I've been moving some radiators and have had the system drained down. Yesterday I put some system cleaner in to the CHW system. I went through the process of filling, and venting the boiler and all seemed to be fine. This morning when I had a shower the water was alternating between hot and cold. I assumed this was a fault on the shower but having checked this happens on all the DHW taps. The tap is opened, the boiler calls for heat and very quickly the water gets over 80 degrees, the heat is cut and the water temp drops rapidly before the boiler then calls for heat again. It repeatedly cycles through this process with the result being hot and cold slugs of water. There are no fault codes and the boiler just plods on. I thought maybe I hadn't properly filled and vented the boiler so I have tried this again but with exactly the same fault. Any ideas what might be causing it?[/b]