HVAC Excellence Master LC Refrigeration cheat sheet

Sensible heat changes temperature without a phase change (Q = 1.08 × CFM × ΔT for air); latent heat changes state at constant temperature (about 1,055 Btu per lb of water vaporized at standard conditions).

Superheat = actual suction line temperature minus saturated suction temperature at the same pressure; subcooling = saturated condensing temperature minus liquid line temperature. Both are measured with pressure converted to saturation temperature.

The four required functions in any mechanical refrigeration circuit are compression, condensation, metering (expansion), and evaporation; removing any one stops the heat pump cycle from moving heat uphill.

TXV/EEV capacity rises as evaporator load rises because the valve modulates to hold superheat; a fixed orifice (piston) flow depends on pressure difference and does not modulate.

Pressure–enthalpy diagram: compression is roughly isentropic up to discharge, condensation is constant pressure desuperheat/condense/subcool, metering is near-isenthalpic, evaporation is constant pressure and temperature.

Compressor types and typical use: hermetic reciprocating for small/medium loads, scroll for light commercial rooftop and split systems, semi-hermetic for serviceable commercial work, rotary for small residential.

Evaporator airflow rule of thumb for comfort cooling: about 400 CFM per ton; low airflow drops suction pressure and superheat and can freeze the coil, high airflow raises both.

Accessory functions: filter-drier removes moisture and acid, accumulator protects the compressor from liquid floodback, receiver stores liquid, muffler reduces discharge pulsation, oil separator returns oil to the crankcase.

Refrigerant glide is the temperature difference between bubble and dew points of a zeotropic blend; charge zeotropic systems as liquid and use dew point for superheat, bubble point for subcooling.

Nitrogen purge while brazing at roughly 1–3 psig flow; the purge prevents internal oxidation (copper oxide flakes) that later plug driers, TXV screens, and capillary tubes.

Use a heat-sink or wet rag on valves and service ports, and flow nitrogen through the tubing with the system open to atmosphere; never braze with refrigerant in the circuit.

Horizontal suction lines should slope about 1/4 inch per 10 feet toward the compressor; traps are used at the base of risers and every 15–20 feet of vertical riser for oil return.

Line sizing balances oil return and pressure drop: keep suction velocity high enough to carry oil, keep liquid-line pressure drop below about 2–3 psi, and size hot gas lines per manufacturer tables.

Evacuation target is typically 500 microns or below with a standing vacuum decay test; triple evacuation with nitrogen sweeps helps remove moisture from large or contaminated systems.

Rooftop/condensing unit clearances: follow nameplate minimums for service access and airflow, mount on a level curb or pad, and isolate vibration with properly torqued spring or neoprene isolators.

Tubing support spacing is roughly 6 feet for horizontal runs and every floor for vertical risers; use hangers that do not crush insulation and allow thermal expansion.

Brazing filler for copper-to-copper is typically BCuP (phosphorus) without flux; copper-to-brass or copper-to-steel requires silver-bearing filler with flux and post-braze flux removal.

Ohm's law and power: E = I × R, P = I × E; single-phase P = V × A × PF, three-phase P = 1.732 × V × A × PF. Locked-rotor current is typically 5–7 times running current.

Motor winding diagnosis: measure winding resistance to common on hermetic compressors; an open winding reads infinite, a shorted winding reads near zero, and a grounded winding reads continuity to the shell.

Capacitor check: use a meter with capacitance function or the microfarad scale; a bulged or out-of-tolerance run capacitor (typically ±6% for motor run) causes high current and poor starting.

Contactors and relays: a pitted or welded contactor contact keeps the compressor running or prevents starting; check coil voltage and pull-in with the control circuit energized.

Defrost and safety controls: high-pressure cutout, low-pressure cutout, oil pressure switch (typically 120-second delay), and freeze stat protect the compressor and coil; bypassing them is a diagnostic shortcut, not a repair.

Thermostat/controller wiring: R to W calls heat, R to Y calls cooling, G runs the indoor fan, and O/B reverses the reversing valve on heat pumps; keep 24 V control wiring away from line-voltage runs.

Electrical troubleshooting sequence: verify supply voltage and phasing, check fuses/breakers, then control voltage, then the load circuit; a single-phase condition on a three-phase compressor will trip internal overloads.

NEC-relevant practice: size branch-circuit conductors and overcurrent protection to the nameplate minimum circuit ampacity and maximum overcurrent protection, and bond the cabinet to the equipment grounding conductor.

Target superheat for a fixed-orifice system is set by indoor wet-bulb and outdoor dry-bulb from the manufacturer charging chart; TXV systems instead set subcooling per nameplate, commonly 8–12°F.

Weigh-in charging is the most accurate method: use the nameplate factory charge plus the additional charge per foot of line length specified for the installed liquid-line size.

Approach method for condensers: condensing temperature minus outdoor ambient; a high approach with clean coils and correct airflow points to overcharge or non-condensables.

Evaporator superheat too high with normal subcooling suggests a restricted metering device or low load; superheat too low with high subcooling suggests overcharge or a stuck-open TXV.

Superheat and subcooling must be read under steady-state conditions: run at least 15–20 minutes, with all doors/panels in place, correct airflow, and stabilized pressures and temperatures.

Airflow verification: static pressure with a manometer, temperature rise/split across the coil (about 18–22°F for comfort cooling), and blower speed taps matched to the required CFM per ton.

Refrigerant pressure corresponds to saturation temperature: R-410A at 118 psig is about 40°F evaporating; R-22 at 68 psig is about 40°F. Use a PT chart for the actual refrigerant, not a generic one.

Non-condensables show as high discharge pressure and high subcooling with a normal-to-high approach; recover, evacuate, and recharge by weight rather than adding refrigerant.

Diagnostic order: gather complaint, verify airflow and filters, measure pressures and temperatures, compute superheat/subcooling, then compare against the manufacturer chart before touching the charge.

Frozen evaporator causes: low airflow (dirty filter/coil, bad blower), low charge, restricted metering device, or a stuck-open TXV; never add refrigerant to a frosted coil before finding the cause.

Compressor won't start: check supply voltage, control voltage at the contactor coil, overload/thermal protector, run capacitor, and winding resistance; a hum with high current suggests a stuck rotor or bad capacitor.

High head pressure: dirty condenser, recirculated air, failed condenser fan, overcharge, non-condensables, or high ambient; low head pressure: low load, low ambient, or undercharge.

Low suction pressure: restricted liquid line/drier, undercharge, iced coil, or a failing TXV bulb; high suction pressure: overcharge, excess load, or a stuck-open valve.

Leak detection: electronic leak detector, soap bubbles, and UV dye; nitrogen pressure test to the manufacturer's test pressure, then evacuate to 500 microns before recharging.

Electrical/mechanical repair verification: after replacing a compressor, confirm correct rotation on three-phase, check oil level, record amps versus nameplate RLA, and recheck superheat/subcooling.

Refrigerant recovery and handling: recover into DOT-approved cylinders, never exceed 80% fill by weight, follow EPA Section 608 rules, and label recovered refrigerant.