Heaters supply heat by convection or radiation or a combination of both.

Convection heaters

In convection heating, air is heated when it comes into contact with hot surfaces in the heater. People feel warmer because of the higher air temperature. Some convection heaters use a fan to draw the cool air in.

Typical residential electric convection heaters include (listed from lowest to highest in terms of cost and performance):

• wall-mounted panel heaters
• wall-mounted or freestanding fan heaters
• oil column or nightstore storage heaters which incorporate a small amount of thermal mass
• ducted central heating systems with floor or low-wall grilles.

Typical residential gas fired convection heaters include (listed from lowest to highest in terms of cost and performance):

• balanced flue gas heaters
• wall-mounted furnaces which may incorporate fans
• boiler supplying hot water radiators
• ducted central heating systems with floor or low-wall grilles.

Characteristics of convection heaters

Convection heaters:

• heat the space from the top down – for larger spaces or spaces with high ceilings, it will take some time for the heat to reach occupant level, particularly when the occupants are sitting
• raise room air temperature more quickly than radiant heaters
• use more energy to achieve the same temperature change as radiant heating in larger spaces
• change the air temperature gradually (fans may increase the rate of air movement) which means that occupants only gradually feel warmer
• make the air warmer close to the heater, so space heating relies on adequate air movement
• create convection currents and temperature gradients as warmer air from the heater rises.

Unflued convective gas heaters are not recommended because of the amount of water vapour and other gases they emit into the space.

Convection heaters with thermal mass

Convection heaters may also incorporate thermal mass. Examples include hot water radiators, nightstore or oil column heaters, underfloor heaters, and solar hydronic heaters. Heaters with high thermal storage generally don’t get as hot on the surface as heaters with low thermal storage, and therefore have steadier heat output.

High thermal storage heaters can be useful for:

• heating spaces with longer periods of occupancy – for example, beyond about 15 minutes
• heating larger spaces that require more even temperature throughout
• providing low impact heating (lower rates of temperature change, and lower air velocity)
• spaces where there is a significant distance to the occupant from the heater
• situations where air temperature needs tighter control over extended periods of time
• rooms where there are high transmission heat losses through walls or the floor or ceiling (for example, a conservatory in winter).

Radiant heaters

In radiant heating, surfaces (including people’s skin and clothing) are heated by infra-red rays of heat from the heater. The air in between is not heated. Typical residential radiant heaters include:

• electric (ceiling) radiant panel or element heaters
• bathroom ceiling-mounted electric heat lamps (with exhaust air duct)
• gas-fired infrared (outdoor) heaters
• gas-fired radiant tube heaters
• gas fires (visible flame) – note: a number of gas fires provide both radiant and convection heating
• electric tubular heaters.

The sun and warmed outdoor surfaces also provide radiant heat.

Unflued radiant gas heaters are not recommended because of the amount of water vapour and other gases they emit into the space.

Characteristics of radiant heaters

Radiant heating provides immediate heat. Someone directly in front of the heater will feel immediately warmer when it is turned on and immediately cooler when it is turned off. Also:

• only surfaces in a direct line of sight to the heater are heated
• though air isn’t directly warmed, it will warm up through contact with surfaces that have been heated
• heating intensity increases as the surface gets closer to the heater.

Uses of radiant heaters

Radiant heaters can be useful for heating spaces with:

• short-term room occupancy (for example, using a bathroom, making a drink in a kitchen)
• a need for immediate warmth
• large size (for example, long rooms, rooms with high ceilings) to directly provide heat to occupants
• high or fast air flow from outside or other cooler spaces (for example, garage, basement, deck)
• high transmission heat loss through walls or the floor or ceiling (for example, a conservatory in winter)
• high humidity (for example, bathrooms) with surfaces that temporarily need to be clear of condensation (for example, mirrors)
• occupants who are very active and mobile while in the room (for example, people doing aerobics or working in a garage workshop)
• rooms with thermal mass (as the heater warms the mass).

A radiant heater will be less effective and less efficient compared to a convection heater if:

• the radiant heat cannot freely radiate into the room (for example, because the heater is behind furniture) and/or directly on to the room occupant
• the heater is distant from the occupant
• the room is already very warm and near to desired air temperature
• the room has longer-term problems with condensation and moisture damage
• tight thermostatic control is required
• safety from touch is an issue.

Combined convection and radiant heaters

Any hot surface provides both radiant heating and convection heating, with the radiant heating component increasing as surface temperature increases. Above about 80C a heater surface is primarily providing radiant heating.

Heaters with heated surfaces that can be touched without causing burns provide fairly equivalent levels of both radiant and convection heating. These heaters typically have a large, warm surface which is oriented vertically. Air that touches the surface is heated and rises, forming convection currents. At the same time, radiant heat is directed across the room. Where air flow is driven by a fan, convection heating will take precedence.

Typical residential mixed convection and radiant heaters (none of which have a fan) include oil column heaters, wall panel heaters, underfloor heating systems, and radiators.

Mixed convection/radiant heaters are typical of heaters used in living rooms and bedrooms. They provide comfort through gentle convective air movement and some radiant heating on the skin.

For your clients

To help your clients make decisions about space heating options, refer them to www.smarterhomes.org.nz/energy/heating.