How Ductless Air Conditioners Work: with Illustrations

When I first saw ductless air conditioners, they were a mystery to me.

“That thing’s cooling the air in here… without a duct?


My curiosity got the better of me, and I’ve researched what a ductless mini-split ac is, how they work, and how to get the most from them.

To make a long story short:

How Do Ductless Air Conditioners Work?

Ductless Air Conditioners work like standard air conditioners. Minisplits remove heat from the room and transport the heat to the outside unit through some small copper, insulated tubes call freon lines. The refrigerant is the medium that absorbs the heat in the indoor coil and releases the heat in the outdoor coil. The compressor is the pump that moves the refrigerant back and forth.

That’s the quick version, but there’s much more to it. Read on for a fuller explanation. As well as pros, cons, installation advice, and best practices.

How Do Mini Split (Ductless) Air Conditioners Work?

Let’s start off by considering a traditional air conditioning system. The general process is as follows:

  1. Hot air is ducted over the indoor coil which has cool refrigerant circulating through it.
  2. The refrigerant – a mixture of liquid and gas – absorbs the heat.
  3. The cool air is ducted back into the living area.
  4. The refrigerant is compressed, the heat extracted and exhausted outdoors, and cooled by the condenser fan.
  5. The cool refrigerant is ready to handle more hot air. The cycle starts again.
Traditional Ducted Central Air Conditioning System Diagram
A (rudimentary) diagram of a traditional air conditioning system.

Now, let’s consider the system in a mini-split AC. Note that the only difference is where things happen. The cooling happens inside the unit in the room. It’s the refrigerant, not the air, that gets transported back and forth.

  1. Hot air is taken by the mini-split unit.
  2. The heat is absorbed by the refrigerant.
  3. The cool air is passed back into the living area.
  4. The refrigerant is pumped to the outdoor unit.
  5. The refrigerant is compressed, the heat extracted and exhausted outdoors, and then cooled via the condenser fan.
  6. The refrigerant is passed back to the mini-split unit to handle more hot air.
  7. The cool refrigerant is ready to handle more hot air. The cycle starts again.

The new steps are numbers 4 and 6.

Okay, that was a lot of steps! Let’s try and simplify it a little bit.

To make a long story short:

A traditional ac system takes hot air out of the living room. It puts it into a central unit. It works it’s magic. Then cold air is sent back into the living room. Done!

A mini-split ac system sits in the living area and takes in hot air. It works a bit of magic. Then cold air is put back into the living room. At the same time, warm gas is sent through a small tube to an outside unit. The other half of the magic happens. Cool liquid is then sent back into the indoor unit.

To make a short story shorter:

Instead of moving hot air around, a mini-split moves refrigerant liquid and gas around. This needs a tiny tube, instead of a big air duct.

Pros & Cons of Ductless Air Conditioners

If you’re weighing up whether to pick up a ductless mini split vs a central ac system, sometimes it can help to consider the pros and cons. I’ve put all the main points in the table below. Hope it helps.

Mini-Split Air Conditioners

Can control cooling per room

Doesn’t bring in fresh air

Better/cheaper if retrofitting without ductwork

More expensive than ducted systems (if you have ductwork)

Easier to install. Doesn’t weigh much.

Visible inside the home.

Multiple options – wall mounted, ceiling recessed, etc.

May need multiple to cover whole home.

Often comes with heater element for winter.

20-30% more efficient than central AC.

Quieter than central AC.

How Do Multi-Zone Ductless Systems Work?

Mini-Splits can cover multiple areas in your home. The way these are split up is (thankfully) quite simple.

A ductless system is made up of indoor units and outdoor units.

The indoor units cool the air, while the outdoor unit pumps out the refrigerant, rejects the heat, and then pumps the refrigerant back in to cool the indoor air.

A multi-zone mini-split system has multiple indoor units for one outdoor unit. Typically up to 8 indoors for each outdoor. The indoor units all send their warm refrigerant. The outdoor unit fans out the warmth, cools the refrigerant, and sends it back to each indoor unit.

Diagram of a multi-zone mini-split ductless air conditioning system.
A basic multi-zone system. Up to four indoor units can operate from one outdoor unit. Indoor units may vary in size/power depending on room size.

How Many Ductless AC Units Do I Need?

If you want your mini-split system to cool your whole home, you will need multiple units. So how many do you need? Well, that’s up to you!

A better question is how many rooms do you want cooled?

Generally, you should expect each mini-split unit to cool the room it’s in. If you cool all the main large rooms, then your whole home will feel the benefit. The question is, which rooms do you need to cool?

There’s two factors to consider here:

  1. What rooms would you need to cool to make your whole home feel comfortable?
  2. What rooms might you want to only cool?

It doesn’t make sense to cool your whole home if you only use one room at a time! If you and your family are typically just in 1 or 2 rooms, it makes sense to target your cooling there. You’ll save tons on energy bills compared to cooling your whole home.

If you do want your whole home cooled, focus on covering the larger rooms. Especially the downstairs, and any main bedrooms. Remember that, typically, 4 indoor units can be handled by 1 outdoor unit.

Once you have an idea of what you need, always consult a specialist before going ahead with a whole-home system.

To get the highest efficiency you can, you’ll want to find the optimum size of mini-split unit for each room. So let’s consider…

What Size of Ductless Air Conditioners Do I Need?

The power you need from your mini-split unit depends on the size of your room. As a quick rule of thumb, you can estimate the requirement by doing the following:

Area of Room in Feet (Length x Breadth) x 25 = BTU Required

For example, a 18′ x 14′ room roughly needs 18 x 12 = 252sq.ft. 252 x 25 = 6,300 BTU per hour of cooling.

As a quick rule of thumb, follow the table below:

Room Size
(sq. feet)
Cooling Required
Up to 2506000
Up to 3007000
Up to 3508000
Up to 4009000
Up to 45010000
Up to 55012000
Up to 70014000
Up to 100018000

However, there are some adjustments to be made!

The above ratings are rough estimates for a standard room with standard height ceilings. It’s not exactly a cover-all!

Take a look at these extra factors below, when figuring out the power you need for each room:

MiniSplitAirConditioner BTU How Much You Need
This great diagram by Senville.

In-case you can’t quite read it – adjust your BTU requirement by the following:

  1. Ceiling Height. If your room extends above 8 foot (2.5m) high, add 20% to your requirement.
  2. Insulation. If your space isn’t fully insulated, add an extra 30% to your requirement.
  3. Climate. If you’re living in a hot climate (often above 90F / 32C), add 30% to your requirement.
  4. Bonus – Shade. If the space is generally shaded and you’re not in a hot climate, you could take 10-20% off the requirement.

Again, always confirm with a specialist before you proceed with buying and installing an HVAC system!

How Much Does A Ductless Air Conditioning System Cost?

I hate this analogy – but how long is a piece of string! System costs can vary from anywhere between a few hundred dollars and over $10,000. This depends on factors like:

  1. Are you just buying an indoor unit? ($200-$500).
  2. Do you also need an outdoor unit? ($700+)
  3. Do you need multiple indoor units? ($1,000+)
  4. Do you need a contractor to install the mini-split system, and connect them together? ($1,600+).

The best way to find out is to figure out what you need, and contact your local HVAC specialists. Or, alternatively, shop around online using websites like Amazon. They often also give you an estimated rate for installation, too.


Mini-Split Air Conditioners are a genuine revolution in the HVAC industry. Instead of complicated, maintenance-prone ducting, modern homes are often being built around ductless systems instead.

I hope this guide has helped you understand how they work, and what you need if you’re looking for a system of your own.

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Have a great day!


I've been helping homeowners with appliance repair since 2016. Starting out as an enthusiastic amateur, I've since worked with many Appliance, HVAC, and DIY experts over the last 7+ years. My mission is to help fix your appliances and prevent future issues - saving you stress, time, and money. Visit my author page to learn more! Read more