I am hoping someone has a solution to this one, two plumbers have been stumped so far. In the process of getting a tankless water heater installed, some debris entered the house pipework. When this hit the taps in the upstairs bathroom, two of them (both hot taps) refused to close all the way. THe plumbers both said the cause is the debris which has entered and wrecked the ceramic cartridges, but neither of them could extract the cartridge as it doesn't appear to come out like they usually do. THe top of the housing has a nut which seems to indicate it should come off with a wrench, but not so! We have tried heating the pipe in the event that Loctite was applied at installation, our best efforts with a vise and wrenches have also failed. Any suggestions?