Boiler turning on with cold tap usage

[CBW]

According to Vaillant the code that runs on my boiler after cold water tap used is “pump overrun following a hot water demand”

That’s correct if you’re running a hot water tap, which is fed from the cold and will only happen after a hot water tap has finished running. You’re staying it’s firing after cold water usage alone - are we correct in saying this?

 

[Nathanlowe25]

That’s correct if you’re running a hot water tap, which is fed from the cold and will only happen after a hot water tap has finished running. You’re staying it’s firing after cold water usage alone - are we correct in saying this?

Yes - after cold water tap in utility room is run and switched off but it only happens occasionally, not always and lasts a couple of seconds.

 

[CBW]

Yes - after cold water tap in utility room is run and switched off but it only happens occasionally, not always and lasts a couple of seconds.

Then it’s as has been diagnosed - trapped air in hot or cold pipe or dead leg in the system.

 

[bernardgreen]

trapped air in hot or cold pipe or dead leg in the system.

Does a expansion vessel act as a "dead leg" ?

Does it push water backwards through the boiler's flow sensor when pressure in the cold water feed drops below the pressure in the the expansion vessel due to a cold water tap being opened. ?

 

[CBW]

Does a expansion vessel act as a "dead leg" ?

Does it push water backwards through the boiler's flow sensor when pressure in the cold water feed drops below the pressure in the the expansion vessel due to a cold water tap being opened. ?

I don’t actually know why it does it, my thought would be movement caused in the pipe causing pressure fluctuations and mimicking that of a hot tap being opened