Side view of a mini-split's compressor unit next to a central AC outside a home

Unless you’ve been living under a rock for the last few years, you have heard that there’s a cost-of-living crisis underway across the world.

With more people looking to make economies in their household budgets, more efficient options for heating and cooling our homes are becoming increasingly popular.

If you want to know more about ways to improve your home’s energy efficiency, read our guide to energy efficiency-improving high ROI home upgrades.

One such option is a mini-split heat pump. This ingenious device uses the same principles that underpin the operation of the humble refrigerator to heat or cool the home.

But how do they work, and why are they so efficient?

The rest of this article will explain what mini-splits are, how they work, and why they are so efficient. So, read on if you want to learn more about this next-generation technology.

How Do Mini-Split Heat Pumps Work?

Mini-splits use the refrigeration cycle to move heat from one place to another to heat or cool the home. They are very efficient because they don’t use ducts, which are a source of significant energy losses, and because moving heat is more efficient than generating it directly.

Heat pump technology has been around for a very long time, with the French physicist Carnot first describing the scientific concepts behind the refrigeration cycle as far back as 1824, which is what underpins the operation of all heat pumps.

Improvements in technology have made heat pumps ever more efficient, to the point now where they can save you some serious money on your heating and cooling (HVAC) bills.

What is a Mini-Split Heat Pump?

Sometimes going by different names, mini-split heat pumps, ductless heat pumps, or simply mini-splits, bring with them a whole heap of jargon that can be confusing at first.

Mini-splits comprise two main parts, the outdoor unit and one or more indoor units.

The outdoor unit houses the compressor, responsible for pumping the refrigerant fluid around the system. The compressor is the most crucial part of the system and the most expensive to replace if anything goes wrong.

The indoor units are often called air handlers, head units, or evaporators and are responsible for directing cool or warm air into the room they serve by blowing air from the room across the coil in the unit.

The outdoor unit is connected to the head units inside the building by copper pipes, called a line set, that contains refrigerant. The line set is generally bundled with electrical cables providing power to and facilitating communication between the components.

An illustrated diagram of how a ductless mini-split system functions

A condensate drain made from plastic pipe is typically bundled with the line set and electric cables and allows the moisture that condenses on the coil in the head unit to drain away safely.

The refrigerant lines are thermally insulated to prevent energy losses as the refrigerant is pumped along the line set.

So too is the condensate drain, which can otherwise sweat during warm, humid weather as moisture from the air condenses onto its outer wall, potentially causing water damage.

A head unit is required in each room or zone that needs heating or cooling. The head units are controlled by a remote, which you can use to set the desired temperature for each zone.

Heating Principles and Mini-Split Technology

Heat will always move from hot to cold; it will not spontaneously flow from a colder body to a warmer one. This principle is the second law of thermodynamics, proposed by Lord Kelvin in the mid-1800s.

How then does a heat pump move heat in the opposite direction?

The answer lies in the refrigeration cycle, which our French friend Carnot came up with in 1824.

Heat pumps use the thermodynamic principles of the refrigeration cycle to move heat from one place to another. In simple terms, they use electrical energy to transfer heat from the cold side of the system to the hot side.

That’s the high-level explanation. Now let’s take a deeper dive into the refrigeration cycle details to understand better what’s going on in our mini-split system.

The Refrigeration Cycle

The refrigeration cycle has four stages, including:

  • Compression
  • Condensation
  • Expansion
  • Evaporation

To understand what happens in a heat pump, we’ll follow the journey of the refrigerant fluid as it is pumped around the system.

The different stages of the refrigeration cycle manipulate the phase of the refrigerant (whether it is liquid, vapor, or gas). The refrigerant has the necessary thermodynamic properties to absorb and release heat optimally for the heat pump to operate efficiently.

We’ll follow the refrigerant around the heat pump as it operates in cooling mode. The system would reverse the process in heating mode, but the principles would be the same.

Compression

We begin our journey around the system as the refrigerant enters the compressor. It enters as low-pressure, superheated gas.

A newer mini-split compressor outside the exterior of a home

The term “superheated” means the refrigerant is above its boiling temperature and is therefore in a gaseous state. The compressor compresses the gas, which changes it into a high-pressure, superheated gas.

Condensation

The refrigerant then enters the condenser. In cooling mode, the condenser coil would be in the outdoor unit.

Here, the gas starts cooling, and as it does, it changes from a gas to a vapor (a combination of gas and liquid).

Outside air is blown across the condenser coil, which further cools the refrigerant inside it and makes it condense into a subcooled, high-pressure liquid.

Expansion

Next, the high-pressure liquid refrigerant travels through the expansion valve (also known as a metering device).

The expansion valve causes the liquid refrigerant to “flash” into a vapor as it passes across the valve into a lower pressure part of the system.

Evaporation

Finally, the refrigerant enters the evaporator in its low pressure, sub-cooled liquid state. Here it boils and loses heat as it evaporates.

As it continues through the evaporator, the refrigerant absorbs more heat and leaves the evaporator coil as a low-pressure superheated gas.

Next on its journey, the refrigerant passes along the line set into the compressor, where the process begins all over again.

Mini-Split Refrigerants

The refrigerant is crucial to the operation of a mini-split. As it passes through the system and undergoes the refrigeration cycle’s phase, pressure, and temperature change, it makes the heating and cooling provided by the heat pump possible.

Refrigerants usually are gases at room temperature and pressure and are man-made, manufactured chemicals.

A row of R-134a or Tetrafluoroethane tanks, a gas used as an eco-friendly refrigerant

Different types of refrigerants are available, but they have distinct properties and cannot be used interchangeably. Homeowners must use the correct refrigerant in each system.

If you use the wrong type of refrigerant, your system won’t work correctly and might be damaged.

Freon

Freon is a trademarked name used to describe a range of different refrigerants. These include chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which are being phased out.

They have been found to damage the ozone layer, which reduces the protection from the sun’s harmful UV rays and increases the risk of skin cancer. 

Freon refrigerants can also be potent greenhouse gases. For example, HCFC-22 has a global warming potential (GWP) almost 2,000 times greater than carbon dioxide.

Chlorofluorocarbons (CFCs)

This refrigerant class contains chlorine, which destroys ozone molecules when it encounters them in the stratosphere.

A single chlorine atom can remove more than one hundred thousand molecules of ozone from the stratosphere, making it a highly ozone-depleting substance.

Because they are so damaging to the ozone layer and are contributors to global warming, these refrigerants are also being phased out.

Examples include:

  • Trichlorofluoromethane (Freon-11, R-11, CFC-11)
  • Bromochlorodifluoromethane (BCF, Halon 1211, H-1211, Freon 12B1)
  • 1,3-Dichloro-1,2,2,3,3-pentafluoropropane (R-225cb, HCFC-225cb)
Open-cell spray foam is shown in use on a ceiling
1,1,1,3,3-Pentafluoropropane is commonly used open-cell spray foam insulation

Hydrochlorofluorocarbons (HCFCs)

The HCFCs contain less chlorine than the CFCs and therefore have a less deleterious effect on the ozone layer.

They can, however, have high global warming potential. For example, monochlorodifluoroethane has a global warming potential of around 2,000 times that of carbon dioxide.

Examples of HCFCs include:

  • HCFC-21 (CHFCl2) Dichlorofluoromethane
  • HCFC-141b (C2H3FCl2) Dichlorofluoroethane
  • HCFC-142b (C2H3F2Cl) Monochlorodifluoroethane

Hydrofluorocarbons (HFCs)

Hydrofluorocarbons do not contain any chlorine and are not harmful to the ozone layer at all.

They do, however, have a significant global warming potential and are therefore being phased down under the Kigali Amendment to the Montreal Protocol.

There has been much innovation and experimentation in the HFC sector, with some used in blends with other refrigerants.

Some of these refrigerants looked promising regarding their thermodynamic properties, with R134a  introduced to replace R22, which was being phased out.

It could be used in existing systems without making many equipment changes, but the efficiency was much lower, resulting in higher running costs.

Examples of HFCs include:

  • R-410A
  • R-407C
  • R-134a

Other Refrigerants

There are other refrigerants that rarely (or never) get used in mini-splits. It’s worth touching on these to understand why they aren’t suitable for a domestic heat pump system.

Ammonia (NH3 or R717)

Ammonia is used in many commercial refrigeration applications but has long been banned in domestic settings due to its toxicity.

It is a very efficient refrigerant with zero global warming potential and is well suited to low and medium-temperature refrigeration.

Hydrocarbons (HCs)

Sometimes referred to as natural refrigerants, hydrocarbons have no ozone-depleting potential and meager global warming potential, which is great from an environmental point of view. The only drawback is they are highly flammable.

They are mainly used in Europe and are banned in many other countries, including the US.

Examples of hydrocarbon refrigerants include:

  • R1270 Propylene
  • R600a Isobutane
  • R290 Propane
A propane tank next to the exterior wall of a home
Propane is still commonly used in residential and commercial heating

For more information about refrigerants, including the most eco-friendly and affordable ones, please read our article covering that, here.

Mini-Split Efficiency

Now we know how a mini-split system functions, let’s look at how efficient they are. Mini-splits are very eco-friendly because they use energy effectively to heat and cool your home.

Three critical metrics describe the efficiency of a mini-split heat pump system. They are:

  • Heating Seasonal Performance Factor (HSPF)
  • Seasonal Energy Efficiency Ratio (SEER)
  • Energy Efficiency Ratio (EER)

Most people tend to focus on HSPF and SEER when deciding on the right system for their home, with greater emphasis placed on the HSPF in colder climates and more attention on the SEER in hot climates where cooling is more important.

But what do these factors mean, and how are they calculated?

Heating Seasonal Performance Factor (HSPF)

The HSPF is the total space heating required during the space heating season (in Btu), divided by the total electrical energy consumed by the mini-split during the same season (in watt-hours).

HSPF gives you an idea of how efficiently your system can heat your home during the winter. The higher the HSPF, the more efficient your heat pump is.

To earn an ENERGY STAR certification, your mini-split must have an HSPF of at least 8.5.

Seasonal Energy Efficiency Ratio (SEER)

The Seasonal Energy Efficiency Ratio (SEER) is the total heat removed from the conditioned space during the annual cooling season (in Btu), divided by the total electrical energy consumed by the mini-split during the same season (in watt-hours).

The SEER ratio indicates how efficiently your heat pump performs when cooling your home over the summer months. The higher this number, the more efficient the system is.

 A mini-split must achieve at least 15 SEER for ENERGY STAR certification.

Energy Efficiency Ratio (EER)

The EER is the ratio of the typical rate of space cooling provided to the average rate of electricity consumption by the unit. This figure is expressed in Btu per watt-hour. The higher the number, the more efficient the system.

A homeowner sits on the couch in her living room, appearing satisfied while adjusting her mini-split evaporator with a remote control

The EER gives a standardized measure of the unit’s efficiency and must be at least 12.5 EER to qualify for an ENERGY STAR certification.

Why Are Mini-Splits So Efficient?

The efficiency of heat pumps is very high, particularly when compared to traditional gas or oil-fired furnaces or electrical resistance heating that uses central air distribution systems.

Let’s look at some main factors contributing to their impressive efficiencies.

Mini-Splits Don’t Generate Heat Directly

Mini-splits do not generate heat directly but use electricity to move heat from outside the building into the home. Moving heat using electricity in this way is far more efficient and can result in significant savings on your heating bills.

Mini-Splits Don’t Use Ducts

Another reason mini-splits are particularly efficient is that they do not require ducts.

Ductwork is responsible for energy losses of around 25-40% of the heating or cooling energy a central AC unit or furnace puts out. This inefficiency can be due to a lack of insulation or because of holes and leaks in the ducts.

Ducting near the ceiling reinforced with metallic tape to cover leakage

Mini-splits eliminate this problem by doing away with ducts altogether.

High Controllability

A further reason ductless heat pumps are so efficient is that homeowners can individually control

 them. This feature means they can be completely turned off when a room is not in use, which saves energy.

Use of Inverter-Driven Compressor Technology

Finally, a modern ductless mini-split uses a variable speed, inverter-driven compressor, which automatically regulates its speed to match demand, ramping up when heating or cooling loads are extensive, such as at switch on.

Once the operating temperature is reached, they dial back their output and can keep ticking over, which allows them to operate very efficiently.

Compared to a single-stage compressor, which is either on or off, this is much more efficient because a lot of energy is consumed at startup when the electrical current peaks.

This approach can also prolong the lifespan of your unit because switching on and off frequently increases wear and tear.

The compressor is the most expensive component in your unit, and if it fails, you will likely need to replace the whole system, so this can be a significant saving.

Summary

Mini-split heat pumps are a fantastic choice for heating and cooling your home.

They comprise an outdoor unit and one or more indoor units that blow conditioned air into your living spaces.

Mini-splits do not generate heat directly but use electricity and the refrigeration cycle to move it from outside your home to inside during the heating season and the reverse in the summer.

Some refrigerants used in heat pumps can cause environmental harm. Still, steps continue to be taken by the government to reduce these harms through the use of more environmentally friendly refrigerants and regulation and licensing of their use.

Mini-splits are incredibly efficient because of the advanced technology that allows them to be precisely controlled, making the best use of the energy they consume.

They also do not require ductwork, a significant source of energy loss in central air systems.

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