HVAC Tech Pocket Card — Air Properties, Duct Velocities & AC Sizing
One-page HVAC card: air properties by temperature, duct velocity limits, ENERGY STAR room AC sizing, and hydronic fluid properties.
Updated August 20, 2026
The numbers an HVAC tech keeps flipping back to, on one printable card: dry-air density and viscosity across the working temperature range from Cengel Table A-15, recommended duct velocity bands by application in both m/s and fpm, the ENERGY STAR room air conditioner sizing ladder by floor area, and the specific heat and density of water and glycol mixes for hydronic flow math.
Every cell is generated from the constants behind the air properties chart and the duct sizing, window AC BTU, and chilled water flow calculators — the fpm column is computed from the exported metric limits with the exact foot definition, never retyped. The linked calculators handle sun exposure corrections, duct friction, and delta-T flow sizing.
HVAC Tech Pocket Card — Air Properties, Duct Velocities & AC Sizing — updated August 20, 2026
Dry air at 1 atm (Cengel A-15)
| T (°C) | ρ (kg/m³) | μ (µPa·s) | ν (mm²/s) |
|---|---|---|---|
| 0 | 1.292 | 17.29 | 13.38 |
| 5 | 1.269 | 17.54 | 13.82 |
| 10 | 1.246 | 17.78 | 14.26 |
| 15 | 1.225 | 18.02 | 14.70 |
| 20 | 1.204 | 18.25 | 15.16 |
| 25 | 1.184 | 18.49 | 15.62 |
| 30 | 1.164 | 18.72 | 16.08 |
| 40 | 1.127 | 19.18 | 17.02 |
| 50 | 1.092 | 19.63 | 17.98 |
| 60 | 1.059 | 20.08 | 18.96 |
| 80 | 0.9994 | 20.96 | 20.97 |
| 100 | 0.9458 | 21.81 | 23.06 |
| 150 | 0.8338 | 23.80 | 28.51 |
| 200 | 0.7459 | 25.77 | 34.55 |
| 300 | 0.6158 | 29.34 | 47.65 |
Thermal conductivity and Prandtl number on the air properties chart. Density scales with pressure by the ideal gas law.
Duct velocity limits by application
| Application | m/s | fpm |
|---|---|---|
| Residence | 3–5 | 591–984 |
| Office | 6–8 | 1181–1575 |
| Retail | 8–10 | 1575–1969 |
| Industrial | 10–15 | 1969–2953 |
| Kitchen exhaust | 7.6–15 | 1496–2953 |
Recommended main-duct bands; fpm computed from the metric limits (1 ft = 0.3048 m exactly).
Room AC sizing (ENERGY STAR)
| Room area (ft²) | BTU/h |
|---|---|
| 100–150 | 5000 |
| 150–250 | 6000 |
| 250–300 | 7000 |
| 300–350 | 8000 |
| 350–400 | 9000 |
| 400–450 | 10000 |
| 450–550 | 12000 |
| 550–700 | 14000 |
| 700–1000 | 18000 |
| 1000–1200 | 21000 |
| 1200–1400 | 23000 |
| 1400–1500 | 24000 |
| 1500–2000 | 30000 |
| 2000–2500 | 34000 |
Base capacity: −10% heavily shaded, +10% very sunny, +600 BTU/h per extra occupant, +4000 BTU/h for kitchens.
Hydronic fluid properties
| Fluid | cp (kJ/kg·K) | ρ (kg/m³) |
|---|---|---|
| Water | 4.186 | 1000 |
| 30% glycol | 3.75 | 1030 |
| 50% glycol | 3.4 | 1060 |
Glycol lowers specific heat and raises density — flow must rise for the same capacity at the same ΔT.
Sources
- Cengel, Heat and Mass Transfer, Table A-15 (dry air, 1 atm)
- Published duct velocity recommendations (see duct sizing calculator)
- ENERGY STAR room air conditioner sizing table
- Water/glycol properties (see chilled water flow calculator)
Try the Calculators
HVAC Tech Pocket Card — Air Properties, Duct Velocities & AC Sizing — reuven.tools/cheat-sheets/hvac-pocket-card — verified against: Cengel, Heat and Mass Transfer, Table A-15 (dry air, 1 atm); Published duct velocity recommendations (see duct sizing calculator); ENERGY STAR room air conditioner sizing table; Water/glycol properties (see chilled water flow calculator)