HVAC Excellence Master LC-AC flashcards

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

Sensible heat ratio

Ratio of sensible heat removed to total heat removed by an evaporator coil. Typical comfort range is 0.70–0.80; lower values indicate more latent (moisture) removal.

Superheat

Difference between actual suction line temperature and saturated suction temperature at the same pressure. Indicates whether the evaporator is properly fed with refrigerant.

Subcooling

Difference between saturated condensing temperature and liquid line temperature. Confirms solid liquid refrigerant at the metering device; typical TXV target is 8–12°F.

Evaporator TD

Return air dry-bulb minus supply air dry-bulb across the evaporator. Typical 15–22°F; high TD suggests low airflow, low TD suggests overcharge or high airflow.

Condenser TD

Outdoor ambient temperature minus saturated condensing temperature. Typical 15–25°F; elevated TD indicates dirty coil, non-condensables, or overcharge.

400 cfm per ton

Standard airflow rule for cooling: one ton (12,000 Btu/h) of capacity requires about 400 cfm. Range 350–450 cfm/ton; low airflow risks coil freeze.

Refrigerant glide

Temperature difference between bubble point and dew point of a zeotropic blend at a given pressure. R-410A glide is about 0.2°F; R-407C is about 10°F.

Static pressure budget

Total external static pressure (TESP) measured across the air handler. Residential target is 0.5–0.8 in. w.c.; values above 0.8 indicate duct restriction.

Temperature rise

Supply air temperature minus return air temperature across a furnace. Must fall within the nameplate range (typically 35–65°F); high rise means low airflow.

Combustion air requirement

About 10 ft³ of air per 1,000 Btu of gas input for complete combustion, plus 50 ft³ per 1,000 Btu for dilution and ventilation.

Manifold pressure

Gas pressure at the burner manifold with the appliance firing. Natural gas typically 3.5 in. w.c.; LP gas typically 10–11 in. w.c.

Flue draft

Negative pressure at the flue collar of a natural-draft appliance, typically −0.02 to −0.04 in. w.c. Positive pressure indicates blocked vent or spillage.

AFUE

Annual Fuel Utilization Efficiency: seasonal efficiency rating for furnaces and boilers. Minimum 80% for standard; 90%+ for condensing appliances.

Condensing furnace condensate

Acidic water (pH ~3–5) produced by condensing furnaces. Must be neutralized before drain disposal and vented with PVC, CPVC, or polypropylene.

Electric heat output

Resistance heat produces 3,412 Btu/h per kW. Output in Btu/h equals volts × amps × 3.412, or kW × 3,412.

Sensible heat formula

Btu/h = cfm × 1.08 × ΔT for standard air. Used to calculate heating or cooling sensible capacity from airflow and temperature difference.

Combustion analysis targets

For natural gas: O₂ 4–9%, CO₂ 8–10%, CO below 100 ppm air-free, excess air 25–50%. Values outside range indicate improper combustion.

Compression ratio

Absolute discharge pressure divided by absolute suction pressure. Ratios above about 10:1 cause high discharge temperature and reduced capacity.

Evacuation standard

Evacuate a refrigeration system to 500 microns or lower and perform a standing vacuum test: rise less than 500 microns in 10 minutes.

Recovery cylinder limit

Refrigerant recovery cylinders must not be filled above 80% of their volume by liquid. Use a scale to verify weight and prevent hydrostatic rupture.

POE oil

Polyolester lubricant used with HFC and HFO refrigerants such as R-410A and R-134a. It is hygroscopic, so keep containers sealed and evacuate thoroughly.

Mineral oil

Lubricant used with CFC and HCFC refrigerants such as R-12 and R-22. It is not miscible with HFC refrigerants and must not be mixed with POE.

Superheat at compressor

Suction line temperature at the compressor minus saturated suction temperature. Typical target 20–30°F; lower values risk liquid slugging.

Nitrogen pressure test

Leak test using dry nitrogen at 150–300 psig on refrigerant lines. Never use oxygen or compressed air with refrigerant; both can cause fire or explosion.

PT chart

Pressure–temperature chart that converts saturation pressure to saturation temperature for a specific refrigerant. Essential for superheat and subcooling calculations.

Ohm's law

V = I × R. Voltage equals current times resistance. Used to troubleshoot electrical circuits and verify motor windings and heater elements.

Locked-rotor amps

Current drawn by a motor at startup with the rotor stationary. Typically 5–6 times running-load amps for PSC compressors; verify against the nameplate.

Capacitor tolerance

Run and start capacitors are typically rated ±6% of nameplate microfarads. Measure with a meter; bulged or leaking capacitors must be replaced.

Motor winding test

Run-to-common plus start-to-common resistance should equal run-to-start. Open winding reads infinite ohms; shorted winding reads near zero.

Thermostat wiring letters

R = 24V power, C = common, W = heat, Y = cool, G = fan, O/B = reversing valve, AUX/E = auxiliary or emergency heat.

Defrost control

Heat pump control that initiates defrost on accumulated compressor run time and coil temperature, and terminates on coil temperature or a time limit.

Three-phase motor current

I = (HP × 746) / (√3 × V × efficiency × PF). Used to verify actual current draw against nameplate full-load amps.