What Is a Mini Split? How Ductless Heat Pumps Work

A mini split — short for "ductless mini-split heat pump" — is one of the most popular ways to add efficient heating and cooling to a home, especially one without ductwork.

If you've seen a slim white unit mounted high on a living-room wall with a small box outside, that's a mini split. Here's what it is, how it works, and when it's the right tool.

What is a mini split?

A mini split is a type of heat pump that delivers heating and cooling without ducts. Instead of pushing conditioned air through a network of ducts, it conditions the air right in the room. A system has two main parts:

• An outdoor unit — the compressor and condenser, about the size of a small AC unit, mounted outside.
• One or more indoor units (heads) — wall-, ceiling-cassette-, or floor-mounted, each blowing conditioned air directly into a room.

They're joined by a line set — thin refrigerant tubing, a power cable, and a condensate drain — that runs through a small (≈3") hole in the wall. A single-zone system has one indoor head; a multi-zone system runs several heads off one outdoor unit, each with its own thermostat.

How does a mini split work?

Like every heat pump, a mini split moves heat rather than creating it. Refrigerant circulates between the indoor and outdoor units:

• In winter, it absorbs heat from the outdoor air (yes, even cold air holds usable heat), and the compressor concentrates it and releases it indoors.
• In summer, the cycle reverses — it pulls heat from inside your home and dumps it outside, acting as an air conditioner.

Because it moves existing heat instead of burning fuel or running a resistance element, it delivers 2.5 to 4 units of heat for every unit of electricity (its COP), which is why mini splits are so cheap to run compared with electric baseboards or a resistance furnace. Modern inverter-driven compressors ramp smoothly up and down instead of cycling on and off, which keeps temperatures steady and efficiency high.

What types of indoor head can you get?

The wall-mounted head is what most people picture, but it isn't the only option — and the alternatives solve the "I don't want a box on my wall" objection.

The slim-duct option is the one homeowners most often don't know exists. It gives you a conventional supply grille instead of a visible head, at the cost of a little efficiency and somewhere to hide the unit.

What electrical does a mini split need?

This is where mini split installs most often go wrong, and it's worth knowing before the quote.

• A dedicated circuit. Most residential mini splits run on 208/240V single phase; some small single-zone units are 120V. Either way the outdoor unit gets its own circuit — it does not share.
• The nameplate sets the sizing, not a rule of thumb. HVAC equipment is sized from two values printed on the outdoor unit: MCA (minimum circuit ampacity), which sets the conductor size, and MOCP (maximum overcurrent protective device), which caps the breaker. The breaker is chosen to the nameplate, not to the wire.
• A disconnect outside. NEC 440.14 requires a disconnecting means within sight of and readily accessible from the outdoor unit — the small pull-out box on the wall beside it.
• On a multi-zone system, the indoor heads are usually powered from the outdoor unit over the interconnecting cable, so they don't each need a circuit.

If your panel is tight, this is the moment to run the numbers — see how to do a home electrical load calculation.

How is a mini split installed?

A single-zone install is typically a one-day job:

1. The indoor head's mounting plate goes up, and a ~3-inch core hole is drilled through the wall, sloped slightly downward to the outside.
2. The outdoor unit is set on a pad or wall bracket, raised above expected snow depth in cold climates.
3. The line set — two copper refrigerant lines, the interconnecting cable and the condensate drain — is run through the hole and connected at both ends.
4. The electrician lands the circuit and the disconnect.
5. The line set is evacuated with a vacuum pump before refrigerant is released into it.

That last step is the one quality check worth asking about. Air and moisture left in the lines degrade capacity and shorten compressor life, so a proper install pulls a deep vacuum and holds it to prove there's no leak. An installer who "purges" the lines with refrigerant instead is cutting the corner that matters most.

The condensate drain should run by gravity to somewhere sensible outdoors. Where gravity isn't available, a small condensate pump does the job — it's a normal solution, but it's one more thing that can fail and flood.

What maintenance does a mini split need?

Very little, but it isn't zero — and the neglected ones develop a distinctive musty smell.

• Rinse the indoor filters roughly monthly in season. They're washable mesh, and a clogged filter costs both capacity and efficiency.
• Clean the indoor coil and blower wheel periodically. Biofilm on the evaporator is what produces the sour, musty odor people describe as "dirty sock" — it's a cleaning problem, not a fault.
• Keep the outdoor coil clear of leaves, grass clippings and snow drift.
• Check the condensate line yearly for blockage, and the pump if you have one.

What are the disadvantages of a mini split?

What does a mini split cost?

Pricing scales mostly with the number of zones and the capacity you need:

• Single-zone: roughly $3,000–$5,000 installed.
• Multi-zone: $5,000–$15,000+, rising with each indoor head.

For a tailored figure, the heat pump cost calculator breaks down equipment, labor, and a cost-by-type comparison (a mini split is the "ductless" option). To get the capacity right first, the heat pump sizing calculator estimates the tons/BTU your space needs — oversizing and undersizing both hurt comfort and efficiency.

Is a mini split right for you?

A mini split is an excellent fit when you:

• Have a home or room without ducts (additions, garages, sunrooms, older houses).
• Want zoned control — different temperatures in different rooms.
• Want to replace expensive electric baseboard heat with something far cheaper to run.
• Want one system that heats and cools.

If your home already has good ductwork and you want to condition the whole house from one system, a ducted (central) heat pump may be simpler — see our ductless vs central heat pump comparison. And to understand the running-cost advantage in dollars, read heat pump vs electric resistance heat. Whatever you choose, size it with a Manual J load calculation from a qualified installer before you buy.