HVAC Excellence AC flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Superheat definition

Suction line temperature minus the evaporator saturation temperature, measured at the evaporator outlet. It confirms the evaporator is fully fed with liquid refrigerant without flooding the compressor.

Subcooling definition

Condenser saturation temperature minus the liquid line temperature. It proves the condenser has fully liquefied the refrigerant and that the liquid line holds a solid column of liquid.

Ohm's law in HVAC

E equals I times R, and power P equals E times I. Technicians use these to compute expected current draw, verify winding resistance, and diagnose open or shorted circuits.

Capacitor tolerance rule

A run capacitor reading within plus or minus ten percent of its nameplate microfarad value is acceptable. Bulging, leaking, or out-of-tolerance units are replaced rather than re-rated.

Locked rotor amps

The current a motor draws when the rotor cannot turn, typically five to seven times rated load amps. Reading LRA with no rotation points to a start circuit or winding failure.

Nitrogen purge purpose

Flowing dry nitrogen through piping during brazing displaces oxygen, preventing internal scale and copper oxide that would later clog the metering device and contaminate the compressor.

Deep vacuum target

Evacuation below 500 microns that holds after the pump is isolated. A rising micron reading indicates residual moisture or an unresolved leak in the circuit.

EPA 608 Type I scope

Covers small appliances containing five pounds or less of refrigerant, including household refrigerators, freezers, and window air conditioners, and allows system-dependent recovery on many of them.

EPA 608 Type II scope

Applies to high-pressure appliances other than small appliances, such as split air conditioners and commercial refrigeration. It requires certified recovery equipment and documented evacuation levels.

R-410A pressure characteristic

A near-azeotropic blend of R-32 and R-125 operating at roughly fifty to seventy percent higher pressures than R-22, so gauges, hoses, and recovery machines must be rated for it.

Liquid charging of blends

Near-azeotropic blends like R-410A must be charged as liquid from the cylinder. Vapor charging can fractionate the mixture and leave the system with an incorrect composition.

Cylinder fill limit

Refrigerant cylinders are never filled above eighty percent liquid by volume. Leaving vapor space allows for thermal expansion and prevents hydrostatic rupture of the cylinder.

Leak detection sequence

Start with an electronic detector, confirm with soap bubbles, then pressure test with dry nitrogen and a trace of refrigerant. Each step narrows the location before repair.

Sensible airflow formula

CFM equals sensible BTUH divided by 1.08 times the temperature difference in Fahrenheit. It converts measured air temperature change into airflow at standard air density.

External static pressure

The total pressure the blower works against, measured across the return and supply taps. It is compared to the fan performance table to confirm delivered airflow.

ECM constant CFM motor

An electronically commutated motor programmed for constant airflow increases torque as static pressure rises. It maintains delivered CFM within limits instead of slowing like a PSC motor.

TXV superheat adjustment

Turning the adjusting stem clockwise reduces refrigerant flow and raises superheat; counterclockwise increases flow and lowers superheat. Adjust in small increments and allow stabilization.

Heat pump balance point

The outdoor temperature where heat pump capacity equals building heat loss. Below it, auxiliary or emergency heat must make up the difference to hold indoor temperature.

Defrost termination check

Verify the coil sensor, timer, and board terminate defrost before the coil overheats, and that the reversing valve shifts properly on demand defrost systems.

Dilution air baseline

The classic draft-hood standard of one hundred cubic feet of carbon-dioxide-free dilution air per cubic foot of flue gas, used to interpret combustion test readings.

VRF charge verification

Variable refrigerant flow systems are charged using subcooling targets plus recorded total and branch pipe lengths. Weigh-in per manufacturer tables is the primary method.

Inverter compressor module

The drive rectifies AC to a DC bus and switches an intelligent power module to vary compressor speed. Check bus voltage and stored fault codes before replacing parts.

Chiller approach temperature

Leaving chilled water temperature minus evaporating refrigerant temperature. A rising approach signals fouling in the evaporator tubes or non-condensable gases in the circuit.

Economizer changeover

The outdoor condition at which the economizer enables free cooling. Verify enthalpy or dry-bulb setpoint, actuator stroke, and damper linkage before blaming the controller.

BACnet object types

Analog input, analog output, binary input, binary output, and analog value objects carry points between devices. Document each point and its instance number before commissioning integration.

Floating suction control

A refrigeration rack strategy that raises suction pressure to the highest level that still holds case product temperature, cutting compressor energy while protecting product quality.

Item difficulty index

The proportion of examinees answering an item correctly. Values around 0.30 to 0.70 usually discriminate best between stronger and weaker candidates on classroom tests.

Point-biserial flag

A correlation between item score and total test score. A negative value means strong students missed it while weak students passed, usually indicating a miskeyed or ambiguous item.

Distractor requirement

A wrong answer option must be plausible to someone with incomplete knowledge yet unambiguously incorrect. Only one option may be defensible as the best answer.

Formative versus summative

Formative assessment is given during instruction to guide teaching adjustments. Summative assessment comes at the end of a unit or course to certify achievement.

Rubric inter-rater agreement

Performance tasks are scored against observable criteria, and multiple raters score sample work first. Agreement is confirmed before scores are released to candidates.

Lab safety prerequisite

Before any hands-on refrigerant work, students don goggles and gloves, confirm ventilation, and set up recovery equipment. Safety instruction precedes tool use in every lab session.