Requirements for thermal comfort study material: Operative temperature

Requirements for thermal comfort study material

2. Requirements

2.3. Operative temperature

There are two parameters that the operative temperature describes – air temperature and mean radiant temperature. The combined influence of these two temperatures is expressed as the operative temperature. For low air velocities (<0,2 m/s), or where the difference between mean. radiant temperature and air temperature is small (<4°C), the operative temperature can be approximated with the simple average of air and mean radiant temperature (ECBSC Annex 37).

This means that the air temperature and the mean radiant temperature are equally important for the level of comfort in a space (Babiak, 2007). Simmonds (1996) found that the traditional design criteria such as dry-bulb temperature and operative temperature were not always sufficient, while mean radiant temperature has an important influence on the comfort results.

Because of the seasonal clothing habits of building occupants, the temperature range for comfort in summer is higher than in winter. The operative temperature range in which, theoretically, no more than 10% of the occupants during light, primarily sedentary activity ( ≤ 1,2 met), in a given level of clothing insulation, will find the environment thermally unacceptable is given in Table 7.

Table 7: Optimum and acceptable ranges of operative temperature for people during light, primarily sedentary activity ( ≤ 1,2 met) at 50% relative humidity and mean air velocity ≤ 0,15 m/s (Ashrae, 1992)

Season	Description of typical clothing	L_cl (clo)	Optimum operative temperature	Operative temperature range (10% dissatisfaction
Winter (°C)	Heavy slacks long-sleeve shirt and sweater	0,9	22 °C	22-23,5 °C
Summer (°C)	Light slacks and short-sleeve shirt	0,5	24,5 °C	23-26 °C
	minimal	0,05	27 °C	26-29 °C

Compared to a convective heating/cooling system a radiant surface heating system can achieve the same level of operative temperature at a lower air temperature and a surface cooling system at a higher air temperature. The transmissions heat loss by radiant heating depends partly on the convective heat exchange between room air and heating/cooling surfaces and partly on the radiant heat transfer between heating/cooling surfaces and the other surfaces in the space (EN 15 377-1). The reference temperature for the transmission heat loss is closer to the operative temperature than to the air temperature. Therefore, there may not be any significant reduction of transmission heat lost with radiation system. It will, however, result in a lower ventilation heat loss due to the lower/higher air temperature by heating/cooling. In buildings with high ventilation rates, such as industrial storage areas this has a significant influence on energy consumption and supports the use of radiant heating and cooling. In modern well insulated buildings, this effect is minor because the difference between mean temperature and air temperature is relatively small (Olesen, 2007).