Air curtains are designed to counteract convection heat losses by using a fan to direct a thin jet of air across the width of the opening. To counter residual heat losses, air curtains normally incorporate an internal heat source to raise the temperature of the air passing through the unit. If the situation is more demanding, then a full door width air curtain is likely to be required. To be fully effective as thermal barriers they should cover the full width of the door, otherwise cold air may penetrate at the edges. If a door opening is double width, then a suitably sized (double width) air curtain will be required. Units in this category require a three phase electricity supply.