Units in this category use an external low pressure hot water supply to raise the temperature of circulating air. 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. Larger units may require a three phase electricity supply.