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My mixer shower that runs from the combi boiler is severely leaking. Any advise on how to fix it? - mixer in saskatoon

My shower mixer was used and therefore not be disabled. In a couple of good hours, but you do not know how to stop it. He stopped the supply of running water, but must get the water back into the rest of the rooster. Is it very difficult to repair? Is it very difficult to install a new mixer shower? I would welcome tips and advice.