Having dealt with the basic elements of control, on/off and PID, it is necessary to appreciate the heating system and its sizing to further our development of its control. The main element of designing a heating system is to determine the heat loss from the building when the inside is maintained at a comfortable temperature. As was briefly mentioned in Chapter 4 on sensors and their responses, an index of comfort is not the inside air temperature, tai, but the dry resultant temperature, or resultant temperature, tres:
where tai = inside air temperature (°C) tr = mean radiant temperature (°C) υ = inside air speed (m s-1)
In typical interiors õ is low, of the order of 0.1 m s-1, so
Typically most design centres on a tres of about 20°C. The CIBSE Guide  gives the recommended design values for tres . The guide has been updated with summer and winter values. Table 7.1 shows the old and new recommended values.