Mini split is truly a wonder of modern HVAC engineering. This is the first ductless air conditioning and heating solution. The ability to cool and heat (at extremely high efficiency) is outstanding. Today, we will look at how does a mini split work.
We will explain how mini splits can both cool (in the summer) and heat (in the winter). It seems paradoxical but the explanation of how this works is dead simple. All this has to do with the dual nature of the compressor; it’s capable of compressing the refrigerant (cooling) as well as expanding the refrigerant (heating).
Note: If anything about how ductless mini splits work is unclear, you can use the comments below to ask questions. We can discuss a bit and we’ll try to help you with the simplest explanation.
Before we check how do ductless mini splits work, let’s list some of its amazing abilities:
- It is ductless. You don’t need ductwork like with central air (cooling) and furnaces (heating).
- Mini split can both provide cooling and heating. It is basically a heat pump; it pumps heat in and out of your home. We will explain how ductless heating and cooling work.
- Very high energy efficiency. Due to how they work, mini splits regularly have a 20+ SEER rating (cooling efficiency) and a 10+ HSPF rating (heating efficiency).
Prior to mini splits, all whole-house HVAC units could provide only heating or only cooling, all of them were ductless, and they were expensive to run.
Mini splits changed all that.
Let’s look at the inner working of how do these ductless air conditioners operate:
How Do Ductless Mini Splits Work? (7 Step-By-Step Cooling Process)
Ductless mini split heat pumps basically pump heat from indoors to outdoors (cooling) and from outdoors to indoors (heating). The reason why they can be so energy efficient is that they don’t create heat (like a furnace burning natural gas would); they merely transport heat between two spaces.
The basic principle of how these split system mini splits work is underpinned by a refrigeration cycle. This is a well-established thermodynamic cycle that uses the compression and expansion of refrigerants (R410A, R22, and so on) to transport heat.
To understand how mini splits AC/heat pump works, we need to be aware of its 3 main components:
- Outdoor unit; here is where the all-important compressor is located.
- Indoor unit; this is an air handler (1 or several of them) mounted on a wall.
- Refrigeration lines (together with communication lines).
Here is the how does a mini split air conditioner works in 7 steps:
- A compressed refrigerant in liquid form (low pressure) flows from the outdoor unit to the indoor unit via refrigeration lines.
- When liquid refrigerant arrives in the evaporator cooling coils (located in the indoor air handler), it expands and turns from liquid to gas. Thermodynamically, this is an endothermic process; this means that the coils are cooled in this process.
- Indoor air blower sucks in warm indoor air and pushes it over these cold coils. This cools the air and removes humidity as well (air moisture is condensed on these cold coils). This cool and dry air is expelled into our home; this is what we experience as the indoor air handler blowing cold air.
- Gaseous refrigerant (high pressure) now travels back to the outdoor unit via refrigerant lines connecting the two units. This is denoted as a pink line on this sketch (cold vapor):
- In the outdoor unit, the refrigerant is compressed (by the mini split compressor) and turned back into liquid. This is an exothermic process; it generates a lot of heat. That’s why outdoor units can be so hot. The job of the compressor is basically to squeeze all the gathered heat from this refrigerant.
- Once the refrigerant is again in cool liquid form, it travels back into the indoor unit via refrigerant lines.
- We are again at the beginning of the mini split refrigeration cycling. This cycle repeats itself over and over again unit we get to the set indoor temperature.
In simplest words, here is the essence of how a ductless mini split AC works:
Refrigerant extracts heat from indoor air (by expansion) and pumps it outside. In an outdoor unit, the compressor squeezes all that heat out of the refrigerant, and this cycle repeats itself over and over again.
Alright, this is the cooling part. How can a mini split air conditioner also heat? It seems paradoxical at first. Let’s explain how mini split heating works:
How Can A Mini Split Provide Heating?
A furnace provide heating. A central AC unit provides cooling. How come mini splits can both cool and heat?
Simple.
The refrigeration cycle, described above, is just reversed. Mini splits use a special switch (reverse valve) that turns the flow of refrigerant around. That’s it; instead of cooling, you now get heating.
While operating as a standard AC, the refrigerant is expanded indoors (providing cooling) and compressed outdoors (expelling heating). When you switch the reverse valve, the mini split heat pump gathers all the available heat outdoors (compressor expands the, absorbing heating) and brings that heat indoors.
As you may have noticed, the whole point of heating is the ability of the compressor to work as an evaporator (capable of expanding the refrigerant gas).
Let’s look at the step-by-step process of how turning the flow of refrigerant in mini splits provides heating (instead of cooling):
- A compressor expands a low-pressure liquid refrigerant into a high-pressure gaseous refrigerant. This provides you with a hot refrigerant.
- Hot refrigerant is channeled indoors and is run through the indoor coils, warming them.
- A blower generated airflow over these now hot coils and blows warm air into your home.
- During this process, the hot gas refrigerant is cooled and turned back into a low-pressure liquid state.
- This liquid refrigerant travels via refrigerant lines into the outdoor unit again.
- A compressor expands the cold liquid refrigerant into a hot gaseous refrigerant, completing the heating cycle.
This is how a mini split can provide cooling in the summer and heating in the winter.
The key thing to remember is that even at low outdoor temperatures (below 50°F), the refrigerant is capable of squeezing all the available heat from outdoor air, bringing the heat indoors.
Hopefully, now you have a better understanding of how mini splits work. Essentially, this is an amazing feat of HVAC engineering that uses the same split system unit to provide both cooling and heating as needed.
If you have any questions about this process, you can use the comments below and we can discuss a bit.
Is a minisplit heat temperature setting actually determining the heat temperature or is it more an on and off heat setting and the room temperature is determined by how long the heat is on?
Hi Stephen, that’s a very good question. The mini split has a temperature sensor measuring temperature. If the registered temperature is below the thermostat set temperature, the heat pump will produce heating (powering on and off, the nature of the refrigeration cycle). If the registered temperature is the same or above the temperature set temperature, the heat pump will shut off. Hope this helps.
i built a 250 sq ft shop inside my garage, there is space between shop ceiling and garage ceiling, could i place the outside unit in this space above the shop, to keep it from extreme cold and heat but also theft..shop has 9 ft ceiling while the garage is 14 ft….thank you
Hi Dale, you can install the outdoor unit in that space between the shop and garage ceiling. Just keep in mind that when you cool the shop with the indoor unit, the outdoor unit in that space will produce heat; the reverse is also true. The mini split moves the heat/cool from one area to the other area, so if your shop becomes cooling, your garage will become hotter.
What is the low temperature range where there is no more available heat in the outside air to cause the liquid refrigerant to vaporize? I am thinking these systems will not keep you warm on a -20F day.
Hi Glenn, you are correct: On a very cold -20F day, the heating output of heat pumps is pretty much diminished. The new modern heat pumps can produce a sizable heating output even at very low temperatures.
Here’s how a mini split thinks about this: Alright, the minimum temperature I can extract heat from is -40F. Even if it’s -20F, I can extract some heat from that; not a lot, mind you, but not 0 as well. This is kind of the gist of mini splits and the refrigerant cycle.
Hi. Our house is long and narrow with one end having a southern exposure with bay windows. The temperature differential between ends of the house can be high. In a multi zone system that cannot heat or cool independently, how are conflicting calls reconciled with the compressor? In other words, if one unit calls for heating at one end of the house, and the other calls for cooling… what happens?
Hi Thomas, that’s a very good question. I am not fully confident that a mini split can both heat (with 1 air handler) and cool (with 1 air handler) simultaneously. Usually, you would set the whole multi-zone mini split system either to cooling (snowflake) or heating (sun). The compressor can do 1 job at the time; so cooling and heating at the same time would be a bit impossible.
Hi. My mini split is used strictly for heating. To make it more efficient, I am considering painting the outdoor unit flat black and building a glass enclosure to capture even more heat from the sun. Is this a crazy idea?
Hi Ted, that’s not a crazy idea, it can turn out to be an ingenious idea! We had a guy who used a heat pump in the 20°F winter conditions; but his outdoor unit was located in the parking garage which had much higher temperature – 40°F or 50°F, can’t remember exactly. He way paying way less for heating due to higher heat pump efficiency.
By painting the outdoor unit black and creating that enclosure, you are creating an enviroment that can be much warmer than the freezing outdoor temperatures. At higher outdoor unit temperatures, the heat pump will always be more energy efficient. Really, this is an ingenious idea, go for it.
I’m in Oregon. We just bought a house with the mini split system and are new to these. My low temp is around 50° at night. Is it ok to leave the ac or cooling feature on or should we switch it? I don’t want to ruin any of the components as I’m used to other compressors not being able to run below 60°.
Hi Alice, well, if the temperature is 50°F at night, you don’t really need AC. In that case, it would be best to turn it off; you are needlessly consuming electricity. In any case, you can run a mini split 24/7, these units are engineered to run quite a lot. Hope this at least partly answers your question.
I have 2 possible locations for the inside unit: the preferred location requires a refrigerant line of 33’; the second location requires only 18’. I can locate the line inside the house (most of it in a walk-in closet)until it goes through the wall directly to the outside unit. I know it’s less efficient with the longer line, but how significant is the difference? Are there other pros or cons I should consider? Does the refrigerant line sweat? Is it hot to the touch?
Hi Paula, I like your reasoning here. The longer refrigerant line have a rather insignificant impact on energy efficiency. If there were even a slight efficiency difference (say 1% or 2%), you would always go for higher efficiency. However, here the efficiency difference is likely less than 1%, and the preferred location with 33’ line is quite an optimum choice. Hope this helps.
In getting estimates for an installation of a mini-split with 2 heads, I had one estimater identify that the minimum distance between the outdoor unit and the indoor unit is 14′. And that should be a consideration in the placement of the units. Nothing I could find addresses minimum distance like this. Most websites I’ve found seem to suggest closer is good and distances shouldn’t exceed 15 meters.
Thank you,
Michael
Hi Michael, you are right, the 15 meters (50 ft) is usually the maximum distance. There should be no minimal distance in theory. In practice, you will have at least a wall (lets say 2 ft) between the outdoor and indoor unit.
I’m curious as to whether the unit should be turned off in the summer when the temperature dips to 60 degrees at night & the mini splits are set to 74. Will they bring in extra moisture inside the house or will they still keep the humidity level down? Thanks
Hi there, mini splits cannot bring humidity inside the house. The indoor-outdoor connection is only the refrigerant lines; these are a closed system. You don’t relly have to turn them off during the night.