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Figure 5.2 illustrates the paths of temperature and water content of air for

heating and humidifying outdoor air in winter, at 08C and 80 per cent relative

humidity, in order to get 50 per cent relative humidity at 208C.

A part of the energy required for heating and humidifying the air in winter

is brought from free sources such as solar radiation or metabolic activity of

occupants. All the electricity used for lighting and other appliances tha t are

not part of the heating system end up as heat, in most cases released into

indoor air. Plants and occupants, as well as activities such as cooking and

drying laundry add water vapour to indoor air. When this is not enough to

reach a comfortable indoor climate, the complement is provide d by a heating

system. In this case, about 0.34 Wh is needed to heat or cool 1 m

of air by

18C, as long as the air is humidiﬁed by ‘free sources’. This value is therefore

used in models calculating the energy for heating buildings. If a humidiﬁer is

used, it will take 2.5 kJ (about 0.7 Wh) per gram of water vapour generated.

This heat is taken in the indoor environment if the humidiﬁer does not generate

water vapour but water droplets (spray humidiﬁers).

The paths in the psychrometric chart for cooling and drying summer

outdoor air from 308C and 70 per cent relative humidity down to 208C and

50 per cent relative humidity are shown in Figure 5.3. Note that, for drying

the air, it should ﬁrst be cooled down at the dew point temperature corre-

sponding to the required speciﬁc humidity, and then reheated to the required

indoor temperature. It should also be noticed that it is impossible to cool the

air below its dew point without drying it.

Hot and humid outdoor air cools down and eventually dries on contact with

cold surfaces, on which excess water vapour condenses. If these surfaces are not

cooled down, such as the building fabric or furniture, their temperature rises

and cooling stops after a while. However, the air temperature rises more

0 10 20 30 °C

Water content [g/kg]

Sat.

90%

80%

70%

60%

50%

40%

30%

20%

10%

Heating

Humidifying

90 kJ/kg

Figure 5.2 Paths in the psychrometric chart for heating and humidifying

outdoor air in winter to reach 208C and 50 per cent relative humidity

Measurements and Measures Related to Energy Eﬃciency in Ventilation 81

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