HVAC Rule-of-Thumb Reference

Airside Rules

Rule	Value / Range	Notes
Supply duct velocity — main trunk	1,000–2,000 FPM	Low-pressure commercial; stay toward the low end above quiet (NC 30–35) spaces
Supply duct velocity — branch / runout	600–1,200 FPM; runouts ≤ 700 FPM	Slower near diffusers to limit noise
Return duct velocity	≤ 1,500 FPM mains; ≤ 1,000 FPM branches	Return and transfer grilles slower still
Duct friction loss target	0.08–0.10 in. w.g. per 100 ft	Equal-friction sizing, low-pressure duct, sea-level air
CFM/ton cooling	350–450 CFM/ton	Comfort-cooling DX and CHW coils; lower when dehumidification governs
Supply air temp (cooling)	55–60°F	Typical coil leaving air; colder where humidity control governs
Supply air temp (heating)	85–120°F	Low end for overhead VAV reheat. Warm air ≥ 15°F above room from the ceiling with a ceiling return gets Ez = 0.8 in ASHRAE 62.1 Table 6-4
OA fraction	Often ~10–20% in offices	An outcome, not a rule: ASHRAE 62.1 sets outdoor air in cfm (V_bz = R_p·P_z + R_a·A_z, then system efficiency), not as a percent of supply. Dense spaces run far higher
Filter face velocity	300–500 FPM	Most filters are rated near 500 FPM; slower means lower ΔP and longer 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/lb) ≈ 4,840 × CFM × ΔW (lb/lb)

Hydronic Rules

Rule	Value / Range	Notes
CHW ΔT (return – supply)	10–16°F	12°F (44/56°F) common; 14–16°F for low-flow plants
HHW ΔT (supply – return)	20–40°F	20°F traditional; 30–40°F with condensing boilers. Higher ΔT = smaller pipe and less pump energy
CHW GPM/ton	24 ÷ ΔT	2.4 GPM/ton at 10°F · 2.0 at 12°F · 1.5 at 16°F
Pipe friction target	1–4 ft per 100 ft	Common design range; about 2.5 ft/100 ft for mains
Pipe velocity — mains	4–8 ft/s	Noise and erosion limit; ≤ 4 ft/s in branches near occupied space
Pipe velocity — pump suction	≤ 5 ft/s	Protects NPSH margin; suction pipe is often one size larger than the pump inlet
Hydronic factor (water)	500	Q = 500 × GPM × ΔT (BTU/hr); glycol mixes are lower
Pump efficiency (typical)	65–80%	Centrifugal pump near its best efficiency point; larger pumps run higher

Psychrometric & Load Rules

Rule	Value / Range	Notes
Comfort RH range	30–60%	Common design practice. ASHRAE 55 sets no lower humidity limit; where humidity is controlled, its upper limit is a 0.012 lb/lb humidity ratio (about 65% RH at 75°F)
Cooling setpoint (occupied)	73–78°F	75°F common; ASHRAE 55 comfort depends on clothing, activity, humidity, and air speed
Cooling load — office	300–450 ft²/ton	Modern code-built offices; the band the AIM Works app benchmarks against. Varies widely
Lighting load density	0.5–1.0 W/ft²	LED office; current ASHRAE 90.1 office allowances are under 1 W/ft². Legacy fluorescent ~1.0–1.5 W/ft²
People load — office	245–250 BTU/hr sensible + 155–200 latent	400–450 BTU/hr total, seated to moderately active office work (ASHRAE Fundamentals Ch.18, Table 1)
Latent load fraction — humid climate	20–35%	Of total cooling load; higher where outdoor air dominates
SHR — typical commercial coil	0.70–0.80	DX and CHW in mixed climates
Enthalpy — summer outdoor (SE US)	38–42 BTU/lb	95°F DB with 75–79°F coincident WB at sea level (enthalpy tracks wet bulb)

Equipment Rules

Rule	Value / Range	Notes
Chiller efficiency — water-cooled centrifugal	≤ 0.610 kW/ton (< 300 tons); ≤ 0.560 kW/ton (≥ 300 tons)	ASHRAE 90.1-2022 Table 6.8.1-3 Path A full-load limits (COP ≥ 5.77 / 6.28); IPLV must also be met. New machines often select near 0.50–0.58 kW/ton
Chiller efficiency — air-cooled	≥ 10.1 EER (≤ 1.19 kW/ton)	ASHRAE 90.1-2022 Table 6.8.1-3 Path A full-load minimum
DX system COP	≈ 2.9–3.8 (EER ≈ 10–13)	Packaged rooftop at the AHRI full-load rating; part-load (IEER) is higher
Boiler efficiency (condensing)	90–98%	Modulating; needs return water below ~130°F to condense
Cooling tower approach	5–10°F	Leaving CW minus ambient WB; 7°F common (85°F at 78°F WB)
Fan motor sizing	BHP × 1.15–1.25	Common selection margin, then the next standard motor size
VFD savings at 80% speed	Up to ~49% (ideal)	Affinity law: power ∝ speed³ (0.8³ = 0.51). Real savings are lower where a pressure setpoint or system head remains
Pump motor sizing	BHP = WHP ÷ pump eff; motor ≈ BHP × 1.15	WHP = GPM × ft head × SG ÷ 3,960

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ℹ️ About This Reference

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.

🔬 Key Constants & Method

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 ≈ 60 × 0.075 × 1,076 ÷ 7,000, and the water factor 500 ≈ 60 × 8.33 × 1.0, follow standard ASHRAE Fundamentals unit relations; the air factors are sea-level values that fall with air density at altitude (about 17% lower at 5,000 ft). The affinity-law cube relationship comes from fan and pump similarity laws. Velocity, friction, and ΔT ranges are common design practice and line up with the default design criteria in the AIM Works app; efficiency limits quote ASHRAE 90.1-2022 where a standard is named. Everything else is a typical range, not a code requirement.

❓ Frequently Asked Questions

What is the sensible heat factor of 1.08?

The factor 1.08 lets you find sensible heat from airflow and temperature difference: Q (BTU/hr) = 1.08 times CFM times delta-T in degrees F. It equals 60 minutes per hour times 0.075 lb per cubic foot of standard air times 0.24 BTU per lb per degree F. Because it depends on air density, 1.08 is a sea-level value and decreases at higher altitude as the air thins.

What is the 500 factor for hydronic loads?

For water, Q (BTU/hr) = 500 times GPM times delta-T in degrees F. The 500 comes from 60 minutes per hour times 8.33 lb per gallon times 1.0 BTU per lb per degree F for water, which rounds to about 500. It is the hydronic counterpart to the 1.08 air factor and is used constantly for coil, chiller, and boiler flow checks.

How much energy does a VFD save at 80 percent speed?

Up to about 49 percent in the ideal case. Fan and pump power follows the affinity law, varying with the cube of speed, so 0.8 cubed is about 0.51, meaning the motor draws around 51 percent of full power and saves about 49 percent. Real systems save less where a static-pressure setpoint or fixed system head remains, and drive losses add a few percent. The cubic relationship is still why modest speed reductions on variable-flow systems produce large energy savings.

Are these rules of thumb a substitute for real calculations?

No. These values are for quick estimating, checking another engineer's work, and catching gross errors. Final design requires proper load calculations, duct and pipe sizing, and equipment selection to the applicable codes and ASHRAE standards. The ranges shown are representative and vary with climate, building type, and project requirements.

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.

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