Combi boiler hot then cold hot water problem/question

[MrHJameson]

Just been reading a few posts, basically I have a combi and the hot water goes hot then after say 20 seconds it gets cooler and runs cold, then runs hot again and carries on this way.

I have heard it could be a number of issues, ie plate heat exchanger blocked/dirty, DHW sensor faulty, microswitch sticking, and also modulating gas valve settings (high and low) incorrect/drifted etc

How does the gas valve settings cause this fault? Is it because the gas valve canot modulate properley??..

The boiler is a worcester cdi non condensing

Thanks for your help and advice

 

[dcawkwell]

Given that is a non condensing boiler and therefore quite old.
My money would go with blocked plate heat exchanger.

 

[MrHJameson]

it does have a modulating gas valve though

How can I test the plate heat exchanger to make sure it is the fault?

 

[dcawkwell]

If my memory is right I think the plate heat exchanger is at the front on the non condensing bosch. So easiest just to drain down remove it flush both side out with limescale remover and replace.
Clean out as best you can and see if there is an improvement.
This could be cased by sludge in the central heating so a flush or two
of the central heat would be recommended.

 

[bengasman]

Symptoms do sound like a sludged up system. Cleaning the plate hex will only help for a while and then the problem will come back; the whole system needs to be cleaned and treated.
Powerflush with acid would be the best way to go about in this case.

 

[ongas]

Just been reading a few posts, basically I have a combi and the hot water goes hot then after say 20 seconds it gets cooler and runs cold, then runs hot again and carries on this way.

I have heard it could be a number of issues, ie plate heat exchanger blocked/dirty, DHW sensor faulty, microswitch sticking, and also modulating gas valve settings (high and low) incorrect/drifted etc

How does the gas valve settings cause this fault? Is it because the gas valve canot modulate properley??..

The boiler is a worcester cdi non condensing

Thanks for your help and advice

Sounds like it needs a new DHW heat exchanger, they are around £90.00 and very simple to fit.

Ongas