Air curtains have the potential to save energy, but they need to be designed properly to address end clients’ environmental concerns. Steve Brown discusses the key criteria.
The principle of using air curtains to prevent heated or cooled air escaping through open doorways has been established for many years.

Indeed, the use of air curtains is now commonplace in many premises where the volume of traffic passing through the doorway makes it impractical to keep the doors closed. Now that building operators are so focused on energy efficiency, however, concerns have been expressed as to whether air curtains are the most efficient option for such applications. Inevitably these concerns will feed back from end clients to building services specifiers and installers - so it's important to understand all of the facts.

Heat will spill out

Just to recap on the general principles. When doors are left open, the heated air inside the building will spill out of the top of the open doorway and be replaced by cold air coming in at the bottom of the doorway. This situation may be reversed in the summer, where air conditioned air leaves the building to be replaced by warm air from outside.

While most buildings prevent this energy wastage by closing doors, this is not practical in shops where there is a steady flow of customers entering and leaving, as the doors would be open for much of the time. A similar situation is found in loading bays and other areas with a high frequency of deliveries entering and leaving the premises.

Comfort maintained

Consequently, air curtains are used to maintain comfortable conditions inside the building and save energy in doing so. In retail applications, they also enable 'open door trading', making the shop more inviting to customers.

The ability of air curtains to save energy is a key consideration for many building operators and retailers in particular are very sensitive about their sustainability credentials. Tests have shown that a properly designed air curtain will reduce heat loss around a door by up to 80 per cent, so that less energy is used by the space heating system.

This more than compensates for the small amount of heat used to temper the air passing through the air curtain.

In fact, air curtains with thermostatic at floor level, so less heat is wasted. Heat pumps are becoming increasingly popular with many retailers because of their low carbon status and the opportunity to receive money from the government through the Renewable Heat Incentive. However, the heat generated by some heat pump systems can sometimes be of 'low grade' that might not be suitable for space heating without additional heating.

Tempering air

With air curtains, though, the requirement is only to temper the air, which makes them ideal for use with low temperature hot water, as might be generated by a heat pump. Similarly, they can also be configured to work with water temperatures that maximise the energy performance of condensing boilers.

In addition, more innovative air curtains may also incorporate electric heating to 'top up' the heat pump's output in particularly cold weather. Clearly, as with any building services plant, it is vital to ensure that the system is designed so that it delivers maximum efficiency.

This means that each air curtain needs to be designed in relation to the height and width of the particular doorway it is protecting and should be wide enough to ensure the air barrier overlaps the doorway at the sides. The velocity of the air also needs to be configured to suit the height of the doorway, so that the air reaches down to the floor but does not come into contact with it at high velocity as this will disrupt the laminar flow of the air.

In situations where it is not practical to mount an air curtain above a doorway, a vertical air curtain to the side of the door can be used instead - blowing tempered air horizontally across the doorway.

Again, the design is all-important so there are very good reasons for specifiers and installers to call in specialist expertise for all air curtain designs. Using UK-manufactured air curtains also helps to improve the overall sustainability of the project and provides the opportunity for more bespoke solutions.

While extolling the virtues of air curtains, I wouldn't suggest they are ideal for all shop doorways. In a shop that only receives occasional customers, a closed door will almost certainly be the most cost-effective option. But in busier shops and shopping centres with frequently opened doorways, an air curtain in conjunction with automatic doors will be the best option for reducing energy consumption and maintaining customer comfort.

• Steve Brown is development director with Envirotec