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.
This calculator handles one step. AIM Works runs the complete MEP design workflow — thermal load calculations, duct & pipe networks, equipment selection, code compliance, and an AI design assistant — in one tool.