What Kind Of Electrical Heater Is Suitable?

Press, Volume CV, Issue 31057, 12 May 1966, Page 10

What Kind Of Electrical Heater Is Suitable?

(By

C. J. SUCKLING,

supply engineer, M E D.)

What type of electrical heater? This is a question frequently asked. Each manufacturer claims special advantages for his type, whether it be radiant, convector, infra-red (or a combination of any of these), storage block or heated floor; not to mention the many models of each type.

All electric heaters deliver 3142 British Thermal Units a kilowatt-hour. A British Thermal Unit is the heat required to raise lib of water 1 deg. F. It would appear, therefore, that all heaters are equally effective. This would be true if all the air in a room were required to be raised to the same temperature and the losses in the room were all constant for all positions and surfaces. A Gradient This is not the case, of course, and because heated air rises, there is a temperature gradient between floor and ceiling. Also different surface materials have different thermal conduction. Therefore a room having large areas of glass (which conducts heat more readily outdoors than other

heating materials) will require a greater input of heat than rooms with a smaller area of window. This is a very small part of the story of the heating requirements for dwellings, but it will serve to show that it is the method of heat distribution which is important and is the basis for comparison of different types of heaters.

Radiant heaters (whether the type with the refractory material heated with an electric element to a high temperature and equipped with a reflector to focus the radiant energy into a beam, or the infra-red heater which concentrates its energy into a longer wave length) have their energy absorbed in solids rather

than the air. This means that they heat the body of a person quickly and without the energy being absorbed by the air through which it passes.

Infra-Red Infra-red heaters are more efficient in this respect, and have greater penetration, thus giving the sensation of over-all warmth, whereas the normal type of radiator tends to make one feel cold on the side facing away from the source of energy. Because of their low storage t capacity they are unsuitable for use with

input control devices such as thermostats. They heat quickly and cool quickly, the sensation of heat being analagous to the sun being obscured by a cloud. Convector heaters are designed to heat the air passing through them and to distribute this heated air into the room with or without the aid of a fan. Unless some velocity is given to the heated air in a horizontal direction, poor distribution will occur, and wall and ceiling staining because of the settling of fine dust carried upward by the air. They are suitable for use with control input devices and generally give more

pleasant conditions than radiant heaters, but are more expensive to run. Storage Block Storage block or heated concrete floors can take advantage of off-peak tariffs, which offset their higher capital cost, if the reduction in tariff is substantial. They heat the air as a convector heater does, although the block type is run at a much higher temperature and delivers some radiant energy. They are unsuitable for quick heating because of the refractory mass necessary to obtain the storage capacity.

Rooms with heated floors can be comfortable with slightly lower air temperatures because if the feet are warm the sensation of comfort is greater. They are frequently installed for background heating with radiant heaters used to supplement the heating where and when required.

Whatever the type of electric heater used, greater economy, lower temperature gradients, and less pattern staining of ceilings can be obtained by the installing insulation in the ceiling. In new houses the expense of full insulation can be justified, although the climate in New Zealand is hardly severe enough to justify the installation of double glass windows.

Heat Wasted

A great deal of heat (whatever the source) is wasted and with the present, and particularly the future high cost of producing electrical energy, losses must be reduced if the use of electrical space heating is to be nationally economical or remain on competitive tariffs with other fuels.