Heat transfer fundamentals: radiant versus convection heat

radiant cooling A kilowatt of radiant heating 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.

Conduction:

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 heating.

Convection:

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:

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.

1kW "Wet" radiator (3433 BTUs) :-

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.

1Kw Herschel Far Infrared panel : -

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.

radiant cooling

Elio Scotti,

President,

Beka USA,

P.O. Box 6087,

Goodyear,AZ 85338-US

In a Comfort Heating situation, Conduction (physical transfer of heat from source to target by direct contact) is not an option