HVAC Excellence Master HP flashcards

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

Sensible heat formula

Q = 1.08 × CFM × ΔT for standard air at sea level. Use it to estimate airflow from a measured temperature rise or drop across a coil.

Latent heat formula

Q = 0.68 × CFM × Δgrains. It estimates moisture removal capacity and is used with the sensible equation to find total coil load.

Superheat definition

Actual suction line temperature minus the saturated suction temperature at the same pressure. Target 8–12 °F at the evaporator outlet on fixed-orifice systems.

Subcooling definition

Saturated condensing temperature minus liquid line temperature. Target 8–12 °F on TXV systems; low subcooling suggests undercharge or a liquid line restriction.

Ton of refrigeration

One ton equals 12,000 BTU per hour, or 200 BTU per minute. A 3-ton system therefore moves about 36,000 BTU/h at rated conditions.

Standard airflow rule

Approximately 400 CFM per nominal ton. A 2.5-ton system should move about 1,000 CFM; below 350 CFM/ton risks coil freeze-up.

Pressure to head conversion

One psi equals 2.31 feet of water column. Use it to convert static pressure readings to inches of water for duct and fan calculations.

Recovery versus recycling

Recovery removes refrigerant to a container. Recycling cleans it for reuse in the same equipment. Reclaim restores it to ARI 700 purity for any system.

SEER2 versus SEER

SEER2 uses the same 0.5 in. w.c. blower static but higher external static, so SEER2 ratings run about 4–5 percent below SEER for the same equipment.

HSPF2 versus HSPF

HSPF2 uses a 5 °F colder outdoor temperature bin than HSPF, so HSPF2 values run roughly 15 percent lower for the same heat pump.

Undercharge symptoms

Low subcooling, high superheat, reduced capacity, and elevated discharge temperature. Confirm with superheat before adding refrigerant to a fixed-orifice system.

Overcharge symptoms

High subcooling and low superheat, often with elevated head pressure and compressor current. On TXV systems, subcooling is the primary charge indicator.

Low airflow effects

Below 350 CFM/ton, sensible capacity drops and the evaporator can freeze. Above 450 CFM/ton, latent removal falls and blower power rises.

Temperature split check

A tuned 3-ton split system holds 18–22 °F across the coil at 400 CFM/ton with an 80 °F return. A low split suggests low airflow or low charge.

Duct leakage limit

The 2021 IECC caps duct leakage at 4 CFM per 100 ft² of conditioned floor area at 25 Pa for new homes. Older homes often test 15–30 percent.

Economizer high limit

ASHRAE 90.1 sets 65–75 °F for differential dry-bulb changeover. Verify the setpoint with a calibrated reference thermometer before adjusting.

EPA 608 certification

Required to buy or handle ozone-depleting refrigerants. Type I covers small appliances, Type II high-pressure, Type III low-pressure, and Type IV all.

A2L refrigerant class

Lower flammability, low toxicity. R-32 and R-454B are A2L and require ignition-source controls, leak detection, and charge limits per ASHRAE 15.

Venting prohibition

Releasing refrigerant to the atmosphere is illegal. Recovered refrigerant must be recycled, reclaimed, or destroyed, and cylinders must never exceed 80 percent fill.

Leak repair deadline

For appliances with 5 or more lb charge, leaks must be repaired within 30 days of discovery. Comfort cooling over 50 lb also requires verification within 10 days.

R-22 phase-out

Production and import ended January 1, 2020. Reclaimed R-22 remains legal for servicing existing equipment but cannot be used in new equipment.

GWP values

R-410A ≈ 2,088; R-32 ≈ 675; R-454B ≈ 466; R-134a ≈ 1,430; R-1234yf < 1. The AIM Act targets an 85 percent HFC reduction by 2036.

Refrigerant PPE

Safety glasses, refrigerant-rated gloves, and a calibrated leak detector. Never use an open flame to detect leaks, especially with A2L refrigerants.

Ohm's law

V = I × R, and power P = V × I × PF. A 240 V single-phase compressor at 12 A and 0.9 PF draws about 2,592 W.

Locked rotor amps

LRA is typically 5–6 times running load amps. A compressor drawing LRA and tripping on overload indicates a seized or shorted winding.

Run capacitor tolerance

A run capacitor more than 10 percent below nameplate microfarads should be replaced. Measure with a meter rated for the capacitor's value.

Thermostat wiring calls

R to W calls heat, R to Y calls cool, R to G calls fan. A blown low-voltage fuse often follows a shorted contactor coil or miswired thermostat.

Gas furnace sequence

Call for heat, inducer proves via pressure switch, igniter energizes, gas valve opens, flame sensor proves, then the blower delays on.

Primary diagnostics

Superheat and subcooling are the primary readings. Static pressure across the coil confirms airflow, and amp draw confirms mechanical load.

True-RMS meter use

Use a true-RMS multimeter and clamp meter. Verify 24 V at the transformer secondary before condemning a control board.

Start relay check

Measure resistance across the start winding. Infinity means an open winding and zero means a short; either reading condemns the compressor.