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70-71 CONDENSATION:SPN Master pages 13/9/08 00:58 Page 3
layout of the air distribution duct channel
SO WHAT IS HUMIDITY? and air outlet grilles is an important HOW DOES A ‘POOL COVER’
Expressed as ‘% Relative Humidity’ this element in condensation control. HELP THE PROBLEM?
way of referring to the quantity of water The air duct channel should be designed Savings on operating costs can come
in the pool room air is misunderstood to ensure: through a pool cover.
and misapplied. •Air is discharged via grilles directly over It acts to ‘seal off’ the pool water
The Relative Humidity Level is an external glass areas prone to surface, reducing evaporation and heat
assessment of how much more water the condensation. loss to a minimum.
air can hold, before ‘saturation’ and • There are no significant parts of the pool When a pool surface cover is in place, the
condensation will occur. Theoretically room through which air is not being pool room air can then be allowed to fall to
100% RH equals saturation, when the air circulated. a temperature below that of the pool water
can support no additional moisture. • That air discharge velocity across or without any adverse effects. This is referred
So, in essence, the cooler the air, the towards the actual pool water surface to as the ‘set back air temperature’.
less moisture can be contained and, is minimal. On pools where a conventional cover
therefore, condensation will occur sooner. • Drainage pipe or small bore pipes cannot practically be used, to help with
cannot be used for air ducting due to energy use, it is worth considering a liquid
the high accumulative resistance / cover, which acts as a partial evaporation
What is the ideal humidity level? pressure drop. inhibitor whenever the water surface is still.
In a typical indoor pool, at 30°C air • Flexible or ribbed air ducting offers a
temperature, the humidity would be very high resistance on the flow of air.
controlled at around 60% R.H., providing • All thermal insulation must be on the can breathe to the ambient outside air,
pleasant conditions for bathers and control outside of the duct. Insulation on the allowing the moisture to escape and not
condensation on the pool room structure. inside is prone to becoming unattached concentrate and condense to water.
and can block the duct. Pitched roofs must have ventilation
Factors Affecting Heat Loss and • Flexible connections in the duct work provision. Flat roofs can either have
Humidity: will help prevent operational vibration breathers in flat surfaces or ventilation
The affect rate of heat loss from the pool transferring into the building structure. through the edges.
room air comes from: • External fresh air intake grilles should be
• Structural / fabric heat loss from the at least 1900mm from any boiler flue. Glazing:
pool room roof and walls etc. • The fresh air intake and air exhaust Even if the pool room environment is
• Amount of out side fresh air ventilation grilles should be kept a distance apart. correctly controlled, some surface
into the pool room. condensation can still be anticipated on
Under Floor Ducting: double glazing during the colder winter
The heat loss from the pool room structure Under floor ducting should not be used if months as the internal surface temperature
is calculated using the probable ground water drainage problems are of the glass can be below the dew point
temperature difference between the out- found to exist on site or a high water table temperature. If this is not acceptable the
side ambient fresh air temperature and the is present. use of triple glazing should be advised to
internal pool room air temperature and the The pool shell should be positioned the client. Due to the comparatively poor
thermal insulation property of the various away from the walls and foundations to thermal insulation value of glass, the
parts of the pool room structure. allow sufficient space for the under floor overall operating costs for the project will
air ducting to be positioned along side largely depend on the amount of glazing
Fresh Air Requirements: where necessary. within the design.
The majority of modern heat recovery The air ducting and discharge grilles Current new building control regulations
systems ‘re-circulate’ the pool room air. The should be located as close to the walls / stipulate the use of high quality glazing
amount of fresh air ventilation which glazing as possible. The grille louvers which should be more than suitable.
requires to be deliberately introduced should be angled towards the glass and
depends upon the nature of the application: be fitted with manually adjustable Roof Windows:
A small lightly used domestic pool using dampers / shutters. The use of roof light glazing is discouraged
pool surface cover will require only minimal as they can often become a trap for
fresh air ventilation. A large commercial Breathing / ventilation of cold roof localised condensation. In addition, it is
pool with high levels of usage will require a space: difficult to provide a good seal between
much higher rate of fresh air ventilation. In practice no vapour barrier will be totally the ceiling vapour barrier and the roof
effective as a small amount of vapour can light, making the surrounding area
Air Distribution Methods: still be passed through the insulation to vulnerable to condensation.
Many indoor pools use a ventilation system cooler areas of the roof. It is therefore If multi-wall polycarbonate sheeting is to
from a dehumidification unit located in the essential within a ‘cold roof’ design to be used, limited condensation can be
plant room, so the correct design and provide a space above the insulation which expected during the winter months. spn
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OCTOBER 08 swimming pool news 71
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