The single most important idea in the trade — and over 40% of your exam comes back to it. Play with the diagram, then test yourself.
Follow the refrigerant around the loop. Cold, low-pressure side is blue; hot, high-pressure side is orange.
Refrigeration doesn't create cold — it moves heat. Heat naturally flows from hot to cold; a fridge, an A/C, a walk-in cooler all do the same trick: they force heat to move from a place you want cold to a place you don't care about. The refrigerant is the delivery truck that carries that heat.
For a saturated refrigerant (liquid + vapor together), raising the pressure raises its boiling temperature; dropping the pressure lowers it. That's the whole game.
A liquid boiling to vapor absorbs a lot of heat at constant temperature (latent heat). A vapor condensing back releases it. We boil where we want to absorb heat, condense where we want to dump it.
Use the diagram above as you read.
Cold, low-pressure liquid enters the indoor coil. Because it's at low pressure it boils at a low temperature — colder than the room air — so heat flows from the air into the refrigerant and it boils off. We let it pick up a little extra heat past that point — superheat — which guarantees only vapor, never liquid, reaches the compressor.
It squeezes low-pressure vapor into a high-pressure, high-temperature vapor (rule 1). Now the refrigerant is hotter than the outdoor air — exactly what the next step needs. The compressor is also the line between the low side and the high side.
Hot high-pressure vapor hits the outdoor coil. Being hotter than outside air, heat flows out and the vapor condenses to a high-pressure liquid. We cool it a bit further — subcooling — so only solid liquid, no bubbles, reaches the metering device.
A deliberate restriction (expansion valve or capillary tube). High-pressure liquid squeezes through and drops to low pressure — and low pressure means a low boiling point, so it's instantly cold again and flashes back into the evaporator. Loop closed.
Every fault — low charge, a restriction, a dirty condenser, a weak compressor — shows up as pressures and temperatures that don't match this picture. Read the loop, read the gauges, find the fault. That's most of your service exam.
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