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AHU sizing calculator in seconds.
Enter an airflow and a target face velocity to size an air handling unit casing — face area, a velocity sanity-check, suggested coil dimensions, and an indicative specific fan power (SFP) and Eurovent energy class. Then audit how your current selection software stacks up.
Operating point
Result
Face area, dimensions and face velocity are exact geometry. SFP and energy class are indicative early-stage estimates from typical component pressure drops — for feasibility, not for a quote. AirSelect3D computes the certified figures from each manufacturer's own calculation engine.
How AHU sizing actually works
Sizing an air handling unit starts from one number: the face area the air must pass through. You choose a face velocity — how fast air moves across the coils and filters — and the casing cross-section follows directly from the airflow.
1. Face area from airflow and velocity
Convert airflow to m³/s (divide by 3600), then divide by the target velocity. At 5000 m³/h and 2.2 m/s that is 0.63 m² — about a 0.8 m × 0.8 m coil face.
2. Why face velocity matters
Face velocity is the single number that trades capital cost against running cost. Aim too low (below ~1.5 m/s) and the unit is oversized and expensive to buy. Aim too high (above ~2.5 m/s) and pressure drop — and therefore specific fan power and energy cost — climb steeply, while wet cooling coils start to carry water droplets downstream.
- 1.5–2.5 m/s — the usual design window.
- ≈ 2.0–2.2 m/s — a common sweet spot for balanced cost and SFP.
- > 2.5 m/s — compact and cheap to buy, but high SFP and carry-over risk.
3. Specific fan power (SFP) and the energy class
SFP is the fan electrical power per unit of airflow, in W/(m³/s). It is driven by the total pressure the fan must overcome — filters, coils, heat recovery, attenuators and ductwork — divided by the combined fan, motor and drive efficiency. EN 16798 / EN 13779 sort it into classes SFP1 (<500) to SFP7 (>4500), and the EU ErP ecodesign regulation caps internal SFP for non-residential ventilation units. The Eurovent energy class then follows from the SFP and the seasonal fs-Pref ratios — which is where a certified engine, not a rule of thumb, is required.
2-minute audit
Is your AHU selection software Eurovent & ErP ready?
The sizing above is a rule of thumb. Turning it into a quotable, submission-grade selection is what your selection software is for. Tick what your current tool actually does — get a score and the gaps.
Questions
How do you calculate AHU casing size from airflow?
Divide airflow in m³/s (airflow in m³/h ÷ 3600) by the target face velocity to get the required face area. 5000 m³/h at 2.2 m/s needs 0.63 m² — roughly a 0.8 m × 0.8 m coil face.
What is a good face velocity for an air handling unit?
Most AHUs are sized for 1.8–2.5 m/s. Below ~1.5 m/s the unit is oversized; above ~2.5 m/s SFP climbs and cooling coils risk moisture carry-over. 2.0–2.2 m/s is a common target.
Is this calculator accurate enough to quote from?
Face area, dimensions and velocity are exact. SFP and energy class are indicative estimates for feasibility — the certified figures come from the manufacturers' own engines in AirSelect3D.