I've just had my hx changed by Vaillant in my 7yr old boiler. The guy that changed it said I had no live feed from the primary circulation pump back to the boiler. He said I should have one as this will allow the boiler to control the pump when the system shuts off via the timer control to allow the heat within the boiler to be removed until the hx temperatures normalised. My circulation pump is a Grundfos ups3 and I have tonight observed that when the timer control switches off, the pump continues to run and circulate for 5 mins before stopping. Will this be sufficient time to normalise the hx temps or will I need a cable running to the boiler? He said because I had no pump overrun controlled by the boiler when the system switched off the water was boiling in the hx and flashing off resulting in my system slowly being topped up with fresh water, which will have accelerated the failing of my hx.