News
05/01/2008
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How Underfloor Heating (UFH) Works
Thermalenergy (heat) transfers in only three ways – by conduction, byconvection and by radiation – and it always moves from a warmer area toa cooler area.
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Conduction is the transferof thermal energy through a material. When you put a poker into a fire,after a while the end of the poker becomes hot.
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Convection uses the air in a room as the transport fluid to transfer energy from one end of a room to the other.
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Radiation is theeffect you feel when you are in the proximity of a warmer (or cooler)mass. On a cool autumn evening, if you walk past a wall that has beenexposed to sunshine during the day and it is still warm, you feel theradiant energy coming from the wall. On a ski slope, if the sky isclear, there can be sufficient radiation coming from the sun to allowyou to sunbathe comfortably, even though the air temperature might below. Radiant energy passes through air without heating it.
Of these three, we have evolved overmillions of years to appreciate radiant energy most because this is theway the sun warms us. Many of us spend a lot of money each year, takingourselves and our families to parts of the world that have high levelsof natural radiation, so that we can bathe in the warmth of the sun.Radiant energy makes us feel good.
Radiators transfers heat mostly by convection.
They are small surface area heaters thatmust be hot by comparison with the temperature of the air in a room.Typically, the power output of a radiator is measured when there is 50Cdifference between mean water temperature and room air temperature(typically 70C radiator and 20C air).
Air in contact with the radiator warms upand then rises to the ceiling. It is pushed across the ceiling by morewarm air coming behind. As it crosses the ceiling, it begins to cooland to drop back into the room. As it descends, it transfers itsthermal energy to the room structure, the contents and the occupants.It is at its coolest when it reaches the floor, across which it returnsfor the cycle – the convection cycle – to be repeated.
With convection, the air in the room isused to heat the room. Stratification of temperature is inevitable –air at ceiling level is always much warmer than it is at floor level.
Radiators are usually sited either beneathor alongside windows in order to generate an upwards stream of warm airacross the cold window. If the radiator were sited on the wall oppositea window, and the air movement cycle was consequently to be in theopposite direction, warm air would arrive at the top of the window andthen accelerate downwards as it was cooled by the glass, drawing coldair onto the floor – which would be uncomfortable.
Siting radiators below windows in order tocounteract downdraught has the consequence of putting the hottest airin the room right by the windows, which are the weakest thermal elementin the room, and ofte