How a car cools, heats, dries and moves the air inside the cabin
Comfortable cabin air is really two separate jobs sharing one box under the dash. Air conditioning is a closed refrigerant loop — a compressor, a condenser, an expansion valve and an evaporator — that moves heat out of the cabin, and as a side effect wrings the moisture out of the air, which is why it also defogs a windshield. Heating is a different job entirely: on a combustion engine it is nearly free, borrowed from coolant that is already hot; on an electric car there is no hot engine to borrow from, so heat has to be made on purpose.
Not the same as engine cooling
This page covers cabin comfort: the refrigerant loop, the blower, the vents and the electronics that control them. Keeping the engine or the battery pack itself at the right temperature — radiators, coolant pumps, thermostats — is a different job, covered in its own section.
From a single dial to a system that reads the sun and argues with itself over two zones
The simplest and cheapest setup: a temperature dial, a fan-speed dial and buttons for recirculation and the compressor clutch. Nothing reads the cabin temperature or adjusts itself — you set the blend of hot and cold air by feel and change it yourself as conditions change.
Components
You set one target temperature and the control unit does the rest: it reads the cabin and outside temperature, blends hot and cold air, chooses blower speed, and cycles the compressor on its own to hold that number — one setting for the whole cabin.
Components
The same automatic logic, duplicated: separate blend doors and sometimes separate blower paths let the driver and front passenger each set their own temperature, with a third zone for the rear seats — more actuators and ducting for a genuinely independent result.
Components
The same refrigerant loop, made reversible with an extra valve. Instead of only rejecting cabin heat outside, it can run backwards and pull heat out of the outside air — even cold air — to warm the cabin using far less energy than a plain electric heater. Increasingly standard on electric cars because it stretches winter range.
Components
Cooling works the same everywhere; heating is where the vehicles diverge
Heating is essentially a free byproduct: coolant on its way to the radiator is diverted through a small second radiator, the heater core, and the blower pushes cabin air across it. There is so much waste heat available that regulating it is the only real design problem.
The engine still supplies a heater core's worth of hot coolant, but it now shuts off at every stop and while cruising on electric power alone, so many hybrids add a small electric heater to cover the gaps. The refrigerant compressor is often electrically driven so air conditioning still works with the engine off, and the same loop may also help manage the traction battery's temperature — though the battery's own cooling hardware belongs to a different section.
No hot engine to scavenge, so heat has to be made on purpose — either with a simple resistive PTC heater or, increasingly, a heat pump, because heating can otherwise be one of the biggest single drains on winter range. The same electrically driven compressor and refrigerant lines often extend into the battery pack's cooling circuit to keep it in its ideal temperature window, though that hardware is covered separately.
The loop that moves heat, and the box that moves air
Click a node to see its description in the cards below
The heart of the loop. It draws in low-pressure refrigerant gas and squeezes it into a hot, high-pressure gas, which is what lets the rest of the loop reject that heat outside. Belt-driven on most combustion cars, electrically driven on hybrids and EVs so it can run with the engine off.
A radiator for refrigerant instead of coolant, mounted at the very front of the car to catch oncoming airflow — often right in front of the engine radiator. The hot high-pressure gas gives up its heat to the outside air here and condenses into a liquid.
Sits inside the HVAC box behind the dashboard. The now-liquid refrigerant expands and boils back into a gas here, and boiling absorbs heat from the cabin air the blower pushes across it. Water vapour in that air condenses on the cold fins too, which is why air conditioning also dries the air.
A precisely sized restriction that meters exactly how much liquid refrigerant reaches the evaporator, creating the pressure drop that lets it expand and boil. A receiver-drier alongside it stores spare refrigerant and scrubs out moisture that would otherwise freeze and block the system.
A four-way valve found on heat-pump systems that swaps which heat exchanger acts as condenser and which acts as evaporator. Flip it and the loop that once cooled the cabin instead pulls heat out of the outside air to warm it — the same hardware, running in reverse.
A squirrel-cage fan that pulls in outside or recirculated air and pushes it across the evaporator and heater core into the cabin. Its speed is what most people call the fan setting, whether chosen by hand or set automatically to hold a target temperature.
A pleated paper or activated-carbon filter sitting in the intake path, usually behind the glovebox. It traps dust, pollen and soot before they reach the evaporator or the cabin, and a clogged one is one of the most common causes of weak airflow.
A set of blend and mode doors inside the HVAC box route air to the face vents, footwells or windshield in any combination, and mix hot air from the heater core with cold air from the evaporator to hit the exact temperature requested — the real mechanism behind the temperature dial.
The electronic brain behind an automatic system. It reads cabin, outside and sunlight sensors, decides blower speed, blend-door position and compressor cycling many times a second, and holds the temperature you asked for regardless of how hot the day gets.
Watches the incoming air for exhaust fumes or high pollution and automatically switches to recirculation to keep them out — for example stuck in traffic behind another car. Higher-end systems add a particulate or CO2 sensor to also trigger fresh air when the cabin itself needs it.