[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

We are air-conditioning a 300 m² multipurpose hall, 4.5 m high, for about 120 seated people, with ducted (or floor-standing) package units. Fresh air is taken through the units.

How should the unit capacity be set: is a W/m² rule enough, and how do sensible and latent loads and the fresh air change the selection? How do I check airflow and throw for a high hall? At commissioning, which values should be measured (supply and return temperatures, airflow, coil ΔT, current) and what tolerances are normally accepted?

[MEP Center Engineering]

Select the unit from a room load calculation split into sensible and latent parts plus the fresh air load, at your design outdoor and indoor conditions, and check its sensible capacity and airflow, not only the nominal total. At commissioning, measure airflow, on-coil and off-coil conditions and electrical data, and compare them with the selection at the measured conditions.

1. Load basis (example)

• People: 120 × 75 W sensible = 9.0 kW; 120 × 55 W latent = 6.6 kW.
• Lighting 10 W/m² = 3.0 kW; equipment 2.0 kW; envelope and solar from the load calculation 12.0 kW.
• Room: 26.0 kW sensible + 6.6 kW latent, room sensible heat ratio ≈ 0.80.
• Fresh air 7 l/s per person = 0.84 m³/s (use the rate of your code, EN 16798-1 or ASHRAE 62.1). Outdoor 35 °C db / 24 °C wb, room 24 °C / 50 %: sensible 1.2 × 1.006 × 0.84 × 11 ≈ 11.2 kW; latent 1.2 × 2501 × 0.84 × (0.0142 − 0.0093) ≈ 12.4 kW.
• Coil load ≈ 56 kW total (57 kW by an enthalpy check), ≈ 37 kW sensible. A 300 m² hall “at 150 W/m²” would give 45 kW and miss this.

Fresh air is more than 40 % of the coil load here: consider heat recovery or a separate fresh air unit with CO₂ control.

2. Airflow and throw

• Supply airflow from room sensible load and a 10 K supply ΔT (24 → 14 °C): 26.0 / (1.2 × 1.006 × 10) ≈ 2.15 m³/s ≈ 7750 m³/h.
• Mixed air on the coil ≈ 28.3 °C; off-coil about 14 °C with ≈ 8.3 g/kg moisture to take the room latent load. The selected unit must deliver this at site conditions and the actual external static pressure of the ductwork.
• Throw: pick grilles or nozzles so the jet reaches about 0.75 of the distance to the opposite wall before its velocity falls to 0.25 m/s, and keep occupied-zone velocity below about 0.25 m/s (ISO 7730 / EN 16798 categories, ASHRAE 55). In a 4.5 m hall, cold air dumps if the throw is too short; long-throw nozzles or side-wall jets above the occupied zone work better than ceiling diffusers.

3. What to measure and accept

Example measurement: airflow 7300 m³/h (94 % of design); on-coil 28.0 °C / 48 % RH (h ≈ 57.1 kJ/kg), off-coil 14.0 °C / 85 % (h ≈ 35.4 kJ/kg). Capacity = 1.2 × 2.03 × 21.7 ≈ 52.7 kW, about 92 % of the 57 kW design coil load (enthalpy method), acceptable within field measurement accuracy.

Measurement	Typical acceptance
Total supply airflow	−5 to +10 % of design (terminals ±10 %)
Fresh air flow	not below design
Coil ΔT, on-coil to off-coil	about 10 to 14 K for DX with fresh air; room return to supply 8 to 11 K
Air-side capacity	within about 10 % of the selection at the measured conditions
Compressor and fan current	at or below nameplate; phase current imbalance within about 10 %
Supply voltage	within ±10 % of rated; voltage imbalance below about 2 %
Superheat / subcooling	per manufacturer, typically 4 to 8 K / 3 to 6 K
Room conditions	setpoint ±1 K, RH within design band

These tolerances are typical values from commissioning practice (CIBSE Code A, NEBB/AABC, manufacturer data); verify the figures required by your specification and local regulations.

Common mistakes

• Selecting on nominal total capacity at 27/19 °C rating conditions instead of site conditions.
• Ignoring fresh air latent load in humid climates.
• Measuring temperatures right at the coil face, in stratified air.
• Accepting airflow without checking filters and external static pressure.

Related: HVAC Commissioning: A Practical Checklist, AHU Sizing: Quick Rules. For deeper study: HVAC Systems Design Course.

Check the edition of the code that applies to your project.

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