Heating, Ventilation, and Air Conditioning (HVAC)
Fundamentals
Know the refrigeration cycle: compressor raises pressure and temperature, condenser rejects heat, expansion device drops pressure, evaporator absorbs heat. Identify system hardware - compressor and clutch, condenser, receiver-drier (expansion-valve systems) or accumulator (orifice-tube systems), expansion valve or orifice tube, evaporator, blower, blend door, and heater core. Refrigerants must never be vented; recover, recycle, evacuate, and recharge by weight.
Reading the gauges
- Both sides low: undercharge or restriction upstream.
- Both sides high: overcharge, non-condensables, or poor condenser airflow.
- High side high, low side low: restriction at the expansion device or orifice tube.
- High side high, low side high: compressor or clutch issue, or overcharge.
- Low side cycling or pulling vacuum: low charge or a plugged screen.
Traps
A system that cools poorly at idle but fine at speed usually has a condenser airflow or fan problem, not a charge problem. An evaporator freezing up points to a cycling switch, thermistor, or blend door - not low refrigerant. Heat complaints are often coolant flow (blend door, heater valve, air pocket) rather than the core itself.
Worked example
Ambient 90°F, center vent 62°F - only 28°F of drop, which is weak (typical target is roughly 30–40°F below ambient). High side reads low and low side reads low, so recover and weigh the charge before condemning the compressor. Add refrigerant to nameplate weight, then recheck.