LA HVAC flashcards

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

Lower Flammability Limit

The minimum concentration of a flammable refrigerant in air that will support combustion; A2L refrigerants such as R-32 and R-454B have LFLs near 12–14% by volume.

Lockout/Tagout

A procedure that isolates hazardous energy with a personal lock and tag before service; verify zero voltage with a CAT-rated meter after isolating the disconnect.

Nitrogen Purge

A low flow of dry nitrogen through copper while brazing; it displaces oxygen and prevents internal oxidation (scale) that would contaminate the refrigerant circuit.

Vacuum Decay Test

Pull the system to 500 microns, isolate the pump, and confirm the vacuum holds for 10–15 minutes; a rising reading indicates a leak or moisture.

Ton of Refrigeration

A cooling rate equal to 12,000 Btu per hour, 200 Btu per minute, or 3.517 kilowatts; it is the standard capacity unit for HVAC equipment.

Superheat

The difference between the actual suction line temperature and the saturated suction temperature at the same pressure; it confirms vapor leaving the evaporator.

Subcooling

The difference between the saturated liquid temperature at the liquid line pressure and the actual liquid line temperature; it confirms solid liquid at the metering device.

Thermostatic Expansion Valve

A metering device that modulates refrigerant flow to hold a set evaporator superheat, responding to bulb pressure, evaporator pressure, and spring force.

Refrigerant Glide

The temperature spread between bubble and dew points of a zeotropic blend; R-407C has significant glide while R-410A has only about 0.2°F.

Manual J Load Calculation

ACCA's residential method for sizing heating and cooling loads from envelope U-values, window solar gain, infiltration, and internal gains at design conditions.

Equal Friction Method

A duct sizing approach that assigns a constant pressure drop per unit length, commonly 0.08–0.10 in. w.c. per 100 ft, to each duct section.

External Static Pressure

The total pressure the blower must overcome in the supply and return ducts, filter, and coil; residential air handlers are typically rated near 0.5 in. w.c.

CFM per Ton

The airflow rate per ton of cooling capacity; about 400 cfm/ton is standard, 350 improves latent removal, and 450 raises sensible capacity.

Temperature Rise

Supply air temperature minus return air temperature across a furnace; it must fall within the rating plate range, commonly 35–65°F, or airflow is wrong.

Stoichiometric Air

The exact air quantity needed for complete combustion; natural gas requires about 10 ft³ of air per ft³ of gas, and burners run 25–50% excess air.

Flue CO2 Reading

The carbon dioxide percentage in flue gas; natural gas at perfect combustion is about 11.7%, and lower readings indicate excess dilution air.

Manifold Gas Pressure

The pressure at the burner manifold; typical natural gas setting is 3.5 in. w.c. and LP gas is 10–11 in. w.c., always confirmed against the data plate.

Flame Sensor Current

The microamp signal a flame rectification sensor sends to the control; readings below about 0.5 µA cause lockout on flame failure.

Section 608 Certification

The federal EPA credential required to buy or handle regulated refrigerants; venting is prohibited and recovery is required for appliances of 5 lb or more.

Superheat Charging Method

For fixed-orifice systems, measure suction pressure, convert to saturated temperature, and subtract it from suction line temperature; target 8–12°F.

Subcooling Charging Method

For TXV systems, convert liquid line pressure to saturated temperature and subtract it from liquid line temperature; target 8–12°F at design conditions.

Recovery Cylinder Limit

A refrigerant recovery cylinder may never be filled above 80% of its capacity by weight; use a scale and respect the DOT working pressure.

Ohm's Law

The relationship E = I × R, where voltage equals current times resistance; combined with P = E × I it supports circuit diagnosis and load checks.

Run and Start Windings

In a single-phase compressor, the run winding has lower resistance than the start winding; the common terminal shows the highest resistance to both others.

Run Capacitor Test

A good run capacitor measures within about 6% of its marked microfarad value on a capacitance meter; a bulged or leaking capacitor is replaced.

Three-Phase Power Formula

Watts equal volts times amps times 1.732 times power factor; a balanced motor draws equal current on all three legs within about 10%.

Condensate Trap Depth

The trap must be deeper than the fan's static pressure, typically 2–3 in., with the drain sloping at least 1/8 in. per ft toward the discharge.

Suction Line Velocity

Suction line velocity should stay between 700 and 1,200 fpm to carry oil back to the compressor without excessive pressure drop or noise.

SMACNA Seal Classes

Duct leakage standards ranging from Class A (tightest) to Class C; residential work commonly requires Class C or better on all joints and seams.

LSLBC Mechanical License

The Louisiana State Licensing Board for Contractors credential required for mechanical (HVAC) contracting above the residential threshold; proof of insurance and bonding applies.

Louisiana Mechanical Code

The state mechanical code is based on the International Mechanical Code, with parish amendments; permits and inspections are required before equipment installation or replacement.

Margin versus Markup

Margin is profit divided by price; markup is profit divided by cost. A 30% margin equals a 42.9% markup, so the two must never be interchanged in bids.