AL HVAC flashcards

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

Superheat definition

Superheat is the difference between actual suction line temperature and the saturation temperature at suction pressure; it confirms the evaporator is fully boiling liquid refrigerant.

Subcooling definition

Subcooling is the saturation temperature at liquid line pressure minus the actual liquid line temperature; it confirms the condenser has fully condensed refrigerant before the metering device.

Compression ratio formula

Compression ratio equals absolute discharge pressure divided by absolute suction pressure, both in psia. Ratios above about 10:1 raise discharge temperature and stress compressor valves.

TXV versus fixed orifice

A thermostatic expansion valve modulates flow to hold constant superheat across varying loads, while a fixed orifice varies superheat with load and is charged by the superheat method.

Natural gas manifold pressure

Most residential natural gas furnaces are set near 3.5 inches water column manifold pressure; propane units typically run near 10 to 11 inches water column.

Furnace temperature rise

Temperature rise is supply air temperature minus return air temperature measured in the same airstream; it must fall within the range printed on the furnace rating plate.

Heat pump balance point

The balance point is the outdoor temperature at which heat pump capacity equals building heat loss; below it, supplementary or auxiliary heat runs to maintain setpoint.

BTU per watt-hour

One watt-hour of electric resistance heat equals 3.412 Btu. Multiply heater nameplate watts by 3.412 to obtain the Btu per hour output.

One ton of cooling

One ton of refrigeration equals 12,000 Btu per hour, the heat needed to melt a ton of ice in 24 hours; it is the standard unit for sizing cooling equipment.

Standard cooling airflow

Nominal airflow for residential air conditioning is about 400 cubic feet per minute per ton, though humid climates sometimes use lower airflow for better latent removal.

Evaporator temperature split

The temperature difference between return and supply air across an operating evaporator coil is normally 16 to 22 degrees Fahrenheit for a properly charged system.

Ohm's law

Voltage equals current times resistance, written E equals I times R. It is used to estimate expected resistance, current draw, or voltage drop in a circuit.

Three-phase power formula

Three-phase power in watts equals the square root of three, about 1.732, times line voltage times line current times power factor.

Motor locked rotor amps

Locked rotor amps is the current a motor draws at standstill, typically about five to six times rated load amps, and appears briefly during every startup.

Run capacitor tolerance

A good run capacitor reads within about six percent of its rated microfarad value; a bulge, leaking oil, or open reading means it must be replaced.

Typical duct friction rate

Residential duct systems are commonly designed at about 0.1 inch water column of friction loss per 100 feet of equivalent duct length using the equal friction method.

Duct velocity equation

Duct velocity in feet per minute equals airflow in cubic feet per minute divided by cross-sectional area in square feet; velocity drives noise and friction loss.

External static pressure

External static pressure is the total pressure drop across the blower from return to supply, typically 0.5 inch water column in residential systems at rated airflow.

Nitrogen purge brazing

Dry nitrogen must flow through the tubing while brazing so oxygen is displaced and no carbon or oxide scale forms inside the refrigerant circuit.

Evacuation target

A system is evacuated when a calibrated micron gauge reads 500 microns or lower and holds steady for at least 10 to 15 minutes after the pump is isolated.

Phosphorus solder on copper

Copper-to-copper joints are brazed with phosphorus-bearing filler that acts as its own flux; copper joined to brass or steel needs a separate flux.

Manual J

Manual J is the ACCA method for calculating residential heating and cooling loads using envelope, windows, infiltration, internal gains, and local design conditions.

Design temperature

Design conditions are outdoor temperatures exceeded only a small percentage of hours, such as the 1 percent summer dry bulb and 99 percent winter design.

Heat loss formula

Envelope heat loss equals overall heat transfer coefficient U times area A times temperature difference; summing all surfaces plus infiltration gives total building load.

Weigh-in charging

When a system has no established charge or a new line set, refrigerant is added by weight using nameplate charge plus the per-foot allowance for tubing length.

Pressure test nitrogen

New piping is pressure tested with dry nitrogen to a level permitted by the equipment manufacturer, commonly 150 to 300 psig, and held to confirm no leaks.

Air balance

Air balancing adjusts dampers and blower speed so each room receives design airflow while total system airflow stays within plus or minus 10 percent of design.

High superheat low suction

High superheat combined with low suction pressure indicates the evaporator is starved of refrigerant, caused by undercharge, a restriction, or a faulty metering device.

Low superheat high suction

Low superheat with high suction pressure points to an overcharged system or a metering device feeding too much refrigerant into the evaporator.

Ice on suction line

Frost or ice forming on the suction line and evaporator usually means low refrigerant charge, restricted airflow, or a blocked metering device.

Recovery cylinder fill limit

A refrigerant recovery cylinder must never be filled above 80 percent of its capacity by weight, measured on a scale, to leave room for liquid expansion.

Section 608 certification

Section 608 of the Clean Air Act requires technicians who service, maintain, or dispose of appliances containing regulated refrigerants to hold EPA certification.