CA C-20 HVAC flashcards

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

Manual J load calculation

A room-by-room method that estimates heating and cooling loads from building construction, orientation, windows, occupants, and climate data to size HVAC equipment.

400 CFM per ton

A nominal cooling airflow rule: one ton of cooling capacity (12,000 Btu/h) generally requires about 400 cubic feet per minute, though latent load and equipment data may require 350 to 450 CFM.

Available static pressure

The blower's rated external static pressure minus pressure losses from filters, coils, registers, grilles, and other components; it is the pressure budget used to select duct size and friction rate.

Sensible heat airflow formula

CFM equals sensible heat load in Btu/h divided by 1.08 times the entering-to-leaving air temperature difference; used to estimate required airflow for a space.

Duct friction rate

The pressure loss per 100 feet of duct, typically near 0.1 inch water column for residential design, used with an equivalent-length layout to size ducts from a duct calculator.

SMACNA duct gauge

A sheet metal thickness standard where smaller gauge numbers mean thicker metal; required gauge depends on duct size, span, pressure class, and support spacing.

Duct leakage test

A pressurization test that measures airflow lost through duct joints and seams at a specified pressure, commonly required by California Title 24 for new or altered duct systems.

Flexible duct sag

Flex duct should be supported so it stays straight and fully extended; excessive sag or sharp bends increase resistance and reduce delivered airflow.

Superheat

Suction line temperature minus the refrigerant's saturation temperature at the same suction pressure; it confirms vapor leaving the evaporator and protects the compressor from liquid.

Subcooling

Liquid line saturation temperature at the liquid pressure minus the actual liquid line temperature; it indicates how much liquid refrigerant has cooled below condensing temperature.

TXV operation

A thermostatic expansion valve modulates refrigerant flow to maintain a preset evaporator superheat, responding to bulb temperature and evaporator pressure.

Compression ratio

Discharge absolute pressure divided by suction absolute pressure; high ratios increase compressor heat and reduce capacity, often indicating low suction or high condensing pressure.

Furnace temperature rise

Supply air temperature minus return air temperature; it must fall within the furnace nameplate range and is used with Btu output to calculate airflow.

Heat pump balance point

The outdoor temperature at which heat pump heating capacity equals the building heating load; below it, supplemental or auxiliary heat is needed to maintain indoor temperature.

Defrost cycle

A heat pump mode that temporarily reverses refrigerant flow or stops the outdoor fan to melt frost from the outdoor coil, usually energizing auxiliary heat to avoid cold supply air.

AFUE

Annual Fuel Utilization Efficiency, a rating that estimates the percentage of fuel energy converted to useful heat over a heating season; higher values indicate greater efficiency.

MCA and MOP

Minimum Circuit Ampacity is the minimum wire ampacity required; Maximum Overcurrent Protection is the largest breaker or fuse allowed, both marked on the equipment nameplate.

Ohm's law

Voltage equals current times resistance (V = I × R); power in a resistive circuit equals voltage times current (P = V × I).

Run capacitor

A capacitor connected continuously in a motor circuit to improve phase angle, starting torque, and running efficiency; a bulged or failed capacitor can prevent the motor from starting.

Thermostat R terminal

The R terminal supplies 24-volt control power to the thermostat; connecting R to W calls for heat, R to Y calls for cooling, and R to G calls for fan operation.

Nitrogen purge brazing

A slow flow of dry nitrogen through the tubing during brazing prevents internal oxidation and scale, which can contaminate refrigerant circuits and damage valves and compressors.

Evacuation hold test

After evacuating to 500 microns or less, isolate the pump and watch the micron gauge; a rapid rise indicates a leak or moisture, while a slow rise may indicate normal outgassing.

Recovery cylinder fill limit

A refrigerant recovery cylinder must not be filled above 80 percent of its capacity by weight, leaving room for liquid expansion and reducing hydrostatic pressure risk.

Weigh-in charging

Charging refrigerant by weight using a scale and the manufacturer's required charge, adjusted for line-set length and component volume; it is the most accurate method when the full charge is known.

Superheat charging method

For fixed-orifice systems, adjust charge until measured superheat matches the manufacturer's target for current indoor wet-bulb and outdoor dry-bulb conditions.

Subcooling charging method

For TXV systems, adjust charge until measured subcooling matches the manufacturer's target, typically using liquid line pressure and temperature at stable operating conditions.

Low suction, high superheat

A diagnostic pattern often caused by undercharge, a restricted liquid line or metering device, or insufficient evaporator airflow; verify airflow and pressures before adding refrigerant.

High head pressure

A condition often caused by a dirty condenser coil, high outdoor ambient, noncondensable gas, or overcharge; check condenser airflow and subcooling before adjusting charge.

Preventive maintenance visit

A scheduled service that includes cleaning coils, changing filters, checking belts and bearings, tightening electrical connections, measuring amp draw, and verifying refrigerant charge and airflow.

Lockout tagout

A safety procedure that isolates electrical or other hazardous energy, applies a personal lock and tag, and verifies de-energization before service work begins.

Refrigerant oxygen displacement

Refrigerant vapor is heavier than air and can collect in low areas, displacing oxygen; use ventilation and a refrigerant monitor in confined spaces.

Carbon monoxide testing

Combustion analysis measures carbon monoxide and other flue gases; high CO or spillage indicates incomplete combustion or venting problems and requires correction.