Environmental and Social Aspects of Domestic Bathing - Ethical Engineering for International Development and Environmental Sustainability - page 206

room to control and it may be well worth considering a fan that is activated by the
light switch, coupled with an 'overrun timer' facility, better still a humidity
sensor'.

Energy Required to Heat Exchanged Air I fi nd that in the (UK) winter, about
three air changes are needed to clear dampness, that is, about 30 m 3 in the case of a
small bathroom. (My studies have mainly been in Norwich, latitude 52° 37

N,
which is about an average latitude, weighted by population, for the UK.) The den-
sity of ambient air is 1.20 kg/m 3 . Making a small correction for humidity and assum-
ing for this purpose a relative humidity of 60%, one may fi nd that, from data in
National Physical Laboratory ( undated, a ), p 124 of Zemansky ( 1957 ) and section
D p 180 of Weast ( 1975 ), the constant pressure specifi c heat capacity of the replace-
ment air is 1.02 J/(g K). Then energy of 0.13 kWh is needed to warm, from 8 to
21 °C, the incoming air which replaces the extracted air. So the 'air exchange' con-
tribution is relatively small, compared with heating the water and also compared
with a lavish person's use of drying energy, but it is not negligible. It will be much
smaller if there is a heat recovery system (heat exchanger between outgoing and
incoming air) in place. Such a system is important because the whole of any habit-
able building needs heating and air changes.

′

Energy Used by Extractor Fan A fan I have used in two dwellings and is satisfac-
tory has a power of 14.5 W, so the energy used is small: 15 minutes of use would
take 0.004 kWh. A more powerful fan, such as what is recommended in The DIY
World ( undated ), would run for a shorter time (unless inadvertently left running)
and use about the same energy. With some fans, noise is an issue. A fast fl ow of air
almost inevitably is noisy. A quiet fan - some are inaudible if positioned suitably -
with humidity sensor is generally best.

Energy Recovery In an airtight building, that is, one in which the necessary air
changes are nearly all controlled, so that windows are not generally opened and
walls do not breathe, the issue of dampness from bathing (also from drying laundry)
becomes especially important, particularly in a cool, damp climate such as the UK
has. For energy effi ciency such a building must have heat recovery ventilation. Also
known as mechanical ventilation heat recovery, this system uses a heat exchanger
between the inbound cold and outbound warm air, thereby warming the incoming
air and saving some of the heat which would otherwise be lost. For more detail from
a US perspective, see NAHB Research Center ( undated ).

Hairdryers The power is usually about 2 kW (on maximum setting) and drying
last typically 10 min (my estimate of others' use). Assuming 5 min on full power
and 5 min on low power (assumed to be about 1 kW) means 0.25 kWh of electricity,
a signifi cant amount in comparison with that needed for the washing itself.

Qualitative Summary

￿ Avoiding long bathing in general, reserving it as something special, helps to save

vaporisation energy and air exchange energy as well as embodied energy.