system is 3 years old in small house, boiler in downstair kitchen. When using shower in bathroom upstairs the water gets hot and then cools after a minute or two before getting hot again. This then continues for as long as you like. When listening to the boiler it sounds like it is constantly firing up and turning off again after a short period. Recently the problem has increased such that there is no hot water at all from the shower when the heating is on, however this is intermitent.
I'm going to make the assumption that you have a mixer shower installed.
What you are suffering is well known, although I don't know the proper term for it myself.
From what I have been told your shower is incompatible with your boiler.
What's happening is that when your mixer shower is adjusting the mix of hot and cold water to make the output temperature what you desire the hot is being slowed to a level that's making the boiler switch off as it doesn't 'see' enough of a demand to run.
Consequently the water in the shower then cools, the mixer then opens the hot feed back up thus starting the boiler back up again and so the cycle continues.
I believe there are mixer showers out there that can be used in these situations but I don't know what they are called.
Thanks for those tips. It happens on all the taps its just more noticeable when getting a blast of cold water first thing in the morning in the shower. Will try the filters and sensor etc on the boiler.