A first flush device is one that takes the first water from your roof and diverts it from your tank. So if effectively flushes your roof by diverting the first of the rain water away from your tank.
If it important to flush the roof and divert water from your tank, because over time your roof gets dust and dirt and debris from the environment. The rainwater effectively washes your roof. So if you do not use a first flush device, the first of the rainwater which will carry most of the dirt will go straight to your tank.
How does a Rainwater First Flush Device Work?
A first flush diverter should always work automatically. It is no good if you have to be watching it every time it rains.
A first flush diverter should always be installed in the downpipes before the water reaches the tank. The first flush diverter has a floating ball inside it. As the first water is delivered to the tank, the water raises the ball to the top of the pipe which then closes off the first flush diverter. Any subsequent rain is then delivered directly to the tank.
The first flush diverter has a very small outlet that allows water to escape slowly over time. that means that after is has stopped raining the first flush diverter will empty itself and be ready for the first flush of water next time it rains.
The bottom of the first flush diverter pipe can and should be removed from time to time to cleans out any dirt or debris that is too large to escape through the small outlet.
How much water should be diverted on the first flush?
The amount of water that is diverted to the first flush device is determined by the length and diameter of the downpipe used.
A "rule of thumb" to determine how much of the first water to divert is to divert the first 1mm of rainwater on your roof. For example:
- assume your roof is 50m²
- 1mm of rain on 1m² of roof is one litre
- so the first flush for a 50m² roof should be 50 litres
See the table below for an indication of the required length of your rainwater downpipe in the first flush to achieve the first flush of 50 litres
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