hi if you are going to fit a shower pump with mixer valve fit a warix flange to your cylinder its the only way i tried to be clever and tap into dhws the pump drew air from the vent i thimk fitted the warix magic
hi i have exactly the same problem but i didnt fit a warix flange that is my next step i teed off the dhws it may be the hot supply is restricted due to limescale on the fitting on top of the tank i will see what happens when i fit the warix but my symptoms are exactly as you describe anyone...