Weird boiler fault/problem?

[ajgas]

Just wondering if anyone with experience with this problem has any decent suggestions?
working on an remeha avanta plus yesterday an everything on it is working normally but if you run a cold tap and then switch off the cold tap then the fan kicks in on the boiler for a minute or so, the boiler fires up fine for hot water.
the boiler does not fire up when the cold tap is ran so its not piped up backwards or anything, its only when you turn off the cold tap that the boiler fan kicks in, would appreciate any suggestions, thanks

 

[Ermintrude]

Water hammer spinning hall sensor. Common fault on combos

 

[happyflyer]

Emi,

If the hall sensor is screwed to the wall properly how can water hammer spin it...:50:

 

[Ermintrude]

The pulse through pipe causes the impeller to move and thus induce signal in Hall effect sensor. And boiler starts ignition sequence

 

[ajgas]

Cheers everyone, someone said to me put a single check valve
in then a half inch shock arrester on the mains goin to the boiler, that sound about right?

 

[mountainman]

Shock arrester alone should be enough!

 

[Ermintrude]

Check valve won't do jack as the shock wave moves in direction of the waters usual flow.

 

[kris]

Im with mr ermintrude , check wont do diddly squat, ive had a few probs of this description and ive always found that its with the cheaper combos, i had a cracker on saturday evening one of my customers phoned me the back of teatime and i could here really loud banging in the background the poor pensioner was scared sh!tless and im not surprised as its the loudest water hammer ive ever experienced, i was at her house ten mins later and the pipes were still bangin away but in this instance her shower mixer valve was faulty causin the bangin but by eck she was loud,

 

[the_ace]

I've fitted check valves to a few combis and its sorted this issue