single air handler on each floor. This configuration works well in linear-shaped homes that allow the unit to be positioned so that it blows conditioned air along the entire house.

However, you might need to leave some internal doors open to allow good air circulation, but this is possible.

When it comes to bathrooms specifically, you can certainly install a mini-split in them, provided the unit is designed to operate in high humidity. Bathrooms get very humid, so a prerequisite for installing a unit would be to put in a bathroom exhaust fan, which will help control the humidity and ensure the mini-split operates well.

Will a Mini Split-Heat or Cool My Garage?

Heating and cooling garages are an excellent application for a heat pump. Garages tend not to have ductwork installed due mainly to the risk that it can draw car exhaust fumes into it and distribute poisonous gases around the home via the HVAC system.

Mini-splits are ideal for adding heating and cooling to a garage because they are easy to install and don’t need ducts to operate. Their head units draw in air from the room and pass it over a coil containing refrigerant to heat or cool it before returning the same air to the room.

A heat pump could be the answer to your prayers if you are sweltering in summer heat or shivering during freezing winter nights while working on your latest project in your garage.

Why Does a Mini-Split Heat Pump Leak Water?

Heat pumps are generally reliable and will provide many years of trouble-free service provided you take care of them with regular servicing and maintenance. However, occasionally they can suffer from issues that could call for a visit from an HVAC professional.

One such problem is a water leak. Leaking water can make a real mess of your walls, causing water marks and potentially leading to mold growth or even structural issues if they are left undetected for a long time.

There are a few possible causes of water leaks on mini-splits, but by far, the most common is a blocked condensate drain.

When in cooling mode under normal operating conditions, the moisture present in the air condenses onto the coil of the air handler.

The condensation drips down from the coil into the condensate pan, enters the condensate drain, and flows under gravity out of the house before being discharged safely to a drain or onto the ground where it soaks away.

Over time, airborne particles such as dust and other debris become trapped in the moisture in the mini-split and find their way into the condensate drain.

If the drain doesn’t have a sufficient slope, this can give rise to accumulating water, where the debris settles out and creates blockages. For example, this occurrence is widespread in parts of the drain that have sagged in between supports.

Once the drain becomes blocked, water backs up and overflows from the condensate drain pan and drips down your wall, making a mess and staining your walls.

An easy fix for this is to attach a vacuum to the end of the condensate drain for a minute or so to pull through any blockages. This technique will work a large percentage of the time and is worth a try before calling out a professional technician to take a look.

There are a couple of other, less common reasons why your mini-split might leak water:

• Poor air circulation can result from dirty filters and lead to ice building on the evaporator coil. When the unit switches off, this ice melts and, in some circumstances, can overwhelm the drain pan.
• Another cause of leaking water is improper installation. If the unit itself is not level, water can drip out from the condensate pan, which is designed to catch all the condensate, but only if it is installed level.

If insufficient care has been taken to ensure the condensate drain is connected correctly, this can cause leaks. For example, if the drain doesn’t slope downwards at the back of the unit, water can flow back into the system and overtop the drain pan.

In some circumstances, a condensate pump is needed to lift the condensate to a level from which it may drain under gravity. Should the condensate pump break down, you will get a leak as the drain pan overflows.

Why Do Mini-Splits Freeze?

During the winter, when your system is operating in heating mode, the evaporator coil in the outdoor unit collects heat from the cold air outside.

The coil is very cold, which can cause moisture in the air or water that is drawn into the unit by the fan to freeze onto it. Typically, your mini-split will detect this and go into a defrost cycle.

The outdoor unit mustn’t allow ice to build up on the coil because that insulates the refrigerant inside the coil from the outside air being blown across it—that makes the unit much less efficient.

The way a mini-split addresses this problem is by going into reverse. The defrost cycle effectively operates the system in cooling mode for a few minutes to melt the ice off the coil in the outdoor unit.

The only difference between standard cooling mode and a defrost cycle is that the fan in the outdoor unit does not blow air across the coil during a defrost cycle.

Once the ice has melted, the system goes back into standard operation and continues to provide heating to your home.

Occasionally, the coils on indoor air handler units can freeze up. Ice is typically caused by poor air circulation in the air handler, resulting from a buildup of dust and other debris on the coils.

Regular cleaning and maintenance can help prevent this. 

However, if your system has this problem, it’s worth bringing in a professional to handle the cleaning because they will have the specialist kit required to do the job properly with minimal mess.

Final Thoughts

Mini-splits are a highly efficient and convenient way to provide heating and cooling for your home.

This guide has covered many of the most common questions that we get asked about these systems, including where they can be installed, common problems they experience, and troubleshooting advice.

If your question hasn’t been answered here, please check our archive of mini-split articles for more information about these flexible, efficient, and convenient systems.