Boiler not heating water correctly?

Your cold water pressure may be too high and this may be causing a bias to cold at the shower. By venting it off when the shower is on you are effectively reducing the cold water pressure to it and this is then allowing the hot water which is at a lower pressure (because of restrictions in the boiler) to flow more freely (hot and cold merge in the thermostatic valve). You may need a pressure reducing valve on the cold supply to your shower. Do not reduce the cold water pressure to the boiler as this will consequentially reduce the pressure out of the boiler. The same applies to any of your taps. If they are mixer taps then the cold water at a higher pressure will override the hot water at a substantially lower pressure so the hot will be difficult to control. Also check your pipe work to ensure that the plumber has not unwittingly connected a cold water pipe into the hot water circuit as you would then get minimal flow from the boiler.

Phil the fluter
January 2010