HVAC Excellence 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 saturation temperature at the measured suction pressure; used to verify charge on a fixed-orifice system and to check TXV operation.

Subcooling definition

Liquid line temperature subtracted from the saturation temperature at the liquid line pressure. Measured at the condenser outlet, it verifies charge on a TXV system.

R-410A liquid charging

R-410A is a near-azeotropic blend and must be removed from the cylinder as a liquid; vapor charging can fractionate the blend and change its composition.

Recovery cylinder fill limit

Cylinders may be filled to no more than 80 percent of their capacity by weight, so liquid expansion at high ambient temperature does not hydrostatically rupture the cylinder.

EPA 608 evacuation levels

Targets are 0 inches Hg vacuum for pre-1993 or under-5-pound systems, 10 inches for 5 to 25 pounds, 15 inches for over 25 pounds, and 20 inches for very-high-pressure refrigerants.

Ohm's law formula

E = I × R, meaning voltage equals current times resistance. Rearranged, I = E ÷ R and R = E ÷ I, which lets you solve for an unknown circuit value.

Run capacitor tolerance

A run capacitor is acceptable when its measured microfarad value is within about plus or minus 10 percent of the nameplate rating; bulging or leaking means replacement.

Thermostat terminal letters

R supplies 24-volt power, W calls for heat, Y calls for cooling, G runs the indoor fan, and O or B energizes the reversing valve on a heat pump.

Voltage imbalance limit

Three-phase supply voltage imbalance should not exceed 2 percent; divide the greatest deviation from average by the average and multiply by 100 to find it.

Compressor winding resistance test

On a single-phase compressor, run-to-common plus start-to-common resistance equals run-to-start; a large deviation or infinite reading indicates a damaged winding.

Complete combustion equation

Methane burns as CH4 + 2O2 producing CO2 + 2H2O. Each cubic foot of natural gas needs roughly 10 cubic feet of air for stoichiometric combustion.

Natural gas manifold pressure

Most natural gas appliances are set to about 3.5 inches water column at the manifold, while propane appliances typically run 10 to 11 inches water column.

Flame rectification current

A flame sensor proves ignition by passing a small DC current through the flame; readings of roughly 1 to 6 microamps indicate a healthy signal on most controls.

Natural draft flue pressure

An atmospheric burner with a natural-draft vent typically shows about 0.02 to 0.04 inches water column negative pressure in the flue, read with a digital manometer.

Furnace temperature rise

Supply air temperature minus return air temperature, compared against the nameplate range of about 35 to 65 degrees Fahrenheit; values outside that range indicate an airflow problem.

Sensible heat airflow formula

BTU/h = 1.08 × CFM × ΔT for standard air. Rearranged, CFM = BTU/h ÷ (1.08 × ΔT), which is how airflow is checked from a measured temperature change.

Fan law for static pressure

Static pressure changes with the square of the speed change, so a 20 percent increase in blower speed raises static pressure by roughly 44 percent while airflow rises only 20 percent.

Nominal cooling airflow

Roughly 400 CFM per ton of cooling capacity is the nominal design airflow; humid climates often use about 350 CFM per ton to improve moisture removal.

Velocity pressure formula

VP = (V ÷ 4005)² for standard air at 0.075 pounds per cubic foot; velocity in feet per minute is found with V = 4005 × √VP.

Blower door test pressure

A blower door measures envelope leakage at 50 pascals, and ACH50 equals CFM50 times 60 divided by the conditioned volume in cubic feet.

High performance duct leakage limit

High-performance programs commonly cap duct leakage at about 6 CFM25 per 100 square feet of conditioned floor area, measured with a duct blaster at 25 pascals.

Duct leakage to outside

A duct blaster pressurizes the duct system to 25 pascals and reads total leakage in CFM25; subtracting leakage to outside leaves leakage into conditioned space.

Tight envelope target

A well-sealed new house often targets 3 ACH50 or less, and natural infiltration can be estimated roughly as CFM50 divided by 20.

Delta T across evaporator

Air temperature drop across a wet evaporator coil is typically 15 to 22 degrees Fahrenheit, depending on indoor wet-bulb; a low drop with full charge suggests low airflow.

Condenser approach method

Approach temperature is the saturated condensing temperature minus outdoor ambient air temperature; typical values run about 10 to 15 degrees Fahrenheit at design conditions.

Sensible heat ratio

SHR equals sensible capacity divided by total capacity. Residential cooling typically runs about 0.70 to 0.75, dropping when the coil removes more moisture.

AFUE definition

Annual Fuel Utilization Efficiency is annual heat output divided by annual energy input, expressed as a percentage; non-condensing furnaces typically rate near 80 percent.

High head pressure causes

A dirty condenser coil, overcharge, non-condensable air, high outdoor ambient temperature, or a failing condenser fan motor will all push head pressure above normal.

Undercharge symptoms

High superheat with low subcooling, low suction pressure, and low compressor amp draw indicate a system short of refrigerant; confirm by weighing in the charge.

Condensing furnace venting

Furnaces rated 90 percent AFUE or higher condense flue gas in a secondary heat exchanger and vent through PVC pipe because the flue gas leaves cool enough to condense.

Non-condensable symptoms

High head pressure together with high subcooling, a bubbly sight glass, and above-normal discharge temperature points to air in the system, requiring recovery and evacuation.

SEER2 rating basis

SEER2 is seasonal efficiency measured at higher external static pressure test conditions than legacy SEER, so SEER2 values run roughly 4 to 5 percent lower for the same equipment.