AHU duty verification against ASHRAE Fundamentals 2021

The heating, cooling and mixing examples printed in Chapter 1 of ASHRAE Fundamentals 2021, run through the same calculation the app uses, and the fan power limit of Approved Document L Volume 2 with the example printed under its Table 5.9.

Result: 8 of 8 checks pass
Cases: 4 worked examples
Version / published: v1.0 · 2026-09-28
Engine fingerprint: abb6de7e2643

Why we publish this

The AHU Duty calculation uses the moist-air equations of Fundamentals Chapter 1 for every state it works out: saturation pressure, humidity ratio, enthalpy and the mixing of two airstreams. Chapter 1 prints worked examples for each process, so the coil duties can be checked against the method rather than against our code. Chapter 1 is not among the chapters revised in the 2025 edition.

How the inputs are entered

Standard air. The calculation quotes coil duties at standard air, 1.2 kg/m³, while the examples convert volume to mass at each stream's own specific volume. The printed dry-air mass flow is therefore entered as its standard-air volume, so both work on the same mass of air.
Wet-bulb states. Example 4 gives the outdoor air as a dry-bulb and wet-bulb pair. The calculator takes relative humidity, so the same state is entered as its equivalent relative humidity, worked out with the Chapter 1 equations.
Condensate. Example 3 subtracts the enthalpy of the condensate leaving the coil, 1.35 kW. The calculation leaves that term out, as coil selection does, and still lands within the tolerance.

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 the coil duties and the air states.
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

Approved Document L, Volume 2: Buildings other than dwellings, 2026 edition
ASHRAE Handbook—Fundamentals 2021 (SI), Chapter 1 Psychrometrics

Printed page numbers refer to the editions above.

Worked examples

AHU-01

Example 2: saturated air at 2 °C heated to 40 °C

Inputs

All outdoor air, saturated at 2 °C, heated to 40 °C
10 m³/s at 0.785 m³/kg = 12.74 kg/s of dry air, entered as 10.62 m³/s of standard air

Checks for case AHU-01
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Heating coil duty q = 12.74 × (51.5 − 13.0) = 490 kW	490kW	ASHRAE Handbook—Fundamentals 2021 (SI), Chapter 1 Psychrometrics Chapter 1, Example 2 (printed p1.12)	486.54kW	0.71 %	±2 %	Pass

AHU-02

Example 3: 30 °C, 50 % rh cooled and dehumidified to saturation at 10 °C

Inputs

All outdoor air at 30 °C and 50 % rh, leaving the coil saturated at 10 °C
5 m³/s at 0.877 m³/kg = 5.70 kg/s of dry air, entered as 4.75 m³/s of standard air

Checks for case AHU-02
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Enthalpy entering the coil	64.3kJ/kg	ASHRAE Handbook—Fundamentals 2021 (SI), Chapter 1 Psychrometrics Chapter 1, Example 3 (printed pp1.12 and 1.14)	64.18kJ/kg	0.19 %	±2 %	Pass
Humidity ratio entering the coil	13.3g/kg	ASHRAE Handbook—Fundamentals 2021 (SI), Chapter 1 Psychrometrics Chapter 1, Example 3 (printed pp1.12 and 1.14)	13.31g/kg	0.07 %	±2 %	Pass
Enthalpy leaving the coil	29.5kJ/kg	ASHRAE Handbook—Fundamentals 2021 (SI), Chapter 1 Psychrometrics Chapter 1, Example 3 (printed pp1.12 and 1.14)	29.27kJ/kg	0.78 %	±2 %	Pass
Humidity ratio leaving the coil	7.66g/kg	ASHRAE Handbook—Fundamentals 2021 (SI), Chapter 1 Psychrometrics Chapter 1, Example 3 (printed pp1.12 and 1.14)	7.63g/kg	0.39 %	±2 %	Pass
Cooling coil duty q = 5.70 × [(64.3 − 29.5) − (0.0133 − 0.00766) × 42.02] = 197 kW. The calculation leaves out the condensate term, 5.70 × 0.00564 × 42.02 = 1.35 kW, as coil selection practice does.	197kW	ASHRAE Handbook—Fundamentals 2021 (SI), Chapter 1 Psychrometrics Chapter 1, Example 3 (printed pp1.12 and 1.14)	198.97kW	1.00 %	±2 %	Pass

AHU-03

Example 4: outdoor air mixed with recirculated air

Inputs

2 m³/s of outdoor air at 4 °C dry-bulb and 2 °C wet-bulb (entered as the equivalent 71 % rh)
6.25 m³/s of recirculated air at 25 °C and 50 % rh

Checks for case AHU-03
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Mixed air dry-bulb temperature Dry-air mass flows 2 ÷ 0.789 = 2.535 kg/s and 6.25 ÷ 0.858 = 7.284 kg/s; the mixed state lies 0.742 of the way from the outdoor to the recirculated state: 19.5 °C.	19.5°C	ASHRAE Handbook—Fundamentals 2021 (SI), Chapter 1 Psychrometrics Chapter 1, Example 4 (printed p1.14)	19.63°C	0.67 %	±2 %	Pass

AHU-04

Fan power limit: central balanced MV with heating and cooling, HEPA filter and humidifier

Inputs

Central balanced mechanical ventilation with heating and cooling, new building
HEPA filter and humidifier in the unit

Checks for case AHU-04
Quantity	Expected	Source	MEPCalc	Δ	Tolerance	Result
Maximum SFP with the Note 3 allowances 2.0 + 1.0 (HEPA filter) + 0.1 (humidifier) = 3.1 W/(l/s), the example printed under Note 3.	3.1W/(l/s)	Approved Document L, Volume 2: Buildings other than dwellings, 2026 edition Table 5.9 and Note 3 (printed p54)	3.1W/(l/s)	0.00 %	exact	Pass