HVAC Excellence AC cheat sheet

Refrigerant states: saturated vapor sits at the evaporator outlet; saturated liquid sits at the condenser outlet; subcooling is liquid below saturation temperature.

Superheat = actual suction line temperature minus evaporator saturation temperature; measured at the evaporator outlet with the system stabilized.

Subcooling = condenser saturation temperature minus liquid line temperature; a normal TXV target is roughly 8-12 °F at design conditions.

Ohm's law and power: E = I × R; P = E × I; single-phase watts = volts × amps × power factor.

Capacitor check: microfarad reading within ±10 percent of nameplate is acceptable; bulged or leaking capacitors are replaced, not re-rated.

Locked-rotor amps typically run 5-7 times rated load amps; a compressor drawing LRA with no rotation indicates a start or winding fault.

Nitrogen flow during brazing prevents internal oxidation; a slight positive pressure with a regulator is standard practice.

Vacuum dehydration target: below 500 microns and holding after isolation; a rising micron gauge reading signals moisture or a leak.

EPA Section 608 Type I covers small appliances; Type II covers high-pressure systems; Type III covers low-pressure; Universal covers all.

Recovery requirements: appliances under 5 lb may use system-dependent recovery; larger systems require certified recovery equipment.

Evacuation level before charging depends on appliance size and refrigerant type; manufacturer instructions govern when they are stricter.

R-410A operates at roughly 50-70 percent higher pressures than R-22 and requires manifold gauges and hoses rated for it.

R-410A is a near-azeotropic blend (R-32/R-125 at 50/50); it must be charged as a liquid to avoid fractionation.

R-22 is a hydrochlorofluorocarbon; production and import for new equipment are phased out, but recovered refrigerant may be reclaimed.

Leak detection order: electronic detector, then soap bubbles, then pressure test with dry nitrogen and a trace of refrigerant.

A refrigerant cylinder is never filled above 80 percent liquid by volume; hydrostatic expansion can rupture it.

Airflow formula: CFM = BTUH sensible ÷ (1.08 × temperature rise or drop in °F) at standard air density.

Total external static pressure is measured across the blower with both return and supply taps; compare to the fan performance table.

A PSC blower motor's speed changes with load; an ECM constant-torque motor holds torque, and a constant-CFM motor holds airflow as static rises.

Combustion analysis: steady-state efficiency derives from flue temperature, ambient temperature, and CO2 or O2 reading.

Dilution air test: 100 cubic feet of CO2-free air per cubic foot of flue gas is the classic draft-hood dilution baseline.

Heat pump balance point is where building load equals capacity; auxiliary heat covers the deficit below it.

Defrost initiation is controlled by coil temperature sensor, time-temperature, or demand; verify termination and reversing valve shift.

Thermostatic expansion valve superheat is adjusted by the stem: turning clockwise (in) reduces flow and raises superheat.

Variable refrigerant flow systems modulate compressor speed and electronic expansion valves; charge is verified by subcooling and total line length records.

Inverter-driven compressors use a rectifier and IPM module; DC bus voltage and error codes precede any parts replacement.

Chiller approach temperature = leaving chilled water temperature minus evaporating refrigerant temperature; rising approach indicates fouling.

Economizer diagnostics: verify outdoor enthalpy, damper actuator stroke, and changeover setpoint before condemning the controller.

Building automation integration relies on BACnet objects (AI, AO, BI, BO, AV) and Modbus register mapping; document points before commissioning.

Commercial kitchen hoods require make-up air balanced to exhaust; a negative space pressure starves the hood and defeats capture.

Refrigeration rack controls use floating suction and floating head pressure to cut compressor energy; verify setpoints against case product load.

Hydronic balancing: proportional balancing of branch circuits precedes pump curve adjustment; measure differential pressure at the far coil.

Assessment items must map to a stated task or competency; each item tests one objective, not several at once.

A distractor is plausible but unambiguously wrong; a key is the single best answer supported by the stated reference.

Item difficulty index p = proportion answering correctly; values near 0.30-0.70 discriminate best in most classroom tests.

Point-biserial correlation flags items where strong students miss and weak students pass; negative values signal a flawed key.

Performance-based tasks are scored with a rubric of observable criteria; inter-rater agreement is checked before scores are released.

Lab safety instruction precedes hands-on refrigerant work: goggles, gloves, ventilation, and recovery equipment in place.

Accreditation bodies evaluate programs on documented student outcomes, faculty qualifications, and adequate lab equipment.

Formative assessment informs instruction during a unit; summative assessment certifies learning at the end of it.