Hot water goes cold - Heat Exchanger?

[Swell1]

Hi, our Worcester Greenstar 24i boiler has an issue, when I run the shower is gets hot and the boiler is running fine, then the boiler stops and a minute later the shower water cools to cold, I hear the boiler trying to fire back up but cannot. If I turn off the shower and start again its the same again.
Does this sound like the heat exchanger or heat exchanger plate or possibly something else?
Thank you!

 

[terryplumb]

Do hot taps suffer from the same problem ? Or do they run continuously hot ?

 

[Swell1]

Do hot taps suffer from the same problem ? Or do they run continuously hot ?

Hi Terry, thank you, I've just tried all the taps and this is what happens (ps its a Bungalow - boiler in the kitchen).

Mixer shower sometimes runs fine, no issues, other times it starts fine then cools to cold and the boiler stops running and tries to fire back up but cannot.

Bath hot tap when turn on full - is cold, when the tap is closed off a bit to slow the flow, the water becomes nice and hot and no issues.

Kitchen tap seems to work fine and often very very hot.

Does this seems strange behaviour?

Thank you!

 

[terryplumb]

Re bath tap.. does the boiler fire up every time that tap turns on ? If it does ,but the temperature of the water is not very hot when bath tap is fully open ,that's fairly normal . The faster the water runs through the boiler ,the less time it has for the heat to be transferred, hence it goes hotter when you partially close the tap.
As kitchen tap is more restrictive than bath tap,and flow rate is slower ,it gives hot water ok.
As your boiler appears to fire up ok using hot taps ,the issue you are having with the shower appears to be shower related rather than a boiler issue .
Is it the shower several years old ? Sometimes the shower head/s can partially block with limescale and restrict flow to a point where the boiler " thinks" there is no longer any demand for hot water.

 

[Swell1]

Re bath tap.. does the boiler fire up every time that tap turns on ? If it does ,but the temperature of the water is not very hot when bath tap is fully open ,that's fairly normal . The faster the water runs through the boiler ,the less time it has for the heat to be transferred, hence it goes hotter when you partially close the tap.
As kitchen tap is more restrictive than bath tap,and flow rate is slower ,it gives hot water ok.
As your boiler appears to fire up ok using hot taps ,the issue you are having with the shower appears to be shower related rather than a boiler issue .
Is it the shower several years old ? Sometimes the shower head/s can partially block with limescale and restrict flow to a point where the boiler " thinks" there is no longer any demand for hot water.

Hi Terry, I've just ran the bath tap fully open and despite it running luke warm the boiler was running then after 2 mins the boiler turned off and tries and fails to restart and the tap ran cold, but I know if I slow the flow it will get nice and hot and the boiler will run fine without stopping.

Yesterday I cooled the shower temp (which I assume slows the hot flow) and it ran fine with no issues.

The mixer shower is quite new and replaced an electric shower.

Thank you again

 

[Johntheo5]

Looks like it could be fouling of the PHEX. Run a hot tap so as its continuously hot then,
In the info menu (press the info button?) and note
I 9 Primary Flowtemp DegC
I 11 DHW outlet temp DegC
I 12 DHW setpoint temp DegC
also measure the hot tap flowrate in LPM.

 

[Swell1]

Looks like it could be fouling of the PHEX. Run a hot tap so as its continuously hot then,
In the info menu (press the info button?) and note
I 9 Primary Flowtemp DegC
I 11 DHW outlet temp DegC
I 12 DHW setpoint temp DegC
also measure the hot tap flowrate in LPM.

Hi, Thank you however we have no display on this boiler to see this info, is this something that can be be cleared/cleaned? Thank you again

 

[dilalio]

Yes it can be cleaned or replaced.
But if your CH system is contaminated it will soon block again.

 

[ericmark]

I had the problem with a flex pipe to the sink, and with the shower, there was nothing wrong with the boiler, it was down to flow rate. With Eco off, the taps worked fine, but shower would go hot then cold then hot again. With Eco on, often could not get hot water at the taps as flow rate too low.

This was an old Worcester Bosch Greenstar 24i boiler. Also had problem with a Main 7 DHW boiler, in summer the shower would go cold, cure was larger shower head, or clean off any lime in the head.