Demand control ventilation saves energy while providing the comfort staff require, says David Cook
The need for energy efficiency and the demand for comfort have led to the development of demand ventilation systems. In contrast, with traditional fixed volume ventilation the system is either on or off irrespective of the number of people in the room. Here, you run the risk of creating a mediocre and unproductive office environment though over or under ventilation, with a resulting waste of energy and poor levels of comfort for staff.

Instead, demand ventilation works by responding to the exact demands of a room, supplying or extracting air only when and to the level it is required. As a result there is greater comfort for building occupiers and the improved energy efficiency of the fans means a reduction in fuel costs and carbon emissions.

Demand ventilation operates according to sophisticated control and sensing options. A range of sensors, such as CO2, PIR occupancy detection, humidity or temperature, are employed to determine the room's air quality, adjusting the ventilation requirements automatically and managing the system's ventilation rates accordingly.

The sensors communicate with the main unit which, in turn, drives the fan to the required speed to deliver the airflow, responding exactly to room conditions. These sensors can be combined to generate a hierarchy of control for the ventilation system and its operation can be easily linked to a building management system for full control and monitoring, if required. Therefore only the energy that is needed to ventilate is actually used.

The latest demand ventilation systems integrate 90 per cent energy recovery into the ventilation system. These demand energy recovery ventilation (D-ERV) systems extract the energy from the room's warm, stale air before it is exhausted outside, while fresh, incoming air is preheated via the high efficiency plate heat exchanger and supplied into the room. This energy recovery process is proven to use up to 94 per cent of the heat energy which would otherwise be wasted.

Demand ventilation control limits summer gains and winter losses as only the required ventilation is delivered based on occupancy and indoor air quality need, reducing summer overheat and heating losses in winter.

As buildings become increasingly airtight, manufacturers have discovered an increasing problem with buildings overheating. Therefore D-ERV systems have been developed with 100 per cent summer bypass. When temperatures rise, an automatic summer bypass closes off the airflow from the heat exchanger, while simultaneously opening a bypass which the air flows through, thus avoiding overheating. This also allows the building to take advantage of any free cooling available when ambient temperature is below the room design condition, typical in spring and autumn. Plus, a userenabled night-time purge facility can reduce the start-up loads for a building's air conditioning plant, helping reduce overheating in summer from non-air conditioned spaces.

In winter, the high efficiency heat recovery of D-ERV systems tempers the air to such a level that a building should not suffer from draughts, eliminating the need for re-heaters.

However, with comfort at the forefront of manufacturers' minds, systems have now been developed to include frost heaters so that in extreme winter conditions, on perhaps a couple of days a year, a system can provide a heating boost to achieve thermal comfort for occupiers.

// The author is product marketing manager - industrial at Vent-Axia //