A quick-reference table of common HVAC engineering rules of thumb — velocity limits, airflow ratios, load factors, and design constants. Bookmark this one.
| Rule | Value / Range | Notes |
|---|---|---|
| Supply duct velocity — main trunk | 600–1,200 FPM | SMACNA; max 1,500 FPM |
| Supply duct velocity — branch | 400–800 FPM | NC-35 spaces |
| Return duct velocity | 400–700 FPM | Lower than supply to reduce noise |
| Duct friction loss target | 0.08–0.10 in/100ft | Equal friction sizing method |
| CFM/ton cooling | 350–450 CFM/ton | Standard DX and CHW coils |
| Supply air temp (cooling) | 55–60°F | Typical LAT from cooling coil |
| Supply air temp (heating) | 90–120°F | Hot water coil or gas heat |
| OA fraction — minimum | 10–20% | ASHRAE 62.1 minimum ventilation |
| Filter face velocity | 300–500 FPM | Higher → shorter filter life |
| Sensible heat factor (sea level) | 1.08 | Q = 1.08 × CFM × ΔT (BTU/hr) |
| Latent heat factor (sea level) | 0.68 | Q = 0.68 × CFM × ΔW_gr (BTU/hr) |
| Rule | Value / Range | Notes |
|---|---|---|
| CHW ΔT (supply–return) | 10–12°F | Typical; 14–16°F for high-ΔT plants |
| HHW ΔT (supply–return) | 20–40°F | Higher ΔT = smaller pipe + lower pump energy |
| CHW GPM/ton | 2.0–2.4 GPM/ton | At 10°F ΔT; 1.5 GPM/ton at 16°F ΔT |
| Pipe friction target | 1–4 ft/100ft | Darcy-Weisbach design range |
| Pipe velocity — mains | 4–8 ft/s | ASHRAE recommended |
| Pipe velocity — pump suction | ≤3 ft/s | Prevent cavitation |
| Hydronic factor (water) | 500 | Q = 500 × GPM × ΔT (BTU/hr) |
| Pump efficiency (typical) | 65–80% | Centrifugal at BEP; VFD reduces at part load |
| Rule | Value / Range | Notes |
|---|---|---|
| Comfort RH range | 30–60% | ASHRAE 55; 40–50% ideal |
| Comfort DB range (cooling) | 73–79°F | ASHRAE 55 operative temp |
| Cooling load — office | 200–350 ft²/ton | Rule-of-thumb; varies widely |
| Lighting load density | 0.5–2.0 W/ft² | LED office; older T8 ~1.5–2.5 W/ft² |
| People load (sensible) | 250–300 BTU/hr/person | Office activity; 750 BTU/hr total |
| Latent load fraction — humid climate | 20–35% | Of total cooling load |
| SHR — typical commercial coil | 0.70–0.80 | DX and CHW in mixed climate |
| Enthalpy — summer outdoor (SE US) | 36–42 BTU/lb | 95°F / 75°F WB typical design |
| Rule | Value / Range | Notes |
|---|---|---|
| Chiller COP (centrifugal) | 5.0–7.0 | 0.5–0.7 kW/ton; ASHRAE 90.1 minimums |
| DX system COP | 3.0–4.5 | Packaged rooftop; varies by SEER |
| Boiler efficiency (condensing) | 90–98% | Modulating; needs low return temp |
| Cooling tower approach | 5–10°F | Leaving CW vs ambient WB |
| Fan motor sizing | BHP × 1.15–1.25 | Motor selection oversizing factor |
| VFD savings at 80% speed | ~49% energy reduction | Affinity law: power ∝ speed³ |
| Pump motor sizing | WHP / pump eff × 1.15 | Safety factor on top of efficiency |
This page collects the rules of thumb HVAC engineers reach for every day — duct and pipe velocity limits, chilled- and hot-water delta-T, load densities, equipment efficiencies, and the sensible and latent heat factors — into four sorted reference tables. It is a bookmark-and-go cheat sheet, not a calculator with inputs.
Rules of thumb are for fast estimating and for checking whether a detailed result is believable. They are deliberately broad, and the right value for a specific project depends on climate, building type, and code. Always confirm with a real calculation before committing a number to a drawing.
| Sensible heat (air) | Q (BTU/hr) = 1.08 × CFM × ΔT (°F), sea level |
| Latent heat (air) | Q (BTU/hr) = 0.68 × CFM × ΔW (gr/lb), sea level |
| Hydronic heat (water) | Q (BTU/hr) = 500 × GPM × ΔT (°F) |
| Cooling capacity | 1 ton = 12,000 BTU/hr = 3.517 kW |
| VFD affinity law | power ∝ speed³ → 80% speed ≈ 51% power |
The air factors 1.08 = 60 × 0.075 × 0.24 and 0.68, and the water factor 500 ≈ 60 × 8.33 × 1.0, follow standard ASHRAE Fundamentals unit relations and are sea-level air-density values that fall at altitude. The affinity-law cube relationship comes from fan and pump similarity laws. Velocity, ΔT, and efficiency ranges are common SMACNA / ASHRAE design practice, shown as ranges because real projects vary.
AIM Works brings peak loads, ASHRAE 62.1 ventilation, equipment and plant sizing, compliance checks, Design Review, and engineering deliverables into one connected mechanical design workspace.
Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.