The Willis Heater is a self contained water heater; it differs from a conventional immersion heater used in a domestic hot water cylinder because the immersion heater is usually inserted into a boss at the top of the cylinder. There are two heating elements one short and one long, the short element is intended to heat up a small amount of water and is switch on at the “Sink” position on the immersion switch, while the long element is obviously intended to heat up a large amount of water and is switched on at the “Bath” position on the switch. It is not possible to use the two elements at the same time. Because the thermostat probe is located fairly high up in the cylinder, and the fact that the hot water rises to the top of the cylinder, causes the thermostat to cut out the electrical supply so it is not possible to heat the full cylinder of water.
An alternative solution to overcome this problem would be to install a Willis Heater, the domestic hot water cylinder is connected to a Willis Heater which contains a similar heat output element. As the water is heated by the Willis element it circulates naturally through the cylinder, in the same way as a cylinder is heated by a conventional boiler, but does so until practically all the water in the cylinder is heated to the thermostat setting on the Willis Heater. Because the circulating loop is also part of the hot water supply pipe from the cylinder, hot water is available very quickly, even if the cylinder has been drained of hot water because the water is drawn through the Willis Heater unit.