Heat transfer fundamentals: radiant versus convection heat
A kilowatt of radiant heat and a kilowatt of convection heat do not have the same heat transfer properties. Whilst both take a kilowatt of energy to produce, their heat transfer properties are markedly different, implying dramatic differences in the amount of heat capacity you need to install and the length of time you need to run them. So as far as heating your home, office or workspace goes, there is a considerable energy difference at stake.
Heat Transfer methods:
There are 3 forms of heat transfer: Conduction, Convection and Radiant heat.
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 is the transfer of heat by movement of liquids or gases. Its properties are:
- 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;
- Air absorbs heat poorly and transfers it back out to other objects poorly;
- Air is not “zoneable” and rapidly cools when the thermostat switches off (i.e. the heater is only effective when on).
Radiant heat has considerable advantages over convected heat for comfort heating because:
- Radiant heat has a higher “flux” (watts output per metre per degree centigrade of the heater) than convection heating. You require less radiant heat for less time to do the job of more convected heat for more time;
- A Radiant heater directly 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. This effect is felt less with heated air which transfers heat into objects poorly;
- 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.
Different effects of the same kilowatt
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”. Is this right?
A kilowatt of heat – whilst indeed being a kilowatt of energy from whatever source it emanates – does not mean the same physical heating effect occurs regardless of the type of heat transfer method. Thinking about a 2kW oven versus an 800W microwave should make this instantly apparent. It’s all a question of what you’re wanting that kilowatt to achieve and employing the correct type of heat for the correct function.
Here is a comparison of different types of heat emitter, all rated 1kW.
Wet radiator panels are assigned a BTU (British Thermal Unit) value rather than a Kilowatt value. 1 kW is roughly equivalent to 3400 BTUs.
A 900mm x 620mm (0.55m2) single panel convector (3433 BTUs) is our closest match to a 1kW radiant heater.
This panel of 0.55m2 frontal area at 60C will radiate only 311 Watts at 8 – 15 microns (i.e. cool) 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 by 1 kilowatt of 25m3.
A Herschel Far Infrared panel of 1m2 area at 90C radiates 0.9kW at 5-12 microns at the panel surface.
The panel has a larger radiating surface than the convector radiator and emits up to 40% higher temperature for the same kilowatt input.
You will feel the heat directly on your skin in the “red” zone in front of the heater shown in the diagram but heat will still be emanating out to heat the building in the orange portion of the heat bloom and beyond.
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 directly into the target at a higher rate instead of into the intermediary of the surrounding air (which absorbs heat poorly and transfers it back out again poorly and is subject to infiltration loss).
This means lower capacity is required from the heater in the first place and running times are significantly shorter than for convection heaters.
The estimating rule of thumb for a Far Infrared panel is 25 watts per cubic metre of room giving a room volume to be heated by 1 kilowatt of up to 40m3. There is also no system loss like there is with a central heating type of system and no degradation of performance over time.
A Herschel Advantage IR2 space heater comprises 2 x 650W ceramic emitters (i.e. 1.3 kW), emitting 400C at 2 – 10 microns. (8 micron bandwidth).
2 – 10 microns is hotter than 8 – 15 microns, but still mostly contains wavelengths of the more “comfortable” heat (3-10 microns).
This sort of heater would be suitable for heating people comfortably in open spaces up to 9m2 from 2.3 to 2.5m away.
Although this heater is “hotter” than the far infrared panel, remember its intended use is in large open spaces and where you may have draughts, open workshop doors etc. so you are going to require more power. It is the most efficient heater in certain space heating applications, where both the preceding types of heater would not be effective.
A 1kW quartz heater emits 1500C at 1-2 microns. (1 micron bandwidth).
1-2 microns is extremely hot (1500C) and is suitable for cooking meat and welding plastic.
At this narrow (hot) bandwidth, you’d cover a lot of ground (throw) with this type of heater, but not much “spread”.
This type of heater is more applicable where you really need the throw, because of distance or air movement and the above types of heater would not be appropriate, but close-too they would feel extremely hot and uncomfortable.
You can get very different physical properties out of heaters each rated 1kW. But some heaters are evidently optimal in certain situations where others are not. And therefore use of that kilowatt really depends what you want it for. Mis-application results in an inefficiency in the use of that energy. As much as heaters that are too hot for a particular application represent wasted energy, heaters that are too cool also represent wasted energy because you need more of them. What you really want is the optimum heater for the correct application. In terms of comfort heating a room, when comparing one kilowatt of Far Infrared against one kilowatt of Convection, you’ll experience up to a 60% difference in the heated area and running times using 1 kilowatt of radiant heat versus 1 kilowatt of convection.