£5 notes the expensive fuel
November 1st 2004

It doesn’t take much common sense to realise that if a factory door is opened many times a day, or stays open for extended periods, a lot of heat is going to escape. In fact, if you don’t protect the doorway, you might as well be burning £5 notes to heat your premises. John Halley of Nordair explains what it could all add up to in lost profits

Adoorway is a protective barrier it keeps cold air out and warm air in.

Take it away, as the majority of industry premises must in the ordinary course of their operations, and the comfort of employees working inside the building will be affected.

Not only that, but the cost of recovering the heat lost when a door is open can amount to as much as £16 770 in the course of a year. That’s money straight off bottom line profits.

There is no getting away from the need to open external doorways to allow essential delivery and despatch, or just to manoeuvre goods in and out. But each time the door opens, warm air flows out through the upper half of the doorway while denser cold air from outside rushes in through the lower half. In summer, when buildings are air conditioned, the opposite applies.

Keeping the interior uniformly warm becomes difficult, time consuming, energy intensive and very expensive.

Nordair has calculated exactly how much money flows out with heated air when the door opens. The calculation is based on BSRIA’s heat loss formula, using a fuel cost equivalent of 1.5p/kWh, and shows how much energy can be lost through open doorways of all sizes in just one hour.

These totals are based on average outside temperature readings taken during January, and relate to a building where a constant internal comfort temperature of 17°C is required.

Seasonally adjusted figures for a 6m by 6m doorway open for 24 hours a day in a building in operation for 215 days during the October-April heating period, the total cost of energy wasted would be £16 770.

The figures only show the cost of heat loss. It is impossible to calculate how much more money is wasted through cold air gain, but this will cause an additional drain on resources. The heating system would need to compensate for both heat loss and cold air gain to restore comfort temperatures.

Air curtains an energy efficient solution

The costs of heat loss and cold air gain can be reduced considerably by fitting an overdoor air curtain. Air curtains protect and maintain internal temperatures when doors must be opened.

The air curtain works by delivering a curtain of ambient or warmed air across the doorway at constant velocity. It effectively separates the internal and external environments, keeping conditioned air inside the building, thereby preventing heat loss and cold air gain.

To achieve an efficient, low energy solution, the air curtain must incorporate high performance features, such as uniform linear flow across the full door width and fan speed control. Airbloc air curtains, for example, which are engineered for low energy consumption, have been shown to reduce the costs indicated in the table by around 80%.

Air curtains have advantages over other types of doorway solution, such as high speed roller shutter doors. A high-speed door cannot prevent cold air infiltration or warm air escape while it is open, however fast it then closes. Air curtains can. They also offer better visibility and safety a stream of air cannot be damaged by fork lift trucks.

Air curtains are a simple cost-effective solution to the problem of open doorways, keeping warmth inside for the benefit of employees, and £5 notes firmly in the company’s coffers.

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