Achieving optimum efficiency of mechanical ventilation systems relies heavily on how effectively the fans are controlled. Alasdair Howie explores some of the control options
As buildings have become better insulated and less 'leaky' there has been growing demand for mechanical ventilation to ensure Indoor Air Quality (IAQ) is maintained. In parallel, the requirement to reduce energy consumption by ventilation fans and other services has also increased, seemingly mitigating against the use of mechanical ventilation.

It is now accepted that a suitable compromise is to introduce Demand Controlled Ventilation (DCV) where ventilation rates are aligned to ventilation requirements and there is no wasted energy through 'over ventilation'. And the energy savings that can be made through DCV are significant. Even conservative modelling suggests an overall saving of between 8-12 per cent on energy bills when ventilation rates are more closely matched to occupancy rates.

In commercial settings there are a number of ways in which ventilation systems can be controlled in relation to demand. So it is important to select the most appropriate method and, ideally, to ensure that the chosen solution combines flexibility and ease of use with sensible costs to maximise the return on investment.

Many controls are simple time clocks, or time control through a building management system (BMS), whereby the ventilation system is switched on during working hours and off overnight, at weekends and during holiday periods. The next level is to provide zonal control by gauging the level of occupancy with passive infra-red sensors (PIR), IAQ with CO2 sensors or perhaps measuring other parameters such as relative humidity.

This principle can be taken even further by installing sensors in individual rooms, and by using smart control modules that take inputs from a number of sensor types (temperature, humidity, CO2, PIR, IAQ, VOCs etc). Such control modules can process the signals from all of these sources to give a complete picture of the indoor environment. Inverter control of the fans will then enable ventilation rates to be adjusted accordingly, thus ensuring there is no energy wasted due to a conflict between demands for heating, cooling and ventilation.

There are also other types of intelligent control that can be deployed, such as intelligent gas interlock controls. These are designed to meet the ventilation requirements for commercial kitchens as detailed in HVCA publication DW/172. As such, they directly monitor the fan speed of both fans in a contra-rotating kitchen extract unit and are able to sense a fan failure, switching the remaining fan to a boost condition to compensate for the loss of ventilation.

Subsequently, the gas interlock valve remains open, an alarm indicates that servicing is required and cooking can continue - keeping the customers (and restaurateur) happy.

So there are a number of options available for introducing DCV and generally taking effective control of ventilation. However, as noted earlier, it's also important to ensure that the solution is as cost-effective as possible.

An example that will have resonance with many HVR readers is the requirement in EN 60335-2-80:2005, which states that all cased axial and in-line fans must be wired via a manual reset facility to prevent automatic restart after a fault has occurred. Historically this has often involved the use of costly contactors with current overload sensors or direct-on-line starters to provide the manual reset facility. However, this feature has now been fitted to some simple and low cost electronic speed controllers so that compliance can be achieved with a lower capital outlay.

Historically, domestic ventilation in multi-residential dwellings has involved the use of individual fans in each dwelling and this approach still predominates. This situation mirrors the typical arrangements with heating, where each apartment has been fitted with a separate boiler.

However, the latter situation is now changing as more such blocks are served by a central energy centre, perhaps using combined heat and power, that supplies hot water to a district heating system. The energy saving advantages of this are now well established and there is no good reason why the same principle shouldn't be applied to ventilation.

A single ventilation installation on the roof of a building, serving several apartments, results in numerous benefits for both installers and users. For example, there are few fan units to maintain and they are much easier to access as there is no need to gain access to individual apartments. Similarly, the occupant doesn't have to worry about an individual ventilator.

Such systems can also provide DCV, as each apartment is fitted with vents that are controlled by use of an appropriate sensor so that when ventilation is required the vents open. Again, inverter control enables the central fan to respond quickly to changes in system pressure or volume by altering the fan speed.

The author is UK marketing manager at S&P