What boiler is this on?
If the diverter has been changed and it's not a puma, my only thoughts are that possibly there is not a suitable enough by-pass fitted to the system, or, possibly that a hot water draw-off is passing, allowing the diverter to move slightly, allowing the heating circuit of the diverter valve to allow a small amount through into the rads.... Possibly a mixer shower on the system can cause this, but is rare.
ccMods, August 2006
