When we rebuilt our kitchen, the contractor suggested attaching a water filter to the incoming water pipes. I was concerned that this might be borer if left on during Shabbos (it would be a pain to disconnect it each week). I declined, letting him only attach it to the line to the ice cube maker - which I shut off before Shabbos anyway. Did I overreact?
In Shemirath Shabbath Kehilchatah 3:56
a. One may use a water tap fitted with a filter.
b. If the filter becomes detached, it may only be replace on Shabbath in a provisional manner ant fixed on tightly, since this may fall within the prohibition against construction.
I should mention that I heard elsewhere that this mainly applies to water that is otherwise fit to drink, and the intent of the filter is to improve the taste, health, (or, in the case of New York water, the Kashrus).