Air conditioners (and heat pumps) employ what is called in the trade a vapor compression refrigeration cycle.  This means that a working fluid, or refrigerant, is compressed to high pressure in the system’s compressor.  The compressor is the heart, or pump, of the system.  Basically the air conditioner pumps heat from the inside of the house to the outside.  Because you won’t ever see an ice cube cool down further when it is placed on a hot plate, nature dictates the direction that heat must flow – from hot to cold.  So if it is hotter outside than it is inside, the inside won’t magically cool down further – enter the compressor.  Its job is to pump the heat from the inside to the outside.

The working fluid in the system is the refrigerant.  Older residential systems use R22, but the US government’s phaseout of this refrigerant in accordance with the Clean Air Act, it is no longer available in new systems.  These systems use R410a which is more environmentally friendly.  Manufacturers all have their trade names for this stuff, but it’s still R410a.  There are lots of different refrigerants out there, but these are the most common in stationary, residential air conditioning applications.

The refrigerant in the system is a two-phase fluid – it boils in the evaporator (inside coil) and condenses from a gas back to a liquid in the condenser (outside coil).  Any time a fluid undergoes a phase change process, i.e. changing from a gas to a liquid or from liquid to gas, far more heat is transferred than if the fluid never changed phase.

A sample schematic of an air conditioner’s refrigerant circuit is shown below.

We’ll start our review of the process in the compressor.  This device, located outside, takes cool suction gas coming from the house and squeezes it to high pressures.  Compression ratios here are about 3 to 1 with R22.  Because the gas leaves the compressor at high pressure, it is also much hotter now.

The hot, high pressure gas enters the condenser coil outside.  The fan on the outdoor unit pulls the outside air over the coil.  Even though it’s hot outside, the refrigerant gas is much hotter, so the outside air cools down the refrigerant gas.  It changes (condenses) into a liquid as it leaves the condenser coil and leaves the condenser as a warm, high-pressure liquid.

The liquid enters the inside unit still as a warm high-pressure liquid.  Just before it enters the evaporator (inside coil), it passes through a restriction.  Many older R22 systems use an orifice.  Newer 410 systems (and high-efficiency R22 systems) use an expansion valve.  Both meter, or control, the refrigerant flow.  When the warm liquid passes through the restriction, its pressure drops (again about a 3-1 ratio).   Just as when the compressor raised the pressure of the gas and it heated up, now the liquid cools down again as its pressure drops.  When it does so, some of the liquid vaporizes.  What enters the evaporator is now a low pressure, low temperature, mixture of gas and liquid.  It is quite cold at this point – right around the freezing point of water.  Since this is a cold mixture at a relatively low pressure, it is right at the boiling point and is much colder than the 75-or-so-degree air inside the space.  So, the warm air from the space is drawn over this cold coil, vaporizing (or “boiling off”) the remaining liquid refrigerant, and cooling down the air in the process.  The cool gas returns to the compressor and that completes the cycle.

The “run” command is really quite simple: the thermostat is supplied with 24 volts from the furnace or air handler’s transformer.  When the thermostat detects that the space temperature has risen above the setpoint, it closes its contacts and sends that 24-volt signal to the furnace to energize the blower relay and to the condenser outside.  This signal closes a magnetic relay outside (called the “contactor”).  When the contactor is closed, high-voltage supply (220 volts) goes to the compressor and fan motor and cooling begins.  When the thermostat is satisfied, the 24 volt control signal is cut off and everything shuts down.

Heat pumps use exactly the same refrigeration principle.  However, they have a few more parts.  One critical component is called the reversing valve.  This valve is located in the outdoor unit.  It reverses the direction of the refrigerant flow through the circuit.  These valves are typically energized when in air conditioning, which means that the unit must see a signal from the thermostat to tell it to run in air conditioning mode; otherwise, it will run in heating mode.