Direct heating appliances fall into two main styles and these are radiators and convectors. Radiators can then be subdivided into two further groups, which are those that are fluid filled and those that are not. The first sub-group, i.e. the fluid-filled variety are most similar to the radiators that are used in a conventional gas or oil-fuelled boiler system and in fact in the case of Acova radiators, the same carcase is used for both types of system. However, unlike in the traditional wet central heating system, instead of being plumbed into a network of pipes around the house, the radiators are filled with a hydro-active fluid at manufacture and sealed for life. A thermostatically controlled resistance immersed in the fluid generates the heat.

In contrast, dry radiators have a solid block of material to act as the emitter. This may be in the form of cast iron, aluminium, stone or glass. The iron or aluminium versions have the resistance inserted into the block of material, whereas the stone or glass would have it attached to their rear surface. An example of this type of element is the rear window heater found in a car, although of a much lower wattage the principle is the same. A further development in these radiators is a second element inside the front panel. The principal resistance is used for rapidly raising the temperature and then it switches off whilst the lower wattage front element maintains a thermostatically controlled cycle and allows a gentle heat to be emitted.

The convector heaters work in a completely different manner. These units have grilles in the top and bottom. Air is allowed to enter the lower grille, pass over and be heated by the element and then by convection, pass out of the upper grille and into the room. Traditionally, convector heaters had one or more wire elements, like springs, stretched across their inner carcase. These were quite fragile and could break after a period of use. This method of construction has been discontinued in all but the cheapest varieties and now the elements are usually encased in a finned aluminium block, which makes them much more robust and durable.