While eating Shabbat lunch at our rabbi’s apartment, my wife noticed that the rebbitzin used the hot water from her sink. When my wife asked how she could be doing that, she was told that since the apartment building was very large (on the UWS of Manhattan) and since the vast majority of occupants were non-Jews, it was permissible. When I asked the rabbi, he did not agree or disagree (in front of his wife). What is the halacha for using hot tap water in a large, non-Jewish building on Shabbat?

If when opening the tap in the flat it does not affect the heating system (i.e. the system heats anyway ect.) then one may use the hot water on Shabbat. If on the other hand turning the hot water tap causes the heating system to start working or cold water to enter the heating system then even if the building is filled with non Jewish tenants it would be forbidden. Rabbi Yosef Weitzen