WISY Rainwater Filter Systems


Complete Drainage Safety - DIN EN 12056 / EN 752, DIN 1989

In standard installations the roof drain is installed as a gravity system.
Rainwater flows through the gutter, downpipes, collecting underground pipes to the sewer or soakaway. Therefore no cross-section pipe reductions are permissable which could impede the continuous flow direction.

WISY filter systems in vertical downpipes or connected to underground pipes do not obstruct the water flow guaranteeing required drainage safety at all times.

At the same time the drainage pipes and the filter systems have to be sized according to the water flow of the connected roof areas.
For orientation, WISY indicates approximate guidelines of the roof area which can be connected to the different filters available.

Maximum Rainwater Flow Rates

The below listed flow rates apply for a maximum pipe slope of 1% and maximum pipe filling-level of 0.7 or 70%:

  • Filter collector (FS) for DN 100 (3.9 in.) = 4,2 l/s
  • Standpipe filter collector (STFS) for DN 100 (3.9 in.) = 4,2 l/s
  • Vortex fine filter WFF 100 (3.9 in.) = 4,2 l/s
  • Vortex fine filter WFF 150 (5.9 in.) = 12,8 l/s
  • Vortex fine filter WFF 300 (11.8 in.) = 80,6 l/s

Efficiency of the WISY Filter System

WISY indicates an average efficiency (hydraulic efficiency) of the filter systems with more than 0.9 or 90%.  This means that more than 90% of the rainwater coming from the roof is filtered and collected into the storage tank. The remaining water along with dirt is diverted to the sewer or soakaway unit.

The stated hydraulic efficiency applies to 99% of all precipitation events in Germany and Central Europe.
Only in 1% of the precipitation events does the filter efficiency lessen to about 40%-60% due to the high flow rate.

The majority of single rainfall events cause a drainage pipe filling-level of less than 0.3 or 30%. 


A building with a projected roof area of 500m² (5,382 sq. ft.) is equipped with an underground WFF 150 Vortex Fine Filter.
In case of medium rainfall with an intensity of 5mm/m² within 15 minutes, that is the equivalent to 5 l/m² in 15 minutes, a flow rate of 2.78 l/s feeds into the WFF 150 Filter. This flow rate correlates to an hydraulic filter efficiency of more than 95% - see efficiency diagram above.

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