Last edited 08 Jul 2014

Operative temperature

Operative temperature (previously known as resultant temperature or dry resultant temperature, but renamed to align with ASHRAE and ISO standards) is a simplified measure of human thermal comfort derived from air temperature, mean radiant temperature and air speed. It can be useful in assessing the likely thermal comfort of the occupants of a building.

Actual thermal comfort is dependent on environmental factors, such as air temperature, air velocity, relative humidity and the uniformity of conditions, as well as personal factors such as clothing, metabolic heat, acclimatisation, state of health, expectations, and even access to food and drink. However as empirical fits to these variables are very complex (see predicted mean vote), a simpler measure can be more useful in practice.

Operative temperature is defined as:

Operative temperature = (tr + (ta x √10v)) / (1+√10v)

Where

ta = air temperature

tr = mean radiant temperature

v = air speed (m/s)

Or:

Operative temperature = (( hr x tr) + (hc x ta )) / ( hr + hc )

Where

hc = convective heat transfer coefficient

hr = radiative heat transfer coefficient

Where the air speed is less than 0.1m/s, (as is typical in buildings) radiative and convective heat transfers may be similar, and so the equation can be simplified to:

Operative temperature = (ta + tr)/2

In many spaces, with low air velocity and where air temperature and mean radiant temperature may be similar, air temperature alone can be a reasonable indicator of thermal comfort. However, in spaces where surfaces may be heated or cooled, where there is significant thermal mass, or where solar radiation is present, air and radiant temperatures may be very different and so it is necessary to take account of radiant temperatures in assessing thermal comfort.

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