Have you ever touched a surface warmed by the sun on an early spring day and felt a pleasant warmth, even when a thermometer indicates cold? The principal behind Infrared heating lies in understanding how the sun’s rays work. They are a mixture of electromagnetic waves ranging from infrared (IR) to ultraviolet rays (UV) which travel through the atmosphere without the need of air or any other medium to carry them.
When the sun’s rays reach the object, they heat it from the inside leaving it warm, without heating the air around it. Infrared rays are just under the visible light range in the electromagnetic spectrum.
Infrared heating works by directly heating walls, floors, objects (and humans!) instead of heating the air. These heated objects then release their warmth into the environment. This method of heating is more pleasant to the human body than convectional heating, which warms the air and relies on air currents to spread the heat.
In comparison with the conventional heating, Infrared heating minimises the difference between floor and ceiling temperatures and significantly reduces heating costs, as it heats square meters instead of cubic metres. Infrared heating acts directly and that means it does not require any contact such as air currents to carry the heat. Therefore, the temperature in a room is easily maintained and restored. The heat is accumulated by the structure of a building and objects within a room and then slowly released into the air.
Infrared heating is a great way of heating domestic, industrial, commercial or any other environments. One of its many advantages is that it does not create air currents that raise dust particles, making it ideal for allergy sufferers.
In terms of heat transfer efficiency, a kilowatt of radiant heat and a kilowatt of convection heat simply do not compare. They have very different heat transfer properties and as far as heating your home, office or workspace goes, it is important to know the difference.
In a Comfort Heating situation, Conduction (physical transfer of heat from source to target by direct contact) is not an option, so whilst it is the most efficient method of the three (presuming a suitable medium to conduct of course), we’re left with Convection or Radiant heat.
Convection currents naturally rise as the hot medium (air in this case) expands and decreases in density and as the cool air increases in density and sinks. Convection in a central heating context, therefore, implies warm air rising to the ceiling and then circulating gradually to lower levels in the room, being at its coldest near the floor. This air movement cannot be controlled and heat transfer always works from hot to cold, which you cannot control in the air. If a door to a cold corridor is opened, draughts exist, etc, the warm air will naturally flow there.
You cannot feel a convection current to the side of a convecting surface (any heat you feel would be radiant), only above it.
Any heating “radiators” with fins (to increase surface area and, therefore, convecting efficiency) and surface temperatures up to 60°C are convective and radiate little because of their low surface temperature. A 1m x 1m wet radiator at 60°C will be emitting roughly 6133 BTUs or approximately two kilowatts of energy. However, its radiant output is only 663 Watts and the rest of the energy is spent heating the air. This is fine if you accept that 1.3kW of the heat is being emitted in an uncontrolled manner to a medium that has poor thermal properties (air) and a poor ability to transfer that heat back into objects in the room. Remember: air retains heat poorly; warm air rises; warm air naturally flows towards cold air; air is not “zoneable” and rapidly cools when the thermostat switches off.
Radiant heat has a higher “flux” (watts output per metre per degree centigrade of the heater) than convection heating. The radiant heat output of an infrared heater of the same surface area as the wet radiator above (1m x 1m) operating at 100°C is 1kW and very little energy is spent heating the air.
A Radiant heater heats objects in an environment, not the air in between. So you are heating surface area of objects in an environment, which warm up and turn the environment into a 360° radiator. You are not heating volumes of air.
Objects retain heat better than air, so residual energy maintains temperature in the environment for longer e.g. if a door is opened to a colder room, or when the thermostat turns off the heat source; you can manage the heater by a thermostat set to a lower air temperature because it is the environment that heats up first, not the air.
In terms of heat transfer efficiency, therefore, a kilowatt of radiant heat and a kilowatt of convection heat simply do not compare. They have very different efficiencies. Let’s now compare the heat characteristics of different types of heater, each outputting 1 kilowatt of energy.
One of the frequent challenges we get is “A kilowatt of heat is a kilowatt of heat: you can’t get more out of one kilowatt from one heater than you can out of another”.
A kilowatt of heat – whilst indeed being a kilowatt of energy from whatever source it emanates – does not mean the same temperature and radiant effectiveness regardless of its source.
A kilowatt of thermal energy has a wavelength, an amplitude and a density, all of which can vary in different proportions to make up that “kilowatt”.
However, it is a mistake only to link temperature to wattage, when temperature relates mainly to wavelength but the overall heat power (that “kilowatt”) can be affected by wavelength plus the other two factors (which collectively we can call “flux”).
It is totally possible, for example, to have low wattage high temperature emitters. They can be extremely hot, but just don’t throw that heat very far (think of a tungsten light bulb, for example). At the other extreme, it is possible to have low temperature emitters with a great deal of “flux” (think of a microwave oven). Even in our every day central heating radiators, if we increase panel size, we don’t increase temperature, but we do increase its overall power. So we need to de-couple the idea of a heater’s temperature as the only thing governed by its wattage.
Most heaters (emitters) are specifically designed to output a certain temperature and this relates mainly to its output wavelength in microns. But we govern its power by other factors including size of the panel, or increasing input power to the emitter. Increasing this input power doesn’t make the emitter hotter, it increases its flux.
So can one kilowatt of one type of heater feel hotter than one kilowatt from another and have different radiant characteristics?
Here is a comparison of different types of heat emitter, all rated 1kW. Remember we’re thinking about achieving a human comfort environment of around 21C.
Wet radiator panels are assigned a BTU (British Thermal Unit) value rather than a Kilowatt value, but 1 kW is roughly equivalent to 3400 BTUs. A 900mm x 620mm (0.55m2) single panel wet heater is our closest match (3433 BTUs).This panel of 0.55m2 frontal area at 60C will radiate only 311 Watts at 8 – 15 microns (Boltzmann’s law) and convect the remainder (689 Watts). 8 – 15 microns corresponds to the relatively low surface temperature of 60C. The standard estimating rule of thumb for a convection panel is 40 watts per cubic metre of room, giving a total possible room volume to be heated of 25m3.(This power requirement relates only to the panel. Remember, the upstream boiler or heat-pump must be able to output this heat requirement plus cater for pipework loss plus factor-in the actual efficiency of the boiler and will, therefore, have to produce even more energy upstream).
A Timbertherm Far Infrared of 1m2 area at 90C radiates 0.9kW at 5-12 microns at the surface. The panel has a larger radiating surface and as such emits up to 40% higher temperature for the same kilowatt input. You will feel the heat directly on your skin.
Heat transfer occurs when the emitted radiant energy meets a target object and the energy is absorbed. This is a more efficient form of heat transfer than convection, because a higher percentage of energy transfers into the target at a higher rate, instead of into the surrounding air. This also means overall running times are significantly shorter than for convection heaters. The estimating rule of thumb for a Timbertherm Far Infrared floor is 25 watts per cubic metre of room giving a room volume to be heated of up to 40m3. There is also no system loss like there is with a boiler-based central system and no degradation of performance over time.