Standard volumetric flow rate of conditioned air delivered by blowers, air handlers, and ventilation equipment.
Rule of Thumb: Standard residential cooling sizing: 400 CFM per nominal ton (350 CFM/Ton in humid zones, 450 CFM/Ton in arid zones).
CFM = Sensible BTU / (1.08 Ã ÎT)
Standard: ACCA Manual D / ASHRAE Handbook of Fundamentals
Linear velocity of air travelling through supply and return ductwork, grilles, registers, and diffusers.
Rule of Thumb: Residential main supply trunks: 700â900 FPM; branch runouts: 600â700 FPM; return grilles: 300â450 FPM for NC < 25 acoustic silence.
Velocity (FPM) = Airflow (CFM) / Duct Area (sq ft)
Standard: SMACNA HVAC Duct Design Standard
Non-SI unit of static pressure and duct friction loss equivalent to the pressure exerted by a 1-inch liquid water column (248.84 Pascals).
Rule of Thumb: Residential duct sizing equal friction standard: 0.08 to 0.10 in. w.g. per 100 equivalent feet.
1 in. w.g. = 248.84 Pa = 0.03609 psig
Standard: AMCA Standard 210 / ASHRAE 51
AirflowTEL
Total Equivalent Length
The physical straight duct length plus the friction resistance of all fittings, elbows, transitions, and dampers converted into equivalent straight feet.
Rule of Thumb: A standard 90° adjustable elbow equals 10 to 30 equivalent feet depending on throat radius and turning vanes.
TEL = Measured Duct Run (ft) + Sum of Fitting Equivalent Lengths (ft)
Standard: ACCA Manual D (Appendix 3)
AirflowESP
External Static Pressure
Total resistance of the external air distribution system (ductwork, filters, coils, and dampers) that the blower motor must overcome.
Rule of Thumb: Typical residential PSC blower rating: 0.50 in. w.g.; ECM blowers modulate up to 0.80â1.0 in. w.g.
ESP = Supply Static Pressure + |Return Static Pressure|
Standard: AHRI Standard 210/240
EfficiencySEER2
Seasonal Energy Efficiency Ratio 2
Wikidata âUpdated 2023 AHRI/DOE metric measuring total seasonal cooling BTU output divided by total electrical watt-hours consumed under M1 static test conditions (0.5 in. w.g.).
Rule of Thumb: National minimum standards: 13.4 SEER2 (North) / 14.3 SEER2 (South); premium variable-speed inverter heat pumps reach 20+ SEER2.
SEER2 = Total Seasonal Cooling BTU / Total Watt-Hours Consumed
Standard: DOE 10 CFR Part 430 / AHRI 210/240-2023
EfficiencyHSPF2
Heating Seasonal Performance Factor 2
Heating efficiency metric for heat pumps measuring total seasonal space heating in BTUs divided by total electrical watt-hours consumed over the heating season.
Rule of Thumb: DOE 2023 baseline standard is 7.5 HSPF2 for split systems; cold-climate Energy Star heat pumps exceed 8.5 to 10+ HSPF2.
HSPF2 = Total Seasonal Heating BTU / Total Watt-Hours Consumed
Standard: AHRI Standard 210/240
Thermodynamic dimensionless ratio of heating or cooling thermal power delivered to the electrical power consumed.
Rule of Thumb: Electric resistance heat has a COP of exactly 1.0; modern inverter heat pumps deliver a COP of 2.5 to 4.5+ depending on ambient outdoor temperature.
COP = Thermal Energy Output (Watts) / Electrical Power Input (Watts)
Standard: ASHRAE Standard 90.1
RefrigerationSuperheat
Vapor Superheat Temperature
Wikidata âDegrees of temperature rise of vapor refrigerant above its boiling saturation temperature at evaporator suction pressure.
Rule of Thumb: Protects compressor from damaging liquid slugging. Optimal range on fixed orifice systems: target SH ¹ 3°F; on TXV systems: 8°F to 14°F.
Superheat (°F) = Suction Line Temp - Saturated Evaporator Dew Temp
Standard: AHRI Standard 540 / EPA Section 608
RefrigerationSubcooling
Liquid Subcooling Temperature
Wikidata âDegrees of temperature drop of liquid refrigerant below its condensing saturation temperature at high-side liquid line pressure.
Rule of Thumb: Ensures a 100% solid liquid column enters the TXV or electronic expansion valve. Manufacturer target is typically 10°F ¹ 3°F under stabilized load.
Subcooling (°F) = Saturated Condenser Bubble Temp - Liquid Line Temp
Standard: AHRI Standard 540
RefrigerationA2L Refrigerants
Mildly Flammable Lower-GWP Refrigerant Blends
Wikidata âASHRAE Standard 34 Safety Group A2L refrigerants (e.g. R-454B with GWP 466, R-32 with GWP 675) mandated under the EPA AIM Act to replace legacy R-410A (GWP 2088).
Rule of Thumb: Require spark-proof service tools, leak detection sensors (A2L RDS), left-hand threaded recovery cylinders, and bubble/dew glide calculation.
Standard: ASHRAE Standard 34 / UL 60335-2-40
RefrigerationTemperature Glide
Zeotropic Phase Change Temperature Span
The temperature change that occurs during boiling or condensing of a non-azeotropic refrigerant blend at constant pressure.
Rule of Thumb: Near-azeotropes like R-454B have small glide (1.5°F); zeotropes like R-407C exhibit large glide (9°F), requiring dew/bubble PT tracking.
Standard: NIST REFPROP / AHRI Guideline N
Measure of a construction material's resistance to conductive heat flow. The mathematical reciprocal of U-Factor.
Rule of Thumb: Higher is better. 2024 IECC Climate Zone 4/5 standards: R-20+5ci exterior wall assembly, R-60 attic ceiling insulation.
R-Value = Thickness (inches) / Thermal Conductivity (k)
Standard: ASTM C518 / IECC 2024
Building ScienceU-Factor
Overall Heat Transmission Coefficient
Wikidata âRate of steady-state heat transfer through an entire building envelope assembly in BTU/hr per square foot per degree Fahrenheit temperature difference.
Rule of Thumb: Lower is better. High-efficiency double/triple pane windows: U-0.20 to U-0.28; Code-compliant exterior walls: U-0.045 to U-0.060.
U-Factor = 1 / Total Assembly R-Value
Standard: NFRC 100 / ASHRAE 90.1
Measure of room or building air volume replacement per hour to maintain indoor air quality, control humidity, and dilute pollutants.
Rule of Thumb: Tight modern residential home: 0.35 ACH; Classroom: 4â6 ACH; Commercial Kitchen: 15â20 ACH; Hospital Airborne Isolation: 12+ ACH.
ACH = (Airflow CFM Ã 60) / Room Volume (cu ft)
Standard: ASHRAE Standard 62.1 & 62.2
Building ScienceEnthalpy
Total Specific Thermal Heat Content
Wikidata âTotal heat content (sensible heat plus latent heat of moisture) contained in one pound of moist dry air, measured in BTU per pound (BTU/lb).
Rule of Thumb: Standard indoor design air (75°F DB / 50% RH) has an enthalpy of approximately 28.1 BTU/lb.
h = 0.240 Ã T_db + W Ã (1061 + 0.444 Ã T_db)
Standard: ASHRAE Handbook of Fundamentals (Psychrometrics)
Building ScienceSHR
Sensible Heat Ratio
Ratio of sensible cooling load (dry temperature reduction) to the total cooling load (sensible load + latent moisture removal).
Rule of Thumb: Typical residential cooling load SHR is 0.70 to 0.80. Humid climates require equipment with lower SHR (0.65â0.70) for aggressive moisture extraction.
SHR = Sensible Cooling Load (BTU/hr) / Total Cooling Load (BTU/hr)
Standard: ACCA Manual S / Manual J