Selecting the Right Centrifugal Fan: Volume Flow & Pressure
A radial fan is the core of every professional ventilation system: it creates higher pressure with the same power consumption as axial fans, making it ideal for duct systems with filters, bends, and long duct runs. Those who dimension incorrectly here pay twice – a fan that is too small does not deliver the planned air volume, and one that is too large wastes electricity and generates unnecessary noise.
The selection follows a clear logic: first, you determine the required volume flow, then the pressure loss of your duct network, and only then do you choose the specific device including motor and control. This guide takes you step by step through precisely this sequence – from volume flow calculation to pressure loss, motor selection, and suitable manufacturers.
Calculating Volume Flow: Rules of Thumb per Application
The first question is: how much air needs to be moved? The volume flow in cubic meters per hour is the basic quantity for any fan design, and it always depends on the use of the room. In living spaces, it's about humidity and CO₂ removal; in kitchens, about cooking fumes and grease; in workshops, about pollutants – the requirements vary accordingly.
Two calculation methods have become established in practice: either person-related (air volume per person present) or room-related via the air change rate, i.e., how often the entire room volume is exchanged per hour. In residential buildings, both methods usually lead to similar results; in special applications like paint shops, the air change rate dominates because the pollutant load, not the number of people, is decisive there. If in doubt, calculate both variants and use the higher value as the design basis.
| Application | Rule of Thumb |
|---|---|
| Residential ventilation | 30 m³/h per person OR 0.4 air changes/h |
| Commercial kitchen | 1,500–4,000 m³/h per m² cooking surface |
| Office | 30–50 m³/h per person (DIN EN 16798-1) |
| Industry/Workshop | 5–10 air changes/h |
| Paint shop | 30–60 air changes/h |
Also, plan for a certain reserve: filters get dirty during operation and increase resistance, so the actual air volume delivered decreases over the service life. A fan that barely reaches its design point only when new is undersized. For residential buildings, the guide Planning a KWL provides the basics, and for commercial kitchens, the guide Planning a Commercial Kitchen.
Calculating Pressure Loss
The volume flow alone does not tell you whether a fan will actually move the air through your duct network. Every component in the air path – from straight ducts to bends to filters – creates resistance to the flow, which the fan must overcome as static pressure. The sum of all these individual resistances results in the total pressure loss of the system, measured in Pascals.
This is precisely where the radial fan demonstrates its strength: while axial fans quickly drop off with increasing back pressure, radial fans maintain their volume flow even against considerable resistance. For design purposes, simply add the pressure losses of all components along the most unfavorable flow path – the following table provides typical values per component.
| Component | Pressure Loss |
|---|---|
| Straight duct | 0.8–1.5 Pa/m |
| 90° bend | 5–15 Pa per piece |
| T-piece | 10–25 Pa |
| Filter F7 | 50–150 Pa (dirty: up to 250 Pa) |
| Silencer | 10–40 Pa |
| Heat exchanger | 80–200 Pa |
| Outlet/Inlet grille | 5–20 Pa |
The large proportion of the filter in the total resistance is striking – and especially how much it increases when dirty. Therefore, always calculate with the end pressure loss of the filter, not the new value, otherwise the air volume will drop towards the end of the filter service life. You can read how filters affect pressure loss and which class you need in the article on Filter Classes for Ventilation Systems.
EC Motor or AC Motor?
Once the operating point is set, the motor question follows – and it significantly determines the operating costs for the coming years. Classic AC motors run on alternating current directly from the mains and are inexpensive to purchase, but they lose considerable efficiency, especially in partial load operation. EC motors, on the other hand, are electronically commutated DC motors with integrated control electronics: they run efficiently at every operating point and can be continuously controlled without additional devices.
Since ventilation systems rarely run at full load, but rather at partial load most of the time, the efficiency advantage of the EC motor adds up considerably in everyday use. In addition, there is less wear and tear: without brushes and without an upstream frequency converter, there are simply fewer parts that can age.
AC Motor (classic)
- Inexpensive to purchase
- Speed control via frequency converter
- Efficiency 60–75%
- Higher wear and tear
EC Motor (modern)
- Efficiency 85–95% – saves 30–50% electricity
- Directly controllable 0–100% (0–10V)
- Low noise
- Low maintenance – no brushes
Pro Tip: Always choose an EC motor for new installations – the additional costs amortize in 2–4 years.
Speed Control: Three Methods
Hardly any system runs permanently with the same air volume: night setback, demand-controlled ventilation, or boost operation in the kitchen require adjustable speed. Which control method is suitable depends directly on the motor type.
| Method | Description |
|---|---|
| Step controller (transformer) | 3–5 steps, simple, AC motor |
| Frequency converter | Continuous 0–100%, AC motor |
| 0–10V signal | Direct for EC motors – best solution |
You can find suitable speed controllers and other climate accessories in our shop.
Noise Level and Manufacturers at a Glance
A technically perfectly designed fan can still be annoying in everyday life – namely, if the noise level does not match the use of the room. Especially in bedrooms, acoustics decide the acceptance of the entire system. Therefore, pay attention to the manufacturer's sound data during selection and compare it with the guidelines for the respective room type.
Noise can be influenced by several factors: a larger fan at a lower speed is quieter than a small one at its limit, silencers in the duct reduce transmission into the rooms, and decoupled mounting prevents structure-borne noise in the building. Those who consider these points in planning save themselves expensive rework.
Guidelines for Noise Level
- 25 dB(A) – Bedroom
- 35 dB(A) – Living Room
- 40 dB(A) – Office
- 70 dB(A) – Commercial Kitchen
Manufacturer Comparison
When choosing a manufacturer, it's worth looking at the application profile: not every brand covers every segment equally well.
| Manufacturer | Strength | Recommended for |
|---|---|---|
| S&P (Soler & Palau) | Huge range, EC motor | Residential and commercial ventilation |
| Blauberg | Made in Germany, Premium | Decentralized ventilation, industry |
| InoxAir | Stainless steel, smoke extract | Commercial kitchen, fire protection |
Models designed for fire protection can be found in the category Smoke Extract Fans.
Frequently Asked Questions
How much volume flow does a single-family house need?
For 150 m², approx. 200–300 m³/h – Calculation: 0.4 air changes × room volume. The actual design depends on the number of people, room layout, and moisture loads.
What is the efficiency of an EC motor?
EC motor 85–95%, AC motor 60–75% at full power point. In partial load operation, the advantage of the EC motor is even more pronounced.
Do I need a frequency converter?
Not with an EC motor – control is direct via 0–10V. For AC motors: step controller or frequency converter.
When do I need a smoke extract fan?
For special constructions, stairwells, and underground car parks. Classification F200/F300/F400 according to EN 12101-3.
Conclusion
Selecting a radial fan is not a guessing game but follows a clear calculation chain: first the volume flow according to the application, then the pressure loss of the duct network, and finally the motor, control, and sound insulation. Those who accurately calculate volume flow and pressure loss and opt for an EC motor will get an efficient and quiet radial fan that operates reliably and economically for years. Always consider the filter reserve and acoustics – both are most often underestimated in practice. All suitable components – from the fan to the controller – can be found in the ventilation assortment of wohnungstechnik.de.
