Heat balance & thermal comfort

In order to stay healthy, we have to keep our internal body temperature at an almost constant 37^oC. The heat produced by the body depends on the person’s activity and must, over time, be balanced by the sum total of the heat lost from it. There are essentially four avenues of heat loss:

Heat is also lost from the lungs during respiration , this loss is partly convective and partly evaporative.

The mathematical expression for the balance between heat production and heat loss is one way of calculating the thermal conditions to provide in a building. This is the Heat Balance approach to setting thermal conditions.

The basic for heat balance equation is

M — W = E_sk + R_a + K + C_o + E_r + S (by convention, all units watts/m² of body surface area)

Where M = Rate of metabolic heat production

W = energy used in doing mechanical work

E_e = Heat loss by evaporation

R_a = Heat loss through radiation

K = Heat loss by conduction (generally negligable0

C_o = Heat loss by convection

E_r = Heat loss by respiration

S = Heat stored in the body (= 0 over time)

Because of the importance of keeping the internal body temperature constant, people have a number of ways of controlling the rate at which heat leaves the body. These actions are referred to as thermo-regulation. Some of these are unconscious, such as changes of blood supply to the skin to increase or reduce heat loss, or the onset of sweating in hot conditions or shivering in the cold. Some are semi-conscious such as the changes in posture as we become cold or warm. Some changes are deliberate such as putting on or shedding clothing to change the rate of heat loss, lighting a fire, or turning on a fan to change the thermal environment or moving to a more comfortable place.

Conscious (or behavioural) thermo-regulation is generally triggered by thermal discomfort . Discomfort is usually the result of a change in the core temperature coupled with information from the thermal sense at the skin that the body risks continued thermal imbalance (eg a draught in a cold room). Generally a situation which is moving in a direction to restore thermal balance is considered pleasurable (eg a cool breeze on a hot day). Thermal sensation is part of a feed-back system to maintain thermal equilibrium, so that a comfortable environment is one where occupants can maintain thermal balance as a dynamic equilibrium. An uncomfortable environment is one in which we cannot prevent discomfort. See part 4 for more information on these behavioural actions.

The complexity of the thermoregulatory processes and their dynamic nature has made them difficult to characterise mathematically. As a result the steady-state formulations for thermal comfort such as that of Fanger (1970) have been found to underestimate the extent to which people will naturally adapt to changes in the environment particularly in variable conditions (Humphreys 1992, DeDear and Brager 1998).

Temperature changes over time

Changes of temperature with time can cause discomfort if they are not under the control of building occupants. Changes of temperature within a day due to the controlling mechanism of air-conditioning or the ramping of temperature on a sunny day in a free-running building are generally little problem if the temperature variation is small (< ± 2^oC ). Baker and Standeven (1995) have suggested that larger temperature variations can be accepted if adaptive opportunities exist, such as the ability to move to another part of the building, to change ones dress or to use a fan or an open window to make use of air movement. The adaptive offsets suggested in this chapter give some idea of the range of temperature changes which various adaptive opportunities can be used to offset.

Temperature changes from day to day (for instance as the result of a heat wave or a drop in temperature in the autumn) are acceptable if they occur sufficiently gradually to allow building occupants to adapt to the change. Nicol and Raja (1996) suggest that a temperature changing at about half the rate of the running mean of outdoor temperature will not cause discomfort.

Seasonal changes in temperature are reflected in normal clothing behaviour. Much of this change is the result of cultural habits so that clothing changes occur among occupants of air conditioned buildings despite a constant indoor temperature. Recent research suggests that these seasonal changes in indoor clothing between summer and winter are small and that the resulting changes in the average preferred comfort temperature in the UK are of the order of 2^oC in air conditioned buildings and 3^oC in naturally ventilated buildings.