A comfortable indoor climate is the result of three variables: air temperature (convection), the temperature of the surfaces in the room (heat-exchange by radiation) and the movement of cold air (draft).
In general, a room with an apparent temperature between 18ºC and 22ºC is perceived as being comfortable. In order to maintain the level of comfort, it’s advisable to minimize the movement of air.
The contribution of convection and radiation to the apparent temperature are roughly similar. An important difference is the instant noticeability of the temperature with radiant heat, whereas convection slowly warms the room by blowing in warm air. This leads to the phenomenon that the apparent temperature in a room with cool air and underfloor heating is perceived as pleasant, while a large window with ice flowers in a room with warm air is perceived as unpleasant.
Systems using air (convection)
To reach the comfort level by means of cooling and heating with air, it’s imperative the surfaces maintain a constant temperature. Especially in winter and summertime, the temperature of the surrounding surfaces may differ substantially from the desired apparent temperature. These surface temperatures vary from approximately 10ºC to over 30ºC. To compensate the difference, the air temperature must be very high or low, respectively. These temperature differences are perceived by the user as being unpleasant. Furthermore, it leads to the movement of relatively large volumes of air, which is disadvantageous for the apparent temperature as well. This often leads to the selection of a top cooling system. The downside is that the desired comfort level isn’t reached several days during the year.
Thermal activation (radiation)
When using thermal activation, the surfaces are maintained at a constant temperature, often approximately 20ºC. With an air temperature between 16ºC and 24ºC, the surfaces give a pleasant (heating and cooling) effect. This increases the level of comfort and keeps the apparent temperature within the desired range.
In addition to radiant heat, thermal activation also leads to convection, as building surfaces partly transfer the heat to the air. Beneficial in this process is that is the bigger the temperature difference, the faster the air temperature adjusts. This prevents the air temperature from leaving the desired range.
Smart thermal activation
Slimline is often equipped with thermal activation. A distinct advantage over other systems with thermal activation is that both the ceiling and the subfloor can be activated to increase the level of comfort. This results in two significant benefits. Firstly, the surface emitting heat or cold is enlarged. Secondly, the convection is amplified because the ceiling is used for cooling (cool air descends) and/or the subfloor is used for heating (warm air ascends). Furthermore, this application fits the known comfort principle of ‘warm feet, cool head’.