The SIA WF100 is a replacement anti-limescale filter that is used with SIA BWT3/4 taps. This filter is used to protect the tank in both taps and also as 1 of the filters needed for filtered drinking water in the BWT4. The filters need replacing approximately every 6 months, however the water tank will have an indicator that shows when the filter needs to be replaced.  Filter Quantities BWT3CH - 1BWT3NI - 1BWT4CH - 1BWT4NI - 1