Demand control ventilation offers a range of solid benefits, according to Alan Macklin.
The primary purpose of ventilation is to provide good indoor air quality. Modern buildings are becoming increasingly airtight as the construction industry looks to meet ever more demanding energy efficiency targets.


However, this can lead to the undesirable effect of exposing occupants to chemicals from modern building fabrics and furnishings. With the average person spending 90 per cent of their time indoors, ventilation is critical in order to prevent adverse health consequences. Inadequate ventilation leads to poor air quality causing occupant discomfort, health problems and damage to the building fabric.

For example, in schools poor air quality has been shown to have a detrimental effect on children's energy and concentration levels, while sick building syndrome remains a problem in the work environment.

Indoor air quality is adversely affected by a number of factors, including:
· Volatile organic compounds from aerosols and modern materials.
· Carbon monoxide from smoking and combustion appliances.
· Humidity from cooking, washing and perspiring.
· Mould spores from dust.
· Allergens from dust mites.
· Carbon dioxide and odours.

Ventilation systems are often based on lowest initial cost, ignoring the fact that during the life of the fan the operational cost far exceeds the capital cost of the fan itself. The fan should be correctly selected to supply the specified volume flow rate against the system resistance at the best possible efficiency and the lowest possible sound levels. In practice, however, fans are normally selected for the worst case scenario and for most of the time a lower air flow rate would suffice. Speed controllable fans are therefore favoured to match the fan duty against the actual demand.

The European Council in 2007 adopted ambitious energy and climate change objectives for 2020 - to reduce greenhouse gas emissions by 20 per cent, rising to 30 per cent if the conditions are right, to increase the share of renewable energy to 20 per cent, and to make a 20 per cent improvement in energy efficiency (Source: Energy 2020 - A strategy for competitive, sustainable and secure energy. Publications Office of the European Union, 2011).

Implementation of the European Union energy objectives in the HEVAC sector is legislated by the Energy Performance of Buildings Directive (EPBD) and the Energy related Products Directive (ErP). The EPBD is applicable to fan systems through Parts F and L of the Building Regulations which specifies minimum ventilation rates and specific fan powers. The ErP Directive is applicable to fans and electric motors through the specification of minimum IE2 efficiency levels for single speed three phase motors from 16 June 2011 and the introduction of mandatory minimum Fan Motor Efficiency Grade (FMEG) fan efficiency values from 1 January 2013.

The Specific Fan Power is a measure in W/(l/s) of the energy input determined by the pressure losses of the duct system, the pressure losses inside the ventilation unit and the fan efficiency. A low system pressure loss combined with an efficient fan will result in a low specific fan power.
SFP = Electrical input power
Air volume flow rate

Direct driven motors eliminate the drive train transmission losses. Typical small AC asynchronous motors up to 1kW can reach efficiencies from 60 per cent up to a maximum of 75 per cent. The highest possible motor efficiencies in this power range of 80 per cent up to a maximum of 90 per cent can be achieved using EC/DC motors with consistently high efficiencies across the speed control range. Larger powers with consistently high efficiencies are catered for by IE2 to IE4 efficiency induction motors coupled to a VSD (frequency inverter).


Traditionally, building services fans have tended to rely on double inlet forward curved centrifugal impellers, which have impeller efficiencies in the order of 60 per cent. Far higher impeller efficiencies of around 80 per cent can be obtained with aerofoil section backward curved centrifugal impellers.

Legislative drivers will force a swing towards high efficiency direct drive axial flow and backward curved centrifugal fans.

An efficient fan and control system will not provide the desired savings if installed in a poorly designed system.

Good system design is critical to reduce the overall energy costs as a reduction in the system resistance will proportionately reduce the specific fan power.

Building ventilation systems often operate at constant or pre-determined ventilation rates regardless of the occupancy level of the building. Ventilation rates are normally based on maximum occupancy levels resulting in consequent energy wastage. The energy wastage is not only due to the fan operation, but also includes the energy used to condition the air, whether in heating or cooling mode.

Demand control ventilation (DCV) is recognised as a method of ensuring a building is ventilated cost effectively while maximising indoor air quality.

Closed loop speed control for both the EC and AC motor options provides major energy savings as the fan power is proportional to the speed cubed. Sensors are used to continuously measure and monitor ambient conditions in the conditioned space and provide real time feed-back to the zone controller which adjusts the fan speed, modulating the ventilation rate to match the specific use and occupancy of the building.

Significant energy savings are made by effective DCV which ensures that the ventilation rate continuously matches the current occupancy rate and varying ambient conditions.