Understanding fan speed control

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Why to control fan speed?

Indoor Air Quality

Fresh air supply is essential to create a healthy and comfortable environment. Since most people spend the majority of their time inside, indoor air quality matters! Temperature, relative humidity, CO2 and TVOC are the most important parameters that determine an occupant's comfort and productivity. When many people share a confined space, fan speed and air volume flow will be maximised in a well-dimensioned ventilation system. This is necessary to guarantee a good indoor air quality and to keep the CO2 and TVOC concentrations at an acceptable level. When fewer people are present in the same room, a lower supply rate of fresh air will be sufficient. This variable regulation of the ventilation system will increase the comfort of the residents and save energy.

COVID-19 contamination via aerosols

The risk of becoming infected with the COVID-19 virus is higher in crowded and poorly ventilated spaces. Transmission of COVID-19 via aerosols rarely occurs outdoor or in enclosed spaces with a large volume. The general recommendation is to deactivate indoor air recirculation, to increase the supply rate of fresh air and the extraction rate of stale air. The ventilation system should remain active on a continuous basis. Most ventilation systems are dimensioned to provide crowded spaces with sufficient fresh air. In case the room is not occupied, the ventilation rate can be reduced.

Energy savings

A slight reduction in fan speed has a major impact on the electrical energy consumption of the fan. A typical HVAC fan follows a quadratic torque curve. Depending on the motor type, a reduction of 25 % air volume flow corresponds with 50 % less energy consumption.

Minimise heat losses

In colder or moderate climates, extracted warm indoor air is replaced by fresh, colder air. Reduced air extraction results in a reduction of energy losses due to heat loss. By continuously optimising fan speed to maintain a good indoor air quality, a significant amount of energy can be saved.

Extended service life

A reduction of the average air volume flow causes the air filters to last longer. The clogging of these filters is related to the total volume of air handled by the system after a certain period of time. A prolonged service life reduces the operational costs.

Increased comfort

Reduced risk for Covid-19 contaminations

Energy savings

Minimise heat losses

Extended service life

Optimal indoor air quality

How to control fan speed?


In general, we distinguish EC fans and AC fans. In the next chapters, we will explain the difference between both motor types in detail. An EC fan can be seen as an AC fan with integrated fan speed controller. So an AC fan requires a separate speed controller.

There are several technologies available to control AC fans. We will explain following types:

These AC fan speed controllers can be controlled either manually - via buttons or a knob on the speed controller itself. Or they can be controlled via an external control signal. EC fans always require an external control signal to specify the requested fan speed. This control signal can be digital (Modbus RTU) or analogue (typically 0-10V). It can be generated manually or automatically (e.g. via an HVAC sensor). Following devices can generate a control signal for an EC fan or an AC fan speed controller:

The next chapters provide an overview of the advantages and disadvantages of these different technologies. First, we will take a closer look at the fan. In general, two types of fans can be distinguished: EC fans and AC fans.