Hot air is lighter than cold air

· A cubic meter of 20 ºC air weighs 1,205 g
· A cubic meter of 60 ºC air weighs 1,060 g
· A cubic meter of 100 ºC air weighs 946 g
· A cubic meter of 200 ºC air weighs 746 g
The hot air coming from the stove rises up directly like the smoke from a chimney on a cold, calm winter day. In a sauna, the hot air does not spread out horizontally, but it rises directly up to the ceiling, spreads out along the ceiling, and falls down mattress-like as there is more hot air coming up from the stove, forcing the cooler air down.

When the sauna heaters were introduced in the seventies, the ventilation pattern remained unchanged. The ventilation was originally designed for the wood-burning stoves:

· The intake was down behind the stove in order to provide oxygen to the fire.
· The exhaust was in the ceiling or up on the wall in order to dry the sauna and to take away excess heat.

Click to see this image bigger... The fire in a wood-burning stove requires a lot of air, and the stove heats up a sauna easily so that the excess heat can be taken away without loss of löyly. The two vents provide effective ventilation.

With the electric heater everything is changed. There is no need for air supply for a fire, and an electric heater does not have an over capacity allowing for a large exhaust of hot air.

Click to see this image bigger... To avoid the loss of hot air, exhaus vent was installed down to the level of lower laude.

Traditional ventilation coupled with an electric heater results in a large temperature differential between the ceiling and the floor.

The differential is diminished by the heat radiating from the heater, which warms the legs of the bathers. As a heater is not an effective radiator, there is a lot of excess heat in the sauna. The ears are burning, which can partially be avoided by increasing the amount of exhaust air through the vent in the ceiling.

This arrangement increases the ventilation in the sauna but at the cost of sharply increasing the energy consumption. Fresh air reaches up to the bathers only if the exhaust vent is larger than the up-flow of air through the stove. In this case there would also be a continuous air flow up past the heater, bringing in fresh cool air. The result is a quite tolerable, even good, löyly at the expense of wasted of energy.

© SaunaSite, 1997

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