fan comes on when shutting off cold water

[mparratt@blueyo]

hello all just had a new intergas boiler fitted my problem is every time i turn off the cold water tap the fan kicks in for a couple of seconds but if i turn off tap slowly it doesnt happen i think the boiler thinks i am turning on hot tap momentaraly because of the hydraulicing in the cold water pipe is there some sort of damper available i noticed that if i introduced/trap air in the cold water pipe it all works fine thats why i am thinking of a damper the boiler was fitted by gas safe engineer but he has gone on holiday and he doesnt seem to think there is a problem or is it just me hope someone can help me regards mick

 

[Rydell]

Maybe try a non return valve on the cold supply to the combi?

 

[GroundControl]

without all relevant info its hard to say for sure !!
But my guess is there is an air pocket on the hot pipe work somewhere and thus when you shut the cold tap suddenly the pressure then squeezes the air pocket giving the boiler a false flow signal to come on !!
have your installer check that he hasn't left what are called "dead legs "!!
Is this a recent install ?? is it a swap ?? was there originally a back boiler ??
there more info you can give the more we can understand )

 

[mparratt@blueyo]

firstly thanks for replying
the boiler was fitted last week using all original pipe work i wondered if it could be the rubber hose going to dishwasher(hot) and washing machine (hot) maybe they are swelling giving giving the same as air trapped i dont know how i can get air out of pipes if any
if i was to fit a piece of say 22mm pipe about 3 foot long with trapped air inside would that act as a damper and therefore equall out any shocks what do you think as you can see i have some understanding of the mechanics but your the experts thanks again for any help mick

 

[GroundControl]

ok
1) you say new last week !!
what did you use before the boiler was fitted ?
2)I would be astounded if the washing machine hose was the fault !!
3)NO NO the idea is to have NO air in the system ~~~~~ therefore no movement of water through the boiler therefore the boiler wont fire !!
4) what is the model of the boiler ?
the water pressure should have been checked by your installer and also the flow rate,do you have the benchmark figures for the install ?
the reason for the old system history is to give an indication of what was fitted and likely to have been overlooked !!
did you have a electric immersion heater ? or is this a totally brand new install ?

 

[Alanka]

Another possible cure is to fit a shock arrester to the cold water pipe (e.g. BES sells them), either near the cold tap that is causing the problem, or in the boiler cold feed pipe, to absorb the pulse and prevent it from operating the hot water demand mechanism.

A possible problem I see with the non-return valve suggestion, is that the unwanted pulse is travelling into the boiler, not out of it, and so would not be blocked by the NRV (?). Just a thought.

 

[GroundControl]

another problem that is really really unlikely is the issue of Legionelle
My work is mainly commercial and there is no way we would ever leave a dead leg in situ on a DHW circuit, otherwise you have a breeding ground
best just get the guy to fix his mistake and the job will be good
why bodge it again ?

 

[mparratt@blueyo]

thanks for all of your answers i will let you know how i get on