AR HVAC flashcards

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

Ton of Refrigeration

One ton of refrigeration equals 12,000 BTU per hour, or 200 BTU per minute, or about 3.517 kilowatts of heat removal.

Sensible Heat Formula

For standard air, sensible heat in BTU/h equals 1.08 multiplied by airflow in CFM multiplied by the dry-bulb temperature difference across the coil.

Latent Heat Formula

Latent heat load in BTU/h equals 0.68 multiplied by airflow in CFM multiplied by the moisture difference in grains per pound of dry air.

Superheat Definition

Superheat is the number of degrees a vapor is above its saturation temperature, calculated as measured suction line temperature minus saturation temperature at suction pressure.

Subcooling Definition

Subcooling is the number of degrees a liquid is below its saturation temperature, calculated as the saturation temperature at discharge pressure minus the liquid line temperature.

Micron Gauge Purpose

A micron gauge measures deep vacuum during evacuation; a proper evacuation reaches 500 microns and holds below 1,000 microns after the pump is valved off.

Manifold Gauge Hose Colors

Blue connects to the low side, red to the high side, and yellow to the center port for a vacuum pump, recovery machine, or refrigerant cylinder.

Oxygen Pressure Test Ban

Oxygen or compressed shop air must never pressure-test a refrigeration system, because mixing it with refrigerant oil can cause an explosion; use dry nitrogen instead.

Refrigeration Cycle Four Components

Compressor, condenser, metering device, and evaporator; the compressor raises pressure, the condenser rejects heat, the metering device drops pressure, and the evaporator absorbs heat.

TXV Function

A thermostatic expansion valve modulates refrigerant flow to hold a constant superheat at the evaporator outlet by sensing suction line temperature and pressure at the bulb.

R-410A Liquid Charging

R-410A is a near-azeotropic blend of R-32 and R-125, so it must be removed from the cylinder and charged as a liquid to avoid fractionation.

A2L Safety Classification

A2L is the ASHRAE class for refrigerants with low toxicity and mild flammability, such as R-32 and R-454B, which carry specific handling and charge-limit rules.

Filter Drier Purpose

A filter drier removes moisture, acid, and particulates from the refrigerant circuit; heat pumps require a bi-flow drier so it works in both flow directions.

Crankcase Heater Function

A crankcase heater warms compressor oil during the off cycle to prevent refrigerant migration and dilution, protecting the compressor from lubrication failure at startup.

Reversing Valve Function

A reversing valve changes the direction of refrigerant flow in a heat pump, swapping the roles of the indoor and outdoor coils between heating and cooling.

Refrigerant Safety Groups

ASHRAE classifies refrigerants by toxicity and flammability: A1 (R-22, R-410A) is low toxicity and nonflammable, A2L is mildly flammable, and B1 is more toxic.

Ohm's Law

Voltage equals current multiplied by resistance, or E equals I times R. Combined with P equals E times I, it lets a technician verify expected circuit readings.

Three-Phase Power Formula

Power in watts for a balanced three-phase load equals 1.732 multiplied by line voltage multiplied by line current multiplied by the power factor.

Run Capacitor Testing

The capacitor is discharged, disconnected, and measured on the meter's microfarad setting; a reading more than about 10 percent from rated indicates replacement.

PSC Motor

A permanent split capacitor motor has a run capacitor in series with the start winding and uses no start capacitor or centrifugal switch.

Locked Rotor Amps

LRA is the current drawn when the rotor cannot turn; it is many times the running current and is used to size overloads and identify a seized compressor.

Compressor Terminal Identification

The highest resistance reading is across common-to-start, the lowest across common-to-run, and the run-to-start reading equals the sum of the other two.

Voltage Imbalance Limit

On three-phase systems the voltage imbalance between phases should not exceed 2 percent; greater imbalance causes motor overheating and shortened service life.

24-Volt Control Transformer

A step-down transformer supplies 24 volts to thermostats, contactors, and safety switches, forming the low-voltage control circuit in residential equipment.

ACCA Manual J

Manual J is the accepted residential method for calculating room-by-room heating and cooling loads from construction, insulation, window, and climate data.

Manual D Duct Sizing

Manual D sizes supply and return ducts using a calculated friction rate, commonly about 0.1 inch of water column per 100 feet of equivalent length.

Airflow Per Ton Rule

Nominal cooling airflow is about 400 CFM per ton of capacity; reducing to roughly 350 CFM per ton in humid climates increases moisture removal.

Total External Static Pressure

TESP is the sum of supply and return static pressure measured across the air handler, commonly 0.5 inch of water column and checked against the fan table.

Recovery Cylinder Fill Limit

A recovery cylinder must never be filled above 80 percent of its capacity by weight, leaving room for liquid expansion and preventing hydrostatic rupture.

Nitrogen Purge While Brazing

A slow nitrogen flow displaces air inside the tubing during brazing, preventing internal oxidation that forms carbon scale and contaminates the refrigerant circuit.

Condensate Trap Slope

A condensate drain needs a trap to overcome cabinet pressure and must slope at least 1/8 inch per foot, without a lift above the drain pan outlet.

Weigh-In Charging Method

Weigh-in charging uses a refrigerant scale and the nameplate charge to add an exact amount, making it the most accurate method, especially with long line sets.