HVAC Excellence Master AC cheat sheet

Sensible heat equation: Q = 1.08 × CFM × ΔT (°F) for air at sea level; latent heat: Q = 0.68 × CFM × Δgrains, both in Btu/h.

Total heat (enthalpy method): Q = 4.5 × CFM × Δh, where Δh is the enthalpy difference in Btu/lb of dry air.

Carnot COP for cooling = T_evap ÷ (T_cond − T_evap) with absolute temperatures (Rankine or Kelvin); real systems run at roughly 40–60% of Carnot.

Standard refrigeration cycle order: compressor → condenser → metering device → evaporator → back to compressor; superheat is measured at the evaporator outlet, subcooling at the condenser outlet.

Superheat = actual suction line temp − saturated suction temp; target 8–12°F at the evaporator outlet for TXV systems, higher for fixed-orifice (check chart).

Subcooling = saturated condensing temp − liquid line temp; TXV systems are charged to 8–12°F subcooling, fixed-orifice systems to target superheat.

Pressure–temperature relationship: saturation temperature rises with pressure, so a low suction pressure with normal superheat suggests undercharge, restriction, or low load.

1 ton of refrigeration = 12,000 Btu/h = 200 Btu/min; 1 psi of pressure ≈ 2.31 ft of water column head.

Compression ratio = absolute discharge pressure ÷ absolute suction pressure; ratios above roughly 10:1 raise discharge temperature and shorten compressor life.

Ohm's law: E = I × R; power: P = E × I = I²R = E²/R. Single-phase watts = volts × amps × power factor.

Three-phase power: P = √3 × E × I × power factor; balanced three-phase line current equals phase current in a wye load.

Locked-rotor amps (LRA) ≈ 5–6 × running-load amps (RLA) for typical single-phase compressors; a reading near LRA means the rotor is not turning.

Motor winding check: resistance across C–S plus C–R should equal S–R (within tolerance); an open winding reads infinite, a shorted winding reads near zero.

Start components: potential (voltage) relay drops out the start capacitor as back-EMF rises; current relay drops out as current falls; PTC thermistor self-heats to open.

Control sequence: thermostat calls → low-voltage signal → contactor coil → compressor/condenser fan contactor closes → safety switches (high pressure, low pressure, float) are wired in series.

Superheat/subcooling and safety limits are set in the control; low-pressure cutout typically opens around 20–25 psig on R-410A systems to protect against low charge.

Defrost controls: time-initiated, temperature- or pressure-terminated; a failed defrost termination sensor causes coil icing and long defrost cycles.

Electrical safety checks: verify 24 V transformer output, check for 24 V at the contactor coil, confirm 240 V (or 208 V) at the line side before condemning a component.

Sensible heat ratio (SHR) = sensible heat ÷ total heat; typical residential design SHR is 0.70–0.80 and drives coil selection and airflow.

Airflow rule of thumb: 400 CFM per ton for cooling (350–450 acceptable); heating airflow is typically 400–500 CFM per ton-equivalent of capacity.

Static pressure budget: total external static pressure (TESP) should stay at or below 0.5 in. w.c. for residential; 0.5–0.8 in. w.c. for light commercial.

Duct velocity targets: supply trunk 600–900 fpm, branch runs 400–700 fpm, return 500–700 fpm; velocities above 1,000 fpm cause noise and high pressure drop.

Equivalent length: add 30–50% to measured duct length for fittings; each 90° elbow ≈ 10–20 ft equivalent, each 45° elbow ≈ 5–10 ft.

Fan laws: CFM varies with RPM, static pressure with RPM², and brake horsepower with RPM³ - doubling RPM quadruples static and octuples horsepower.

Return air path matters: undersized returns starve the blower, causing high static, low airflow, and evaporator icing; check filter pressure drop (clean filter ≈ 0.10–0.20 in. w.c.).

Balancing: measure airflow at each register with a flow hood; adjust dampers so each room gets design CFM; total supply CFM should equal return CFM plus outside air minus exhaust.

Duct leakage: typical new residential duct systems leak 10–30% of total airflow; sealing to ≤5% leakage is the target for high-performance homes.

Duct R-value: R-8 is common for supply ducts in unconditioned attics; R-6 minimum in many climate zones; bare sheet metal has negligible R-value.

Duct surface area rule: exposed duct area in unconditioned space should be minimized; each 10 ft of 8-in. round duct has about 21 ft² of surface.

Envelope leakage: blower door test measures CFM50; a tight home is ≤3 ACH50, average existing home 5–8 ACH50, leaky home >10 ACH50.

Insulation R-values by climate: attic R-49 to R-60, walls R-13 to R-21, floors R-19 to R-30; check local code for the exact requirement.

Thermal bridging: wood studs reduce effective wall R-value by roughly 15–25%; continuous exterior insulation or advanced framing reduces the penalty.

Window performance: U-factor measures heat loss (lower is better), SHGC measures solar gain (lower is better in cooling climates, higher in heating climates).

Duct location: ducts in conditioned space lose the least energy; ducts in attics can lose 20–30% of cooling capacity in hot climates due to conduction and leakage.

ASHRAE 34 safety groups: A1 (lower toxicity, no flame propagation), A2L (lower toxicity, lower flammability), A3 (lower toxicity, higher flammability).

A2L refrigerants (R-32, R-454B, R-452B) have burning velocity ≤10 cm/s and minimum ignition energy >10 mJ; they are flammable but hard to ignite.

A3 refrigerants (R-290 propane, R-600a isobutane) are highly flammable; charge limits are small (typically ≤150 g for R-290 in residential appliances under IEC 60335-2-40).

A2L handling: no open flames, no smoking, use ignition-proof tools, ventilate the work area, and leak-check with an electronic detector rated for A2L.

Recovery: EPA Section 608 requires recovery of refrigerant before opening a system; do not vent. Recover to a dedicated cylinder and never overfill (80% fill rule).

Cylinder safety: store upright, below 125°F, away from heat; A2L cylinders may have a pressure relief device and require a specific recovery machine.

Leak detection: A2L and A3 refrigerants are heavier than air in many cases; check low points and use a detector calibrated for the specific refrigerant.

Labeling: A2L systems require a yellow flame symbol and a specific charge limit label; service techs must be certified under EPA 608 Type II (or universal) for high-pressure systems.

Target superheat for fixed-orifice systems: use the manufacturer's charging chart; typical values are 10–20°F depending on indoor wet-bulb and outdoor dry-bulb.

Target subcooling for TXV systems: 8–12°F at the condenser outlet; low subcooling suggests undercharge, high subcooling suggests overcharge or a restriction.

Delta-T across the evaporator: 18–22°F for a properly charged residential system; below 15°F suggests low airflow or low charge, above 25°F suggests low airflow.

Compressor amp draw: compare to nameplate RLA; a reading well below RLA may indicate low charge or a failing compressor; well above suggests high head pressure or a bad capacitor.

Condenser approach: condensing temperature minus outdoor ambient should be 10–20°F; higher approach means dirty coil, low airflow, or non-condensables.

Evaporator approach: return air wet-bulb minus evaporator saturated suction temperature; typical 15–25°F; higher approach means low airflow or dirty coil.

Airflow verification: measure external static pressure and compare to the blower performance table; a dirty filter or coil raises static and reduces CFM.

Performance test sequence: steady-state operation ≥15 minutes, then measure ambient, return wet-bulb, supply dry-bulb, suction and liquid pressures, superheat, subcooling, and amp draw.

Energy audit levels: walk-through (Level 1), energy survey and analysis (Level 2), detailed analysis of capital-intensive modifications (Level 3), per ASHRAE procedures.

Blower door and duct blaster tests quantify envelope and duct leakage; a duct blaster measures duct leakage in CFM25.

SEER2 and HSPF2 are the current efficiency metrics (2023+); EER2 is used for commercial and high-temperature applications.

Minimum efficiency: 14.3 SEER2 for split systems in most of the U.S. as of 2023; higher in the northern region.

Load calculation: use ACCA Manual J (residential) or Manual N (commercial); never size by rule of thumb (e.g., 500 ft² per ton) alone.

Equipment selection: ACCA Manual S matches equipment to the Manual J load; oversizing causes short cycling, poor dehumidification, and higher energy use.

Duct design: ACCA Manual D sizes ducts to the blower's performance curve; Manual T selects registers and diffusers.

Green awareness: refrigerant GWP values - R-410A ≈ 2,088, R-32 ≈ 675, R-454B ≈ 466, R-290 ≈ 3; low-GWP alternatives reduce climate impact but change safety classification.

Energy savings measures: air sealing, insulation upgrades, duct sealing, HVAC tune-up, programmable or smart thermostats, and equipment replacement with high-efficiency units.