Refrigerant
Superheat
Vapor temp above its saturation temp at that pressure.
Measured at the suction line near the compressor. Target 8–12°F on fixed-orifice systems; TXV systems target 8–15°F.
Subcooling
Liquid temp below saturation at that pressure.
Measured at the liquid line at the condenser outlet. Target 8–12°F on most residential TXV systems.
TXV
Thermostatic expansion valve — meters refrigerant to evaporator.
Maintains constant superheat by modulating flow based on evaporator outlet temp/pressure via a bulb strapped to the suction line.
EEV
Electronic expansion valve — stepper-motor metering device.
Piston / fixed orifice
Passive metering device sized to a specific tonnage.
Reversing valve
4-way valve that swaps evaporator/condenser for heat pump mode.
Filter-drier
Removes moisture and debris from the liquid line.
Always replace when the system is opened. Suction-line drier after a burnout.
Accumulator
Suction-line trap that prevents liquid slugging on heat pumps.
Micron
Unit of vacuum. Evacuate to <500 µm and hold.
R-410A
HFC blend (~2088 GWP) — phasing down under AIM Act.
R-32
Lower-GWP (~675) A2L single-component refrigerant.
R-454B
Low-GWP (~466) A2L blend replacing R-410A on many new systems.
A2L
Mildly flammable refrigerant class — requires updated tools and leak detection.
Airflow
MER-V
Filter efficiency scale 1–16. Residential typically MERV 8–11.
ESP
External static pressure — pressure the blower must overcome.
Measured across the air handler (return + supply). Target ≤0.5" WC total on residential.
CFM
Cubic feet per minute of airflow. Target 350–400 CFM/ton cooling.
Manual J
ACCA residential load calculation.
Manual D
ACCA duct-sizing procedure.
Manual S
ACCA equipment-selection procedure.
Electrical
ECM
Electronically commutated motor — variable-speed DC blower.
PSC
Permanent split capacitor — older fixed-speed motor.
Dual-run capacitor
Two capacitors in one can — fan and compressor.
Hard-start kit
Adds start capacitor + potential relay to help weak compressor start.
Contactor
24 V-controlled relay switching line voltage to the compressor.
MCA / MOP
Minimum Circuit Ampacity / Maximum Overcurrent Protection on nameplate.
Wire per MCA; size breaker at or below MOP.
Megger
Insulation resistance tester (500 V DC). <0.5 MΩ = grounded compressor.
Controls
24 V control voltage
R (24 V hot), C (common), Y (cool), W (heat), G (fan), O/B (reversing valve).
C-wire
Common return that powers smart thermostats.
Defrost board
Times/initiates outdoor coil defrost on heat pumps.
Sequencer
Time-delay relay staging electric heat strips.
Combustion
HSI
Hot-surface igniter (silicon-carbide or silicon-nitride).
Flame rectification
Flame conducts a small DC current proving flame is present.
Draft inducer
Fan that pulls combustion products through the heat exchanger.
Pressure switch (gas)
Proves inducer draft before allowing ignition.
General
SEER2 / EER2
New (2023) US cooling efficiency ratings with M1 external static.
HSPF2
Heating Seasonal Performance Factor v2 — heat pump seasonal efficiency.
COP
Coefficient of performance — instantaneous efficiency (heat out / work in).
Ton of cooling
12,000 BTU/hr.
Balance point
Outdoor temp where heat pump capacity = building load.