A2L/Crosswalk
A2L/Crosswalk

How a heat pump moves heat

The refrigeration cycle, the reversing valve, and where every part of a split and a mini-split lives.

Checked against the sources below on October 7, 2026

The short answer

A heat pump does not make heat and it does not make cold. It moves heat from one place to another with a refrigerant, a compressor and two coils. Cooling moves heat from the house to the outdoors; heating runs the same loop the other way and moves heat from the outdoor air into the house.

The cycle, in four parts

The refrigeration cycleRefrigerant flows in a loop: evaporator (indoors, absorbs heat) to compressor (raises pressure) to condenser (outdoors, rejects heat) to metering device (drops pressure) and back. The compressor and metering device divide the high-pressure side from the low-pressure side.LOW-PRESSURE SIDEHIGH-PRESSURE SIDEcold · indoorshot · outdoorsEVAPORATORindoor coilHEAT INfrom room airCONDENSERoutdoor coilHEAT OUTto outside airCOMPRESSORraises pressure — the only part that uses real powerMETERING DEVICETXV or piston — drops the pressurelow-pressurecold liquid + vaporlow-pressurecool vaporhigh-pressurehot vaporhigh-pressurewarm liquidNothing here makes cold. Heat is moved from the left coil to the right coil.
The refrigeration cycle. The dashed line splits the loop into a high-pressure side and a low-pressure side; it passes through the only two parts that change the pressure — the compressor and the metering device.
PartWhat it does to the refrigerantWhat you would notice
CompressorTakes in cool low-pressure vapor and squeezes it into hot high-pressure vapor.The only part that uses real power. It pumps vapor, never liquid.
Condenser coilThe hot vapor gives up its heat to the air passing over the coil and turns into liquid.Warm air blowing off the coil. Heat leaves the system here.
Metering deviceA restriction (TXV, EEV or piston) that drops the liquid from high pressure to low pressure.The line goes from warm to cold across it.
Evaporator coilThe cold low-pressure refrigerant boils, absorbing heat from the air passing over the coil.Cold coil, and water condensing on it in cooling. Heat enters the system here.

The idea that trips up beginners

The compressor does not “make cold”. It creates the pressure difference; the cold happens at the evaporator because a liquid that boils absorbs heat. Follow where the heat goes in and where it comes out and the rest of the system makes sense.

The reversing valve: what makes it a heat pump

An air conditioner can only move heat one way. A heat pump adds one part — the reversing valve — that swaps which coil the hot gas from the compressor goes to. The compressor always pumps the same direction; the valve changes where the discharge is sent.

The reversing valve: cooling mode and heating modeTwo side-by-side views of the same heat pump. The compressor always discharges hot gas the same way; the four-way reversing valve sends it either to the outdoor coil (cooling mode: outdoor coil is the condenser, indoor coil is the evaporator) or to the indoor coil (heating mode: indoor coil is the condenser, outdoor coil is the evaporator and can frost). The thermostat's O/B wire switches the valve.COOLING MODEmetering deviceindoor coilevaporator · coldoutdoor coilcondenser · hotreversingvalvedischargesuctioncompressoralways pumps the same wayHEATING MODEmetering deviceindoor coilcondenser · hotoutdoor coilevaporator · coldcan frost → defrost cyclereversingvalvedischargesuctioncompressoralways pumps the same wayThe thermostat's O/B wire flips the valve. Most brands: O, energized in cooling. Rheem/Ruud: B, energized in heating.
Same compressor, same refrigerant. In cooling the outdoor coil is the condenser; in heating the indoor coil is the condenser and the outdoor coil becomes the evaporator.

That is why “condenser” and “evaporator” are roles, not places. On a heat pump the fixed names are outdoor unit and indoor unit: in summer the outdoor coil condenses, in winter the same coil evaporates. It is also why the outdoor coil frosts in heating mode — it is the cold coil — and why the unit runs a defrost cycle to clear it.

Where the parts live in a split system

A residential split systemIndoors: the air handler holds the evaporator coil, the metering device and the blower, connected to supply and return ducts and a condensate drain. Outdoors: the condensing unit holds the compressor, condenser coil and fan. Two copper lines connect them: a small bare liquid line and a large insulated suction line. A low-voltage thermostat wire runs to the air handler.INDOORSOUTDOORSreturn ductroom air insupply ductconditioned air outAIR HANDLERor furnace + coilevaporator coilmetering deviceblowercondensatedrainthermostat24 V wireliquid linesmall · bare · warmsuction linelarge · insulated · cooldisconnect240 VcoilfancompressorOUTDOOR UNITcompressor · condenser coil · fanHeat pump: same layout, plus a reversing valve in the outdoor unit.
A ducted split system. The outdoor unit holds the compressor, the outdoor coil and the fan; the indoor unit holds the indoor coil, the metering device and the blower.
  • The two copper lines between the units are not the same. The liquid line is small and bare; the suction line (vapor line) is large and insulated.
  • The condensate drain leaves the indoor coil. Water forms there in cooling, and it has to go somewhere.
  • The thermostat talks to the equipment on 24-volt control wiring. The power that runs the compressor arrives separately, through the disconnect.

Ductless mini-split: same cycle, no ducts

A ductless mini-splitA wall-mounted indoor head conditions the room directly, with no ducts. A single wall penetration carries the refrigerant line set, the condensate drain and the power-and-communication cable to the outdoor unit, which holds the inverter compressor, the condenser coil, the fan and the electronic expansion valve. Multi-zone systems connect several indoor heads to one outdoor unit.INDOORSOUTDOORSroom air inconditioned air out — no ductsINDOOR HEADevaporator coil · blower · filterremoteno thermostat wireone 3" holeeverything passes hereIN THE BUNDLEliquid linesuction linecondensate drainpower + comms cableindoor unit is fed from outdoorsdrain outcoilfanEEVexpansion valvecompressorOUTDOOR UNITinverter — modulates, doesn't cycleMulti-zone: one outdoor unit, two to five indoor heads, one line set each.
A ductless mini-split. The indoor head hangs on the wall and blows straight into the room; the line set, the condensate drain and the power and communication cable all pass through one hole in the wall.

A mini-split is the same refrigeration cycle with the air handler shrunk to a wall-mounted head and the ducts removed. The metering device is usually an electronic expansion valve in the outdoor unit, and the indoor head is powered from outdoors. A multi-split connects several indoor heads to one outdoor unit. Most are inverter-driven: instead of switching on and off, the compressor changes speed to match the load.

Why this matters when you read a rating

On every one of these systems the efficiency rating is certified for a specific pair — this outdoor unit with that indoor unit. Change the indoor unit and the certified SEER2 changes with it. That is the whole reason the lookup lists every certified indoor match instead of one number per outdoor model.

Sources

Federal rules change. This page reports what the sources said on the date above; the source is the authority, and state or local rules can be stricter. It is reference material, not legal advice and not installation guidance.