IA HVAC cheat sheet

Sensible heat ratio (SHR) = sensible heat / total heat; total = sensible + latent. Coil SHR must match or be below the space SHR to control humidity.

Dry-bulb (DB) is air temperature; wet-bulb (WB) is the lowest temperature reachable by evaporative cooling; dew point (DP) is the temperature at which air reaches saturation (RH = 100%).

Relative humidity = (vapor pressure / saturation vapor pressure at same DB) x 100; RH changes with temperature even when moisture content (humidity ratio W) is constant.

Humidity ratio W (lb water/lb dry air) = 0.622 x Pw / (P - Pw); at 70 F DB and 50% RH, W is roughly 0.0078 lb/lb.

Sensible heat: Qs = 1.08 x CFM x delta-T (standard air, ~sea level). Latent heat: QL = 0.68 x CFM x delta-W (grains/lb). Total: Qt = 4.5 x CFM x delta-h.

At altitude, replace 1.08 with 1.08 x (actual density / 0.075); standard air density is 0.075 lb/ft3 at 70 F and sea level.

Enthalpy h (Btu/lb dry air) = 0.240 x T + W x (1061 + 0.444 x T); total heat change on a coil follows delta-h between entering and leaving air.

1 ton of refrigeration = 12,000 Btu/h = 200 Btu/min = 3.517 kW; 1 kW = 3,412 Btu/h.

Psychrometric processes on the chart: horizontal right = sensible heating, horizontal left = sensible cooling, vertical up = humidification, vertical down = dehumidification, sloped line = coil process.

ACCA Manual J is the residential load procedure; Manual S selects equipment, Manual D sizes ducts, Manual T selects registers/diffusers.

Design conditions use ASHRAE 1% cooling DB / 0.4% WB and 99% heating DB for most residential work; use local code-adopted tables.

Conduction load: Q = U x A x delta-T; U = 1/R. Add solar gain through glass via SHGC: Q = A x SHGC x transmitted irradiance.

Infiltration estimate: Qs = 1.08 x CFM x delta-T with CFM from air-change rate or blower-door results; tight homes ~0.35 ACH natural, leaky ~1.0 ACH.

Internal gains: people ~250 Btu/h sensible + 200 Btu/h latent (office, light work); add appliances, lighting, and duct gains (typically 5-15% for ducts in unconditioned space).

Oversizing causes short cycling, poor latent removal, and higher humidity; target equipment capacity close to calculated load, not nameplate tonnage.

Heating load omits solar and internal gains (conservative); cooling load is calculated at design-day peak hour, not the hottest outdoor temperature.

Diversity: not all appliances/people run simultaneously; apply schedule factors per Manual J tables rather than summing worst-case.

Furnace efficiency: AFUE (Annual Fuel Utilization Efficiency); 80% category is non-condensing (vented with metal flue), 90%+ is condensing (PVC vent, condensate drain).

Gas furnace combustion air: 50 ft3 per 1,000 Btu/h input for Category I appliances in a confined space, or use outdoor air openings sized per code (NFPA 54 / IFGC).

Hydronic heating: boiler output Btu/h = GPM x 500 x delta-T; 1 GPM at 20 F delta-T delivers ~10,000 Btu/h.

Radiator/baseboard output depends on average water temperature; a 180 F average with 20 F drop is the standard rating point; lower water temperature reduces output.

Heat pump balance point = outdoor temperature where heat pump capacity equals building load; below it, supplemental (electric resistance or gas) heat runs.

Electric resistance heat: 1 kW = 3,412 Btu/h at 100% efficiency; COP = 1.0. Air-source heat pump COP typically 2-4 at mild temperatures.

Steam vs hot water: steam systems use traps and vents, operate near 212 F, and require pitch back to boiler; hot water systems use circulators and expansion tanks.

Combustion safety: measure CO in flue gas, verify draft, and check for spillage at draft hood; a cracked heat exchanger requires shutdown and replacement.

Vapor-compression cycle order: compressor (low pressure vapor to high pressure vapor) -> condenser (rejects heat, becomes liquid) -> expansion device (drops pressure, part-flash) -> evaporator (absorbs heat, becomes vapor) -> compressor.

Superheat = evaporator outlet temp - evaporating (suction saturation) temp; target 8-12 F at the evaporator for a TXV system at design conditions.

Subcooling = condensing (liquid saturation) temp - liquid line temp; target 8-15 F for TXV systems; high subcooling indicates overcharge or restricted liquid line.

TXV/EEV maintains constant superheat by modulating refrigerant flow; a fixed orifice (piston) system charges by superheat and varies with load.

Pressure-temperature relationship: at R-410A, ~118 psig suction corresponds to ~40 F evaporating; ~340 psig corresponds to ~100 F condensing (approximate).

Compression ratio = absolute discharge pressure / absolute suction pressure; ratios above ~10:1 stress compressors and reduce capacity.

Low suction pressure causes: low airflow, undercharge, restricted liquid line, or low load; high head pressure causes: dirty condenser, overcharge, non-condensables, high ambient.

Refrigerant glide: R-410A is near-azeotropic (small glide); R-407C has significant glide (~10 F), so use dew-point and bubble-point values when charging.

Duct sizing methods: equal friction (constant pressure drop per 100 ft, typically 0.1 in. w.c.), velocity reduction, and static regain.

Duct friction loss: use the Darcy-Weisbach or ASHRAE friction chart; pressure drop rises roughly with the square of airflow.

Equivalent length adds fitting losses: a 90-degree round elbow equals ~5-10 ft of straight duct; transitions and tees add more.

Return air path must be sized; undersized returns cause high static pressure and reduced airflow. Static pressure budget: ~0.5 in. w.c. total for residential (0.1 supply + 0.1 return + 0.3 equipment).

Duct leakage: seal all joints; SMACNA leakage classes define allowable leakage; duct leakage to outside should be minimized (target <5% of system airflow).

ASHRAE 62.2 residential ventilation: whole-house mechanical ventilation rate based on floor area and occupants; typically 0.01-0.03 CFM/ft2 plus 7.5 CFM per occupant.

Bathroom exhaust: 50 CFM intermittent or 20 CFM continuous; kitchen range hood 100 CFM intermittent (or per code).

Airflow measurement: use a balometer or pitot traverse; face velocity x free area = CFM; register throws and velocities should match manufacturer data.

Thermostat control: single-stage (on/off), two-stage, or modulating; deadband prevents short cycling by requiring a temperature swing before restart.

24V control circuit: R (24V hot), C (common), W (heat), Y (cool), G (fan), O/B (reversing valve); a blown low-voltage fuse often indicates a shorted control wire.

Line voltage vs low voltage: compressors and fans use 208/240V single-phase or 3-phase; controls use 24V; never mix conductors in the same raceway without proper rating.

Electrical safety: lockout/tagout before service, verify with a meter, discharge capacitors, and confirm grounding and GFCI protection where required.

Motor protection: thermal overloads, internal overloads in hermetic compressors, and time-delay fuses; check amp draw against nameplate RLA/LRA.

Refrigerant safety: R-410A is A1 (low toxicity, no flame propagation) but displaces oxygen; use leak detectors and ventilate confined spaces.

Combustion safety: CO detector placement near sleeping areas; flue gas CO should be <100 ppm air-free for natural gas appliances; draft must be verified.

Electrical code basics: conductor ampacity per NEC 310.16, overcurrent protection sized to nameplate MCA/MOP for HVAC equipment.

Commissioning sequence: verify installation, measure airflow and static pressure, check refrigerant charge (superheat/subcool), verify electrical, then document performance against design.

Airflow verification: measure external static pressure and compare to manufacturer fan curve; target 350-450 CFM per ton for cooling, ~400 CFM/ton nominal.

Charge verification: use superheat for fixed orifice, subcooling for TXV; always verify airflow first, since low airflow causes low suction and misleading charge readings.

Preventive maintenance: replace filters, clean coils and drain pans, check refrigerant pressures, inspect belts and bearings, verify safeties (high-limit, float switch).

Troubleshooting order: confirm power, thermostat call, control voltage, then component operation (contactor, capacitor, compressor, fan).

Capacitor testing: measure microfarads with a meter; a bulged or out-of-tolerance capacitor causes hard starts or no-start.

Common cooling faults: frozen evaporator (low airflow or low charge), high head (dirty condenser), no cooling (bad capacitor, contactor, or compressor).

Documentation: record baseline readings at startup; compare future service readings to baseline for trend analysis.

ASHRAE 90.1 sets minimum energy efficiency for commercial buildings; IECC/IRC Chapter 11 covers residential; state and local codes adopt or amend these.

SEER2, EER2, and HSPF2 are the current efficiency metrics (2023+); SEER2 values are generally lower than legacy SEER for the same equipment.

Minimum efficiency: split-system central AC typically 14.3 SEER2 / 11.7 EER2 (varies by region and code edition); heat pumps 14.3 SEER2 / 7.5 HSPF2.

Refrigerant transition: R-410A is being phased down under the AIM Act; R-454B and R-32 are lower-GWP replacements with A2L flammability classification.

A2L refrigerants require updated handling, leak detection, and ventilation per ASHRAE 15 / 34; do not assume R-410A procedures transfer directly.

Duct insulation and sealing requirements: IECC mandates duct sealing in unconditioned spaces and minimum R-values for ducts and pipes.

Ventilation code: ASHRAE 62.1 (commercial) and 62.2 (residential) set outdoor air rates; economizers required above certain cooling capacities in many climates.

Energy code compliance paths: prescriptive (component R-values, equipment efficiency) or performance (whole-building energy modeling).