If the situation is relatively undemanding, an overdoor heater mounted above the door may be a practical solution. These units provide provide heating close to the point of heat loss, thereby eliminating cold spots. However, they generally have only limited effectiveness as thermal barriers (especially against wind) as they are unable to produce the aerodynamic flow across the full door width required of a 'true' air curtain. However they do have a number of benefits such as small size, low capital cost and ease of installation (being typically rated at 3kW they can be wired to a fused spur from a standard ring main). 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. If the unit is to be mounted significantly above a doorway, or if the door opening is unusually tall, then a more powerful unit is likely to be required to maintain the airflow. Typical applications for single phase (230V) electric units include include doorways in small shops, hotels and other commercial premises.