Boiler starts when COLD tap is opened then shut.

[Chris@@@]

I have a Potterton Puma 80 Combi. I recently turned off the mains water supply to replace hot and cold sink taps. Now when I draw COLD water the boiler starts to fire up when I turn the cold tap off. It only gets as far as opening a relay and I guess from the sound opening a valve (not actually firing) before shutting down again. It's like turning the hot tap on for just a second or so. I assume it's something to do with water pressure??? Any ideas.
Thanks, Chris

 

[COG]

Sound as if you have water hammer and if it only started after you turned the stop cock off then its probably needs a new washer or new tap itself.The only problem with the boiler flow switch being tripped is that it will interrupt the ch heating cycle every time.

 

[DP]

It is more likely to be due to a dead leg in the hote water plumbing. Imagine a length of pipe with air in it. When no taps are running, this trapped air is at mains pressure.

When you run any tap, the compressed air will push the water back towards the running tap. At the instant when the tap is closed, water will continue to flow through the boiler compressing the air plug. This will be seen by the boiler as a demand, so it will fire until 'demand' is no more when air plug reaches mains pressure.