Ductwork verification against CIBSE Guide B2, Guide C and ASHRAE Fundamentals

The supply index run that Guide B2 works through in Appendix 2.A5, its outdoor-air inlet, nine fitting loss factors read from the printed tables of Guide C, the circular equivalents of Table 2.A1.1, the condensation example of Guide B2 and the flexible duct of the ASHRAE Handbook's Chapter 21, run through the same calculations the app uses, line by line against the printed figures.

Result: 16 of 16 checks pass
Cases: 6 worked examples
Version / published: v1.0 · 2026-09-25
Engine fingerprint: 5f45d84eff4e

Why we publish this

Guide B2, Guide C and the ASHRAE Handbook are the references these calculations cite. Guide B2 prints a full worked index run with every fitting priced, Guide C prints the loss-factor tables the pricing comes from, and the Handbook prints its own friction examples, so a calculation can be checked against the method rather than against its own code. Each case above names the standard, the table or example and the printed page; a printed table value has to match exactly, and a figure worked from printed inputs has to land within 2 %.

What is not compared, and why

Appendix 2.A5's end-to-end total (70.4 Pa). The print reads its straight-duct friction off the Guide C chart (Figure 2.33); the calculation works it from Colebrook–White on the hydraulic diameter, and the two sit 5 % apart on this run (66.6 Pa). The fittings, which the example exists to demonstrate, are compared.
Guide B2 Example 2.1, the temperature change along an insulated duct. The print reads a chart and takes the transmittance on the duct's inner area; the calculation is BS EN ISO 12241 on the insulated outer perimeter, and they differ by 13 % on the example.
ASHRAE Table 13 and the spiral duct of Example 6. Both are built on an absolute roughness of 0.12 mm; the catalogue's spiral duct carries Guide C's 0.075 mm, 6 % apart on friction. The flexible duct of Example 6, whose 0.9 mm roughness the catalogue shares, is compared.
ASHRAE Examples 1 to 5, the thermal gravity effect. The calculation does not model stack pressure in ductwork.

How to read the tables

Expected is the value printed in the standard, or a hand calculation from printed values. The working is shown under each quantity.
Tolerance is exact for tabulated values and selected sizes, and ±2 % for calculated quantities such as fitting losses, circular equivalents, the dew point and the flexible-duct friction.
Every check re-runs automatically whenever the calculation code changes, and this page is regenerated from those results. The engine fingerprint above identifies the calculation code the results came from.

Sources

ASHRAE Handbook—Fundamentals 2021 (SI)
CIBSE Guide B2 2016, Ventilation and ductwork
CIBSE Guide C 2007, Reference data

Printed page numbers refer to the editions above.

Worked examples

DW-01

Guide B2 Appendix 2.A5 — the supply index run E–V6, fittings priced from Guide C

Inputs

1.2 m³/s supply at 30 °C from the AHU, index run E–V6 as Figure 2.A5.1
1000 × 600 to 400 × 500 rectangular, then Ø355 and Ø315 circular, 54 m of duct in eight sections
Damper, 45° contractions, vaned bend, shoe tees, double bend, segmented bends, fire damper, flexible and diffuser
Fittings with a Guide C table in the catalogue are resolved from it; the rest carry the ζ the print quotes

The straight-duct friction is not compared: the print reads it off the Guide C chart and the calculation works it from Colebrook–White, 5 % apart on this run.

Checks for case DW-01
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Fitting losses along the index run Table 2.A5.1 fittings total 36.0 Pa plus the flexible connection's 1.1 Pa	37.1Pa	CIBSE Guide B2 2016, Ventilation and ductwork Appendix 2.A5, Table 2.A5.1, printed p2-137	37.4Pa	0.81 %	±2 %	Pass

DW-02

Guide B2 Appendix 2.A5 — the outdoor-air inlet: louvre, mesh, damper and expansion taper

Inputs

0.48 m³/s of outdoor air at 10 °C through 500 × 500, 1.92 m/s, velocity pressure 2.29 Pa
Louvre ζ 4.8 (Table 4.129), mesh 70 % free area ζ 0.58 (Table 4.130), open opposed-blade damper ζ 0.52 (Table 4.126), expansion taper ζ 0.33 as the print quotes

Checks for case DW-02
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Outdoor-air inlet fittings 11.0 + 1.3 + 1.2 + 0.8 Pa	14.3Pa	CIBSE Guide B2 2016, Ventilation and ductwork Appendix 2.A5, Table 2.A5.1 items A1, A2, B and C, printed p2-137	14.27Pa	0.21 %	±2 %	Pass

DW-03

Guide C §4.11 — fitting loss factors read from the printed tables

Inputs

Nine printed cells across Tables 4.42, 4.44, 4.49, 4.56, 4.66, 4.112(a), 4.115, 4.126 and 4.130
Each resolved by the catalogue at the printed row and column

Checks for case DW-03
Quantity	Expected	Source	MEPCalc	Tolerance	Result
ζ, smooth 90° bend r/d 1.5, d 200 mm	0.11	CIBSE Guide C 2007, Reference data Table 4.42, r/d 1.5 row, d 200 mm column (held copy p.254)	0.11	exact	Pass
ζ, segmented 90° bend r/d 1.5, 5 segments, d 150 mm	0.28	CIBSE Guide C 2007, Reference data Table 4.44, r/d 1.5, 5 segments, d 150 mm column (held copy p.255)	0.28	exact	Pass
ζ, mitred 90° circular elbow, d 250 mm	1.23	CIBSE Guide C 2007, Reference data Table 4.49, d 250 mm column (held copy p.255)	1.23	exact	Pass
ζ, mitred 90° rectangular elbow, h/w 1	1.19	CIBSE Guide C 2007, Reference data Table 4.115, 90° row, h/w 1 column (held copy p.295)	1.19	exact	Pass
ζ, opposed-blade damper, x 0.5, 40°	12.3	CIBSE Guide C 2007, Reference data Table 4.126, x 0.5 row, 40° column (held copy p.302)	12.3	exact	Pass
ζ, mesh screen, 70 % free area	0.58	CIBSE Guide C 2007, Reference data Table 4.130, free area 0.7 column (held copy p.306)	0.58	exact	Pass
ζ, vaned radius bend r/w 1.0, h/w 1.25	0.05	CIBSE Guide C 2007, Reference data Table 4.112(a), r/w 1.0 row, between the h/w 1.0 and 1.5 columns (held copy pp.293 to 294); the value Guide B2 quotes for bend H	0.05	±2 %	Pass
ζ, symmetrical contraction 60°, A1/A2 3	0.029	CIBSE Guide C 2007, Reference data Table 4.56, 60° row, A1/A2 3.0 column (held copy p.258)	0.029	exact	Pass
ζ, 90° tee, diverging branch, Ab/Ac 0.5, qb/qc 0.5	1.4	CIBSE Guide C 2007, Reference data Table 4.66, Ab/Ac 0.5 row, qb/qc 0.5 column (held copy p.264)	1.4	exact	Pass

DW-04

Guide B2 Table 2.A1.1 — circular equivalents of rectangular ducts

Inputs

600 × 300 mm and 1000 × 400 mm, Guide C equation 4.19 (equal friction at equal flow)

Checks for case DW-04
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Circular equivalent of 600 × 300 1.265 × (600 × 300)^0.625 / (600 + 300)^0.25	462mm	CIBSE Guide B2 2016, Ventilation and ductwork Table 2.A1.1, printed p2-127	461.7157mm	0.06 %	±2 %	Pass
Circular equivalent of 1000 × 400 1.265 × (1000 × 400)^0.625 / (1000 + 400)^0.25	683mm	CIBSE Guide B2 2016, Ventilation and ductwork Table 2.A1.1, printed p2-127	682.4487mm	0.08 %	±2 %	Pass

DW-05

Guide B2 Example 2.2 — insulation to prevent surface condensation on a chilled-air duct

Inputs

Circular duct Ø800 carrying air at 12 °C, in a ceiling void at 35 °C and 85 % RH
Glass wool, λ 0.045 W/m·K; outside surface coefficient 10 W/m²·K

Checks for case DW-05
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Dew point of the ambient air Psychrometric chart at 35 °C dry bulb and 85 % RH	32.1°C	CIBSE Guide B2 2016, Ventilation and ductwork Example 2.2, printed p2-46	32.091°C	0.03 %	±2 %	Pass
Insulation thickness to avoid surface condensation (32.1 − 12) × 0.045 / ((35 − 32.1) × 10) = 0.030 m	30mm	CIBSE Guide B2 2016, Ventilation and ductwork Example 2.2, printed p2-46	30mm	0.00 %	exact	Pass

DW-06

ASHRAE Ch.21 Example 6 — flexible duct, fully extended, against spiral steel

Inputs

470 L/s through Ø250, 1.8 m installed, fully extended (no compression)
Absolute roughness 0.9 mm (Table 1, flexible duct); air density 1.204 kg/m³

The spiral duct of the same example (7.7 Pa at 0.12 mm) is not compared: the catalogue's spiral duct carries Guide C's 0.075 mm, 6 % apart on friction.

Checks for case DW-06
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Flexible duct, fully extended, 1.8 m Table 2, flexible CD11-2 fully stretched	11.4Pa	ASHRAE Handbook—Fundamentals 2021 (SI) Chapter 21 Example 6 and Table 2, printed pp21.6 to 21.7	11.2473Pa	1.34 %	±2 %	Pass