How does a mini split heat pump work? Heat, cold, inverter

30-second answer

12,800 BTU/h at 5°Fverified Aug 2026

A mini split heat pump does not make heat. It moves heat, and that one fact explains both how it works and how to size it.

• It moves heat, not makes it. A refrigerant loop runs one way to cool a room and the other way to warm it, so one box on the wall does both jobs.^[1]
• Capacity falls with temperature. The MRCOOL DIY 5th Gen 18k is rated 18,000 BTU/h of heating at 47°F and a maximum of 12,800 BTU/h at 5°F.^[3]

Size for the cold number, not the nameplate.

How does a mini split heat pump move heat instead of making it?

A mini-split heat pump is an air conditioner that can run in reverse: in summer it pulls heat out of the room and rejects it outside, and in winter it pulls heat out of the outdoor air and delivers it inside. ENERGY STAR’s own explainer puts it plainly: the heat pump uses the same cycle “run in reverse” in winter to extract heat energy from the outside and transfer it into your home.^[1] The trick is pressure: refrigerant boils at a low temperature when its pressure is low, and it releases heat when its pressure is high, and the compressor in the outdoor unit is the component that sets that pressure. Here is the loop in heating mode, in four steps:

• Outdoor coil. Low-pressure refrigerant is colder than the outdoor air, even on a cold day, so it soaks up heat from that air and boils into a gas.
• Compressor. The gas is squeezed. Squeezing it raises its temperature well above room temperature.
• Indoor coil. The hot gas flows through the line set to the head on your wall. The indoor fan blows room air across it. The air warms, the gas cools and turns back into a liquid.
• Expansion valve. The liquid drops back to low pressure, gets cold again, and heads back outside to repeat.

Cooling mode is the same loop with the two coils trading jobs, because a reversing valve in the outdoor unit swaps the direction of refrigerant flow. That one component is the difference between an air conditioner and a heat pump.

If you want the parts list before the physics, the what is a mini split guide walks through the indoor head, the outdoor unit, and the line set between them.

Why does it still heat when it is cold outside?

Cold air still holds heat: air at 5°F is far warmer than the refrigerant in the outdoor coil, so heat still flows into the loop, and what changes is how much the unit can move per hour. The MRCOOL DIY 5th Gen 18k carries a rated heating capacity of 18,000 BTU/h at 47°F and a maximum of 12,800 BTU/h at 5°F.^[3] Same unit, colder air, less heat delivered.

Two things drive the drop:

• A smaller temperature gap. The colder the outdoor air, the less difference there is between that air and the refrigerant in the coil, and the less heat flows across it each minute.
• Thinner gas. Cold refrigerant gas is less dense, so each stroke of the compressor moves less of it.

Cold-climate models fight back with bigger compressors, different coils, and controls that let the compressor overspeed for a while. That is why cold-climate ratings belong to a model tier, not to a brand.

The efficiency follows the same curve. Because the unit moves heat instead of burning fuel for it, it delivers more heat energy than it draws in electricity. The NEEP listing rates the 18k unit at a COP of 3.91 at 47°F, meaning roughly 3.9 units of heat for every unit of electricity.^[3] That ratio shrinks as the outdoor air cools, which is the bill-side reason capacity at temperature matters.

ENERGY STAR now tests cold-climate heat pumps down to 5°F for its certification.^[2] When you compare models, look for that 5°F figure, or the lower one the maker publishes, rather than the headline BTU. Below 5°F, the maker’s own performance tables are the only honest source, and many standard models publish nothing for 0°F at all. Match the published capacity at your area’s design temperature against the room’s heat loss, and plan backup heat if the two do not meet.

What does “inverter” mean, and why does it matter?

An inverter-driven mini split changes the speed of its compressor instead of switching it on and off. The MRCOOL DIY 5th Gen 18k lists a DC inverter and a rotary compressor on its product page.^[4] The inverter turns your incoming 208/230V power into a variable frequency, and the compressor speed follows it. This is the component that makes a mini split feel different from a window unit, because an on-off compressor blasts at full power, overshoots, stops, and lets the room drift before it starts again, while a variable-speed compressor finds the speed that matches the room’s heat loss and holds it there — three things follow:

• Steadier temperature. The room stops swinging between too warm and too cool, because the unit is never fully off and never fully on.
• Quieter running. Most hours are spent at low speed, where both fans and the compressor are at their quietest.
• Better dehumidifying. In summer the indoor coil stays cold for longer at a gentler airflow, so more moisture condenses out of the air.

It also changes what “oversized” means, because an inverter unit can turn down only so far: put a big unit in a small, tight room and it runs at its minimum speed, short cycles anyway, and pulls little moisture out of the air. That is why sizing starts with the room’s heat load and not with the biggest unit you can afford. The sizing calculator runs the screening version of that load math in a minute, and a load calculation settles it for real.