Electric space heaters use either convection or radiation (or a combination of both) to transfer heat to their surroundings.

Radiant Heating

Radiant (infra-red) heat is produced by anything hot, a familiar infra-red source being the sun. In electric radiant heating, an electric current is used heat an element. They are quite similar in operation to the electric light bulb, although designed to produce as much infra-red heat as possible.

Advantages of Radiant Heating

  • Thermal radiation is not absorbed by air, and so they can efficiently warm people or objects in exposed and draughty locations, or infrequently occupied buildings with high ceilings, without incurring high 'heat-up' energy costs
  • Rapid operation means that heaters can be switched on or off to match occupancy patterns, for example using suitably rated PIR sensors
  • Can provide different levels of heating for different zones
  • High reliability, having no moving parts

Disadvantages of Radiant Heating

  • Line-of-sight operation may result in cold spots in any 'shadow' areas
  • Quartz units do produce some visible glare.

Common Types of Radiant Heaters

Quartz

These heaters use high temperature tungsten filament heating elements and operate at high temperatures. The element is encapsulated in a quartz tube and coated with a special material designed to maximise the output of infrared heat.

  • Very high radiant efficiency
  • Short heat-up and cool-down time -- measured in seconds
  • Weather proof versions for outdoor use
  • Quartz tubes are relatively fragile
  • Typical bulb lifetime 5,000 hours
  • Visible glare can sometimes be considered obstrusive, although lamps are available which are designed to make this aestheically more pleasing, for example the 'gold' qaurtz lamps which are often used in churches.

Ceramic

The ceramic heater element is produced using a specialised process in which an alloy resistance wire is cast a ceramic body. The ceramic element is normally glazed to reduce moisture ingress into the ceramic material.

  • Lower radiant efficiency than quartz
  • Rugged and durable
  • Long heat-up and cool-down time -- measured in minutes
  • Little or no visible glare

Metal

The metal sheathed element has resistance wire encapsulated in a metal sheath, similar to the element of an electric oven.

  • Rugged design with high resistance to vibration
  • Long heat-up and cool-down time -- measured in minutes
  • Little or no visible glare

Glass

  • Metal resistance wire wound around a heat resistant mandrel (often porcelain) and encapsulated in a glass/silica sleeve.

Estimation of Radiant Heat Load

The amount of radiant heat required for comfort heating is generally in the range 100W to 200W per square metre. Estimating the radiant heat load is not an exact science however, and depends on both the nature of the activity taking place in the heated area as well as its size. For example 150 W per square metre may be quite adequate for a busy shop, whereas over 200 W per square metre may be required for church to compensate for the relatively low levels of (physical) exertion. In situations that are exposed to particularly cold and strong draughts up to 400W per square metre of radiant heating may be required.

The directional nature of radiant heating leads itself particularly well to 'zoning' where heating requirements vary within a larger (e.g. open plan) area.

Example:

Radiant heating is required in an industrial unit loading bay with a large roller shutter door that is generally open all year round. The bay has dimensions of 4m x 5.5m. The area to be heated is 22 m2 (4m x 5.5m). As the location has little shelter from outdoor conditions, the required radiant heat load is taken to be 400 W per square metre. The total radiant power required is therefore estimated to be 400 x 24 = 9600W (9.6kW). In practice, a more comfortable and uniform environment would be created by using a number of smaller radiant heaters rather than relying on a single higher powered heater. The siting of any radiant heaters needs to comply with manufacturer's installation instructions; these usually stipulate stand-off distances from walls or ceilings and may also include addtional requirements, for example that the unit is tilted to ensure adequate circualtion of air to prevent over-heating.


Convection Heating

Heating by convection takes place when a hot object heats the air which then circulates around the room, either naturally due to the buoyancy of heated air or driven by an electric  fan.

Advantages of Convection Heating

  • It provides a comfortable and uniform environment generally preferred for home or office environment, rather like natural indoor summer conditions

Disadvantages of Convection Heating

  • Large spaces require a long and potentially expensive heat-up period, which can be especially problematic if the building is only intermittently used, for example, churches or similar buildings.
  • In poorly insulated buildings the energy losses can be high. In extreme cases, such as near to open roller shutter doors, the loss of heat can make any form of convection heating impractical

Common Types of Convection Heater

Fan Heaters

  • Rapid bursts of localised heating
  • Compact, light and transportable
  • Cooling (fan only) operation in summer

Convection Heaters

  • Silent and quick to heat up
  • Designed for whole room heating
  • Can be wall mounted

Oil Filled Radiators

  • Eliminates dust burning on high temperature elements
  • Provides a comfortable balance of both convected and radiated heat
  • Portable

Panel Heaters

  • Compact size
  • Can be wall mounted
  • Avoids dust burning on exposed high temperature elements

Estimation of Convection Heat Load

It takes more power to maintain a comfortable temperature in a large, poorly insulated room than it does in a small, well insulated one. How much heating power is required can be estimated by first determining:

  1. The volume of the room (cubic metres)
  2. The difference between the required internal room temperature and the coldest outside temperature that is anticipated (oC)
  3. How well insulated the room is. This is expressed as a simple number which varies from about 1 if the insulation is considered good (a typical modern house) to about 2.5 if the insulation is poor (an older draughty house, e.g. with large windows and high ceilings).

The heating requirement for the room is then estimated by multiplying the volume, temperature and insulation numbers together, and adding another 20%.

Example:

Suppose we have a poorly insulated room of dimensions of 5m by 5m by 2m which needs to be maintained at 20oC. An external outside temperature of 0oC is assumed (the 'coldest design day').

We multiply the volume of the room (5m x 5m x 2m = 50 m3) by the required temperature difference between indoors and outside (in this case, 20 oC) and the insulation factor (2.5 in this case) and add 20%. The value obtained (50 x 20x 2.5 x 1.20) is 3000W (3kW).

The room could be heated with a single 3kW convection heater, but in practice it is generally better to have a number of lower power units rather than one high power unit to avoid hot or cold spots, and also to locate heat sources close to naturally cooler parts of the room such as the external walls and windows. A total convection heating load of 3kW could be satisfied by using, say, two 1.5kW heaters.


This guide explains some of the general principles of space heating, but does not relate to any specific products. All electrical products must be installed in accordance with the manufacturer's specific instructions, relevant regulations and by competent persons.