The principles of radiant heating and cooling
We spend virtually three quarters of our lives in buildings either at home, at work or at leisure. The indoor climate, temperature and air quality has a considerable influence on our general well being…
In common with other forms of heat transfer, radiant heat transfer occurs whenever there is a temperature difference between two objects. Unlike other forms of heat transfer these objects need not be in direct contact with each other or indirect contact via a moving fluid. Radiant heat transfer involves the net electromagnetic energy transfer from one object to another object at a lower temperature. Air is effectively transparent to this electromagnetic radiation and is neither heated nor cooled directly.
As objects absorb electromagnetic radiation their temperature increases and the increased surface temperatures lead to indirect heat transfer to the air. An important distinction between radiant systems in which objects heat/cool the air indirectly and conventional systems relying on the air to heat/cool objects, is the difference between the actual air temperature and the perceived air temperature. For a conventional, convective heating system the perceived or resultant air temperature will be equal to the actual air temperature as this air is providing the heating of bodies. A radiant system’s perceived air temperature is associated with not just the temperature of the air but the temperatures of the surfaces, some of which will be heated. Using radiant heating a perceived temperature can be achieved with actual air temperatures around 2°C lower than for conventional systems. This reduced air temperature will reduce heat losses and energy bills.
Radiant cooling systems share the above advantages in reverse with the same perceived temperature being achieved while air temperatures are around 2°C higher than for convective systems.
When a room is reliant on radiant heating there will be a much reduced difference in air temperature between floor and ceiling. Convective systems suffer from the effects of warmer, more buoyant air migrating towards the ceiling and being displaced by cooler air at low level. This stratification will also lead to increased heat losses particularly at high level and through roofs/ceilings. A common misconception is that radiant heating via ceiling panels is ineffective as heat rises – heat doesn’t rise but warm air does as it is displaced by cooler, less buoyant air. Radiant heat transfer is unaffected by the density of the air and occurs from object to object totally unaffected by their elevation i.e. radiant heat can be transferred down from a ceiling just as effectively as it is transferred upwards from the floor or laterally from a wall.
While radiant panels predominantly transfer heat through radiant exchange with objects there is still some convective heat transfer as air is in contact with the panels and their surface temperatures are different to the air temperature. This air is heated or cooled by contact with the panels and rises or drops into the room due to its higher or lower buoyancy. The additional convective heat transfer allows the panel to transfer heat at a rate which is higher than it otherwise would be capable of; around 70 to 80% of heat transfer from a panel is by the more efficient radiant means.
When designing a radiant solution the best results in terms of comfort are achieved when the radiant heat is evenly spread. If too many panels are concentrated in too small a space then occupant discomfort can result due to the effects of asymmetrical radiation; panels should be spread in much the same way as light fittings are distributed – both are sources of electromagnetic radiation. If panels are run at excessively high water temperatures then discomfort due to ‘hot head’ syndrome can result. If mean water temperatures are maintained around 70°C then a panel mounting height of 2.5m will prevent this becoming an issue.
A radiant system does not rely on the movement of air to be effective so there are savings that accrue from the lack of energy required to power the fans and blowers. Additionally, the lack of fans ensures that the radiant system will not add to the noise within the space. Indeed, in some applications, free hanging radiant panels can be utilised in such a way as to absorb noise by using perforations and acoustic insulation on the back.