What Is A Mini Split Air Conditioner? Split System Units Explained

“What is a mini split?” Many HVAC experts would say a mini split is the best thing that has happened in HVAC in the past 20 years. And it’s not hard to see why.

Essentially, mini splits are heat pumps that can provide both cooling in the summer and heating in the winter. In fact, these are generally considered to be the best ductless heating and cooling system.

These are split systems HVAC units. What is a split system HVAC system? In it’s most basic formulation, it is a system that consist of:

• 1 outdoor unit; referred to as condenser unit.
• 1 or more indoor units; referred to as indoor air handlers. If a mini split has more than 1 air handler, we talk about multi-zone mini splits.

One of the advantages of mini splits – and not even the bigger one – is the fact that they require no ductwork. In the 20th century, most HVAC systems were ducted (central air, furnaces).

With the invention of ductless mini split heat pumps, there is no need for ductwork. That saves you quite a lot of problems, space, and $1,000s of dollars you would have to spend for ductwork and installation.

That’s not the only big advantage mini splits have. Incredibly high energy efficiency (20+ SEER and 10+ HSPF ratings) heavily reduce cooling and heating costs.

“Once we started using a mini split, our cooling bills were slashed from $120 per month to less than $70. Best HVAC unit I could have purchased 100%”, is just one of our reader’s accounts of how much you can save on electricity by installing a mini split.

To best present what mini splits are, we have prepared an overview of all things you need to know about these modern air conditioning and heating units. This includes:

• How do mini splits work? We will explain how these ductless HVAC split system units are capable of both cooling and heating (refrigerant-cycle based operation).
• Types of mini splits heat pumps. In most cases, we talk about air-to-air mini split heat pumps. We will familiarize ourselves with water-to-air and ground-to-air heat pumps as well.
• Basic components of mini splits. Internal parts of outdoor and indoor unit explained.
• Advantages and disadvantages of mini split heat pumps. Needless to say, pros of mini splits (extremely high energy efficiency, easier installation (even DIY), record-low noise levels, etc.) heavily-outweigh the cons of mini splits (high upfront cost, lower heating efficiency in very cold weather).
• Where can you install mini splits? These units can be used as whole-house HVAC systems. Many homeowners install them in the bedroom, living room, garage, or even attic, to name just a few rooms.
• Comparison of mini splits vs. central AC units, and even portable and window AC units.
• Which are the best mini splits units currently on the market?

The goal here is to get all the information on what mini splits are, including the key specifications and features. Equipped with this knowledge, you will be able to make an informed decision if a mini split is a right choice for your home:

How Do Mini Splits Work? (It’s All About Heat Transfer Via Refrigeration Cycle)

The working principle of ductless mini splits is based on a refrigerant cycle. Mini splits don’t ‘generate’ cool or heater; they only facilitate the transfer of heat. This is also why mini splits are so incredibly cheap to run; you don’t have to burn natural gas, oil, or propane to generate heat.

All this transfer is facilitated by the expansion and compression of refrigerant gas (R410A or R22, for example):

Mini Split Air Conditioner Working Principle

In the summer, inside coils (located in the air handler) gather heat in your house. This is done by channeling cold refrigerant (liquid state) through the indoor coils. A blower (powered by a blower fan) generates airflow over these cold coils and blows cold air.

Eventually, however, the refrigerant in the heater up and turned into gas. We usually say that indoor coils ‘absorb’ the heat. The refrigerant is channeled through refrigerant lines (they go through to wall) to the outdoor unit. The compressor, found in the outdoor unit, compresses this hot gaseous refrigerant into cold liquid form (a lot of heat is exhausted outdoors during this process). The cold liquid refrigerant travels via refrigerant lines indoors again, and we have a complete cycle.

Here is the key feature of mini split air conditioner:

The refrigerant cycle can be reversed.

When we need cooling, the mini split works as an air conditioner. It absorbs heat from indoors and exhausts it outdoors.

When you reverse this cycle (this is as simple as switching the reversing valve; usually via mini split remote control), you turn a mini split air conditioner into a mini split heat pump:

Mini Split Heating Working Principle

Reversing the valve switches the mini split working principle from cooling to heating. Now, the hot gaseous refrigerant is found in the indoor coils (in the air handler). Blower generates airflow over these hot coils and you enjoy a warm breeze.

Of course, eventually, these hot indoor coils are cooled down (turned into liquid form), and the refrigerant liquid travels outdoors where it is expanded (heated up) by the available outdoor heat. Refrigerants can absorb heat from outdoor air even if the outdoor air’s temperature is below 0°F (at extremely low temperatures, this is not the most efficient process, however, as we will point out further in the cons of using a mini-split for heating).

This hot refrigerant in the gaseous form again travels indoors where it provides heating. Here is a diagram that illustrates how do mini splits work when in ‘Heating’ mode:

This dual nature of mini splits – being able to provide cooling as well as heating – is one of the best features mini splits offer. You don’t need a central air + furnace HVAC combo; you can just use 1 mini split.

From the diagram above, you see the title ‘Air Source Heat Pump’. To properly look at what mini splits are, let’s just quickly go over types of heat pumps currently on the market:

Types Of Mini Split Heat Pumps

By far the most common mini split is an air-to-air heat pump. This is what we have been looking at thus far. It is a HVAC device that gathers heat/coolness from outdoor air and transfers it indoors via the indoor air handler (unit that utilizes blower-generated airflow to heat/cool our homes).

In more than 80% of cases, when people ask what is a mini split air conditioner or what is a mini split heat pump, they have air-source or air-to-air heat pumps in mind.

Two other types of heat pumps include (much less common):

• Water-to-air heat pumps.
• Ground-to-air heat pumps.

In both of these types, the indoor air handler is the same. It is an indoor unit with hot or cold coils and a blower that either blows hot or cold air.

The source of heat/coolness, however, is not air anymore.

In the case of water-to-air heat pumps, we have an outdoor unit that extracts heat/coolness from water. This is actually better because the temperature of the water is much more constant than the temperature of air (outdoor air is hot in the summer and cold in the winter; it’s harder to extract coolness from hot air, it’s easier to extract it from mildly warm water).

Ground-to-air heat pumps are the best there is (they also cost $10,000+, require massive installation, and are not really that in demand). These units extract heat/coolness from the ground. The advantage here is that the ground temperature is constant. That increases the energy efficiency of mini split ground-to-air heat pumps.

With this covered, let’s look at the basic parts of mini splits:

What Is Mini Split Composed Of?

For all technical enthusiasts, it is quite useful to see the basic parts that make up a mini split.

Here is a very simple diagram showing what components mini split is made out of:

Let’s first look at the parts in the outdoor unit:

• Compressor. This is the most important part of every mini split (also the priciest and the one thing you don’t really want to break down). It compresses the refrigerant and converts it from a gaseous to a liquid state.
• Outdoor coils. These serve as refrigerant coils (winter) or condensation coils (summer). They are usually made out of copper and serve as a heat-exchanger. Naturally, they have superb heat transmission qualities.
• Outdoor fan. Generates an airflow over the coils.

The outdoor unit is connected with one (single-zone mini splits) or several (multi-zone mini splits) indoor units or air handlers. Parts that make an air handler are:

• Indoor fan (blower). This generates airflow over indoor coils.
• Indoor coils. These serve as refrigerant coils (summer) or condensation coils (winter). They have the same superb heat-conducting qualities as the coils in the outdoor unit.
• Capillary tube. This gathers the condensation that might be gathering and channels it via the tubes in the wall outside.
• Dust filter. A well-known dust filter all mini splits have. These are usually 13 MERV filters.

As you can see, the basic outline of mini splits is not all that complex. Especially if you compare them with the complexity of central AC units + ductworks.

A lot has been said on just how great mini splits are. Let’s clearly look at the advantages and disadvantages of mini splits:

Pro And Cons Of Mini Split Systems

It is not without reason that many HVAC specialists agree that mini splits are the best thing that happened in HVAC for quite a while.

Advantages Of Mini Split Systems

Most homeowners are aware of what pros mini split systems bring to the table. Let’s highlight just the biggest advantages mini splits have:

• Extremely high energy efficiency (and low running costs). Among all types of air conditioners, the ductless mini split units are the most energy efficient. They can easily surpass a 20 SEER rating. In fact, the most efficient energy unit is a 33 SEER rated Mitsubishi mini split. Due to this energy efficiency, you are likely to literally save $1,000s of dollars in the next 10-20 years.
• Wide range of capacities. Mini splits can produce as little as 3/4 tons of cooling effect or as much as 6 tons. You can always install more units as well. That means that you can use mini splits to adequately cool a very small 300 sq ft place or even a 10,000 sq ft mansion. This is the range that covers both central air and room air conditioners (window or portable units).
• No ducts required. This goes without saying.
• Easier installation. If you compare mini split to central air units, you can see that mini splits are much easier to install. With central air, you need elaborate ductwork. With ductless mini splits you only need a hole in the wall (for refrigerant and communication lines).
• Cheaper than central air. Mini splits AC units still cost in the $1,000s but they are, on average, cheaper than central air for the same SEER rating and tonnage. On top of that, the installation costs are lower as well.
• Extremely quiet. The noisiest part – the compressor – is located in the outdoor unit. That means that you get very low noise levels indoors.
• Mini splits can provide cooling and heating. This is an all-around HVAC system; not just an air conditioner. In the winter, they can provide heating as well. This is not just some supplemental heating. You can, in fact, use a mini split heat pump as the main and only source of heating.

These are just a few of the advantages mini splits have.

Disadvantages Of Mini Split Systems

Homeowners are generally less aware of the disadvantages of using mini splits. The pros mini splits bring heavily outweigh the cons but it is still important to understand what the cons of mini splits are when figuring out if a mini split is right for your home.

Disadvantages of mini splits AC units include:

• High upfront cost ($1,000s). There is no denying that most HVAC system is quite costly. Mini splits are no exception. To get all the pros mini splits offer (especially long-term saving), you will have to first invest $1,000s for the unit and the installation. Of course, you can limit the costs by installing a mini split yourself (check here for the easiest and best DIY mini split systems).
• Lower heating efficiency in cold weather. This is the biggest disadvantage mini splits have; the efficiency (Coefficient Of Performance or COP) is reduced from 300% at 27°F of higher to about 100% when the temperatures hit 0°F. This will increase the heating costs in very cold weather. You can read more about this greatest disadvantage of air-source heat pumps here. The best way to alleviate this is to install a mini split furnace combo. The mini splits does all the work at higher temperature; when it gets very cold, the furnace takes over the heating.
• Dependency on electricity. All mini split heat pumps are powered by electricity. And electricity is not exactly the cheapest energy source. You will also have problems during power outages.

  

All in all, the main disadvantage people are worried about is the lower heating efficiency in very cold weather. If you were to ask a pessimistic person ‘What is a mini split heat pump?’ the answer would probably be ‘That expensive thing that doesn’t provide heating when you need it the most’.

One part of understanding what is a mini split system air conditioner is checking where you can install one:

Where Can Ductless Mini Split Heating And Cooling Systems Be Used?

With central air units, we understand these units can provide cooling to any room with ductwork. The point about mini splits is that they are ductless. That means that split systems can literally provide cooling to any room imaginable.

Mini split heat pumps are used to provide cooling and heating to:

• Bedrooms. You can check out the best bedroom AC units here to get a clearer understanding of how to choose an AC unit for a bedroom.
• Living rooms.
• Even bathrooms.
• Kids rooms.
• Kitchens.
• Garages. You can check the best mini split systems for garages here.
• Offices. These ceiling cassette mini split systems are regularly used in offices.
• Attics. You can check out the best air conditioners for attics here.

  

Basically, you can put a mini split anywhere. You only need:

1. A wall to mount the indoor air handler.
2. An outdoor space where you can place the outdoor unit.
3. A wall between the two for refrigerant and communication lines.

That’s pretty much it. That’s why mini split systems are so versatile and widely used.

Which Are The Best Mini Split Systems On The Market?

Of course, when homeowners understand what mini split systems are and see the pros, they usually want to get one. Or at least to look at some prices to see if installing a split system is a financially viable option.

We look at specs here. When determining which mini split is the best on the market, you should look for adequate cooling/heating capacity, low noise levels, easy installation (DIY), and, most importantly, a high SEER rating.

Based on these primary (and several secondary) specs we have created a helpful list of the best mini split systems (you can check it here). This list and explanation will illustrate what the mini split systems actually are in a more practical way.

Hopefully, even this theoretical explanation of the nature of mini split systems in HVAC might be helpful for you. If anything is still unclear, you can use the comment section below to pose a question and we will try to help you out as best we can.

Thank you.