Air Conditioning Systems
The cycle first
The A/C questions almost always trace back to the vapor-compression cycle. The compressor raises pressure and temperature, the condenser rejects heat and subcools the liquid, the metering device drops pressure and creates flash gas, and the evaporator absorbs heat and delivers superheated vapor. Learn what failures look like at the gauges: a restricted metering device drives superheat up and suction pressure down; an overcharge shows high subcooling and high head pressure; a dirty condenser shows high head with normal superheat.
Charging and airflow
Two numbers dominate: superheat (fixed-orifice/piston systems) and subcooling (TXV/EEV systems). Superheat is suction-line temperature minus saturation temperature at suction pressure. Target superheat is not a fixed value - it moves with indoor wet-bulb and outdoor dry-bulb, so a blanket "10° superheat" answer is a trap. Airflow is the other half of the diagnosis: roughly 350–450 cfm per ton, with 400 cfm/ton as nominal design. Low airflow drops suction pressure, raises superheat, and can freeze a coil; a question that blames charge when the filter or return is restricted is testing this.
Worked example
A 3-ton split system moving 1,200 cfm (400 cfm/ton). Return air 75°F dry-bulb, supply air 56°F. Delta-T is 19°F - inside the normal 16–22°F band, so capacity and airflow are broadly sane. A 10°F delta-T points to low charge, duct leakage, or excess airflow; a 25°F delta-T points to low airflow.
Traps: sensible versus latent capacity, SEER2/EER2 rating questions, and confusing a heat pump's heating cycle with cooling.