kettling combi: scale, a dodgy valve or something else?

[caractacus]

i've looked in the archive, but can't quite find the answer...

it's a worcester greenstar he plus boiler, and occasionally it overheats. the scenario last night was this: a bath being run upstairs [no problem when done on its own] + i started doing the washing up downstairs: so extra demand on the hot water.
various crunching pops emitted from the boiler. the temp read 113 degrees on its little screen. this fell quickly when the water was turned off, and everything was ok after. normal peak temp for running the bath was 65-75 degrees.
it's serviced regularly, is fairly new, had a valve changed when this problem occurred once before..
any views on the cause? scale? thermostat? i'm not sure i want to try experimenting, and trying to make it happen - it's an alarming noise - but is it possible that the system was getting its 'flow demand' from the kitchen [ie low] and high 'temperature demand' from the bath? & therefore superheated water..

 

[Agile]

The boiler has only one DHW outlet!

It has no idea where the water is to be used.

Usually higher demand cools down the boiler.

I dont guess at boiler faults but would expect it would take quite a few minutes to diagnose at the boiler.

Perhaps some others will immediately recognise the senario you describe.

Tony

 

[caractacus]

thanks..
i've just been trying to get a repeat performance, without success, if success is the right word..
i'm wondering how cpmplex these boilers are: how many information inputs and how much computer processing. is the water flow linked to the gas flow by a mechanical method or an electronic one?... if the answers's mechanical then the cause is mechanical..
you know that situation where you turn the hot tap slower and the water temperature builds before the gas shuts off... like there's a rather clumsy thermostat in charge... is our problem something to do with that?
thanks for any further opinion!

 

[Agile]

The boiler senses the demand for hot water, turns on and sets the temperature electronically.

Most things work electronically now.

Tony