Sir: David Nowell (letter, 2 January) condemns the fact that retailers appear to be "wasting large amounts of energy" by leaving shop doors open in sub-zero temperatures.

Mr Nowell may not have fully appreciated the major advances in air separation technology made in recent years. Modern "air curtains" effectively separate the internal and external climates, preventing major heat loss through an open doorway, reducing uncomfortable draughts and, most importantly, making a positive contribution to the overall heating efficiency of the building heating system.

While there are no doubt many thousands of old-technology door heaters still in use, major British retailers have recognised the energy-saving benefits of the new air separation technology and have invested heavily in such equipment.

Mr Nowell may be interested to learn that shops with "open door trading" attract more potential customers than retail outlets where the door is closed. The benefits of air separation systems have also been welcomed by those who operate buildings where continually-opening doors are a necessity - hospitals, clinics, hotels, leisure centres, banks and building societies.

PAUL DONOVAN

Biddle Air Systems

Nuneaton, Warwickshire