Fresh air volume is calculated as the larger of two values: the minimum fresh air per occupant, and the air-change rate multiplied by the room volume — general offices need 30 m³/h per person, high-grade offices and residences 30–50 m³/h per person, shopping malls 15–25 m³/h per person, and the air-change reference is 1–2 ACH for general rooms, 2–4 ACH for computer rooms and 3–8 ACH for smoking-intensive meeting rooms. Whichever calculation gives the larger figure governs.

 

 

 

 

 

The two formulas

 

 

There are only two ways to establish a fresh air requirement, and you take the larger result. Getting this wrong in the direction of choosing the smaller number is the single most common sizing error, because it produces a system that ventilates below the required rate and quietly becomes a compliance problem later.

 

 

 

Formula 1 — occupancy basis

 

 

The first formula is driven by how many people are in the space:

 

 

 

Vpeople = N × R

 

 

Where N is the number of occupants and R is the minimum fresh air rate per person for the occupancy type.

 

 

 

Fresh air rate per person — ROTER product manual

 

Occupancy type Minimum fresh air per person Source

General office 30 m³/h ROTER product manual

High-grade office 30–50 m³/h ROTER product manual

Residence or villa 30–50 m³/h ROTER product manual

Shopping mall 15–25 m³/h ROTER product manual

 

 

For large shopping malls there is an alternative basis: set the fresh air volume at 30% of the central air conditioning total supply air volume. That approach is often more practical than a per-person figure in a retail space where occupancy fluctuates, because it ties the ventilation rate to the cooling plant that is already being sized.

 

 

 

Formula 2 — air-change basis

 

 

The second formula is driven by the room itself:

 

 

 

Vach = ACH × Vroom

 

 

Where ACH is the required air-change rate per hour and Vroom is the room volume in m³.

 

 

 

Air-change rate reference — ROTER product manual

 

Room type Air-change rate Source

General rooms 1–2 ACH ROTER product manual

Computer rooms 2–4 ACH ROTER product manual

Smoking-intensive meeting rooms 3–8 ACH ROTER product manual

 

 

 

 

Worked examples

 

 

Example 1 — a general office

 

 

A general office of 120 m² with a 3 m ceiling and 10 occupants.

 

 

 

Example 1 — general office, 120 m², 3 m ceiling, 10 occupants

Basis Working Result

Occupancy 10 occupants × 30 m³/h per person 300 m³/h

Air change 2 ACH × (120 m² × 3 m = 360 m³) 720 m³/h

Governing figure Larger of the two 720 m³/h

 

 

 

Interesting result: the occupancy basis is nowhere near governing in this case. The room volume drives it, which is why a low-density office can end up with substantially more ventilation than its headcount suggests.

 

 

 

Example 2 — a shopping mall

 

 

A shopping mall of 2,000 m² with a 6 m ceiling, 100 occupants, and a central air conditioning plant supplying 20,000 m³/h.

 

 

 

Example 2 — shopping mall, 2,000 m², 6 m ceiling, 100 occupants

 

Basis Working Result

Occupancy 100 occupants × 15 m³/h per person 1,500 m³/h

Air change 1 ACH × (2,000 m² × 6 m = 12,000 m³) 12,000 m³/h

30% of supply air 30% × 20,000 m³/h 6,000 m³/h

Governing figure Larger of the three 12,000 m³/h

 

 

The air-change basis governs here by a wide margin. Note that the 30%-of-supply-air alternative does not always win either — it sits between the other two. This is worth checking explicitly, because in retail design the 30% rule is often assumed to govern.

 

 

 

Example 3 — a computer room

 

 

A computer room of 60 m² with a 3 m ceiling and 4 workstations.

 

 

 

Example 3 — computer room, 60 m², 3 m ceiling, 4 workstations

 

Basis Working Result

Occupancy 4 occupants × 30 m³/h per person 120 m³/h

Air change 4 ACH × (60 m² × 3 m = 180 m³) 720 m³/h

Governing figure Larger of the two 720 m³/h

 

 

Again the air-change rate governs, and again by 6×. The lesson across all three examples is that the air-change basis usually governs in real buildings — the per-person figure is a floor, not a target.

 

 

 

 

 

Industrial and special cases

 

 

For industrial workshops where harmful substances are released, neither formula applies. The fresh air volume should be sized according to the airflow required to dilute the pollutants to safe concentrations, which is a calculation from the emission rate and the permissible exposure limit rather than from occupancy.

 

 

 

Two related special cases are worth noting. For a hospital isolation room or a surgical area, the requirement follows the air-change schedule set for the room classification, not the general table above. And for a data centre, the outdoor-air requirement comes from the IT cooling load rather than from occupancy at all.

 

 

 

 

 

Matching the calculation to a product

 

 

Once the airflow is established, the unit selection follows. ROTER's published ranges are given below so the calculation can be carried through to a model directly.

 

 

 

Published ROTER ranges for matching a calculated airflow

 

Product range Airflow (m³/h) Static pressure (Pa) Efficiency

Single-flow fresh air ventilator (RS series) 200–1,000 120–140 Heat recovery

Standard haze-removal ventilator (RT-A02) 200–1,000 100–120 70% enthalpy

DC inverter ventilator (RT-D02) 200–500 100 70% enthalpy, 40 dB(A)

Medium ceiling-mounted total heat recovery (CXH) 1,000–15,000 120–400 68–76% total

Vertical total heat recovery (CHX-L) 1,500–15,000 158–460 73–76% sensible / 58–70% total

Long-range jet air handling unit 1,200–12,000 0 Heat recovery section available

Modular building-block AHU 2,000–80,000 Model selection Heat recovery option

 

 

Two practical points on the selection. First, an airflow figure calculated at the top of a range usually means one unit cannot carry it, and multiple units in parallel cost more in ductwork than a larger single unit — worth asking about. Second, and more importantly, the fresh air figure is a ventilation requirement, not a heat recovery requirement: the ERV that delivers it should also recover heat, and the ERV sizing guide covers that decision.

 

 

 

 

 

Frequently asked questions

 

 

How do I calculate the fresh air volume for a room?

 

Take the larger of two values: the minimum fresh air volume per person and the air-change-based requirement. Reference figures from the ROTER product manual are general office 30 m³/h per person; high-grade office 30–50 m³/h per person; residence or villa 30–50 m³/h per person; shopping mall 15–25 m³/h per person. For air-change rate: general rooms 1–2 ACH; computer rooms 2–4 ACH; smoking-intensive meeting rooms 3–8 ACH. Large shopping malls can alternatively set fresh air volume at 30% of the central air conditioning total supply air volume. For industrial workshops where harmful substances are released, size the airflow to dilute the pollutants to safe concentrations.

 

 

 

What is the difference between ACH and fresh air per person?

 

The per-person rate is set by how much fresh air each occupant needs for comfort and health. The air-change rate is set by how many times the whole room volume of air must be replaced in an hour, and it exists to control things like odour, heat and humidity accumulation. The per-person figure is a floor for health; the air-change figure is usually the one that actually governs in real buildings, because it scales with room volume rather than headcount.

 

 

 

How many air changes per hour does a home need?

 

The general reference from the ROTER product manual is 1–2 ACH for general rooms. For a residential installation the per-person basis often governs instead — 30–50 m³/h per person for a residence or villa — because the occupancy is known and the room volumes are typically modest relative to the headcount.

 

 

 

Does the room ceiling height matter?

 

Yes, directly, because the air-change formula multiplies the floor area by the ceiling height to get the room volume. A room with a 4 m ceiling has 33% more volume than the same floor area at 3 m, so at the same ACH it needs 33% more airflow. This is frequently overlooked in warehouse and retail calculations.

 

 

 

How do I size fresh air for an industrial workshop?

 

Neither the per-person nor the air-change formula applies. The airflow should be sized according to what is required to dilute the pollutants to safe concentrations, which is calculated from the emission rate and the permissible exposure limit. Send us the emission rate, the substance and the target concentration and ROTER will size the unit and the ductwork around it.

 

 

 

What if the calculated airflow exceeds the largest available unit?

 

Use multiple units in parallel, or move to the modular building-block air handling range, which covers 2,000 to 80,000 m³/h and is assembled to order. Multiple parallel units often cost more in ductwork and maintenance than a single larger unit, so it is worth asking about the modular option. Send us the airflow and static pressure and the selection comes back within 24 hours.

 

 

 

Can I get a quotation based on my calculation?

 

Yes. Send us the calculated airflow, the design static pressure, the supply air set point, the outdoor and indoor design conditions and the exhaust conditions. ROTER returns the sizing calculation, a model selection with the efficiency computed at your conditions rather than the catalogue value, and a quotation — normally within 24 hours.