Greenstar Junior 24i. Boiler runs, red hot water. Shuts down.

[JohnHuson]

Boiler has been causing problems for a few weeks, with poor supply to domestic hot water, hot warm then cold.
I Have descaled and had system flushed. No improvement. Boiler fires on demand, DHW gets very hot, too hot! then shuts down with residual short term warm water to cold.

I do not think this is a problem with the exchanger. Boiler green light goes off after about 30 seconds output of very hot water then shuts down. Central heating is circulating and getting hot. Pump is running. I have not yet checked pressure in expansion vessel

Is it the DHW sensor; the thermistor? or something I have missed?

Thanks for your help, much appreciated. John.

 

[Stadan20]

Yeah get a gas safe engineer to look at it

 

[terryplumb]

When hot tap is opened ,does it run continuously getting very hot then cold ,very hot then cold repeatedly,and without boiler closing down ( so it will run as above perpetually) ??

 

[JohnHuson]

Yes, got it in one!

 

[terryplumb]

Blocked plate HEX would be suspect.

 

[JohnHuson]

I have today drained the system and added the usual descaler stuff, perhaps I am expecting things to happen
too quickly from the result of what they do?

 

[JohnHuson]

I really do appreciate your help.

 

[terryplumb]

The plate hex needs removing from the boiler to effectively clean it. To be frank ,I would replace it . Strong chemicals are required to clean it , British gas use sonic cleaning equipment. And I doubt either are as good as exchanging for a new one.

 

[JohnHuson]

Thank you for your advice. I will replace it. It looks very similar from first look to the Ideal HEX?

 

[JohnHuson]

I do not understand the sequence of events. If I have now inserted the chemicals to clear the HEX which is in a water
scale free area. How long should I give this stuff to di its job>

 

[terryplumb]

Plate hex has very fine waterways,and there are dual tracks. The primary systems water ( same as what runs round your radiators) passes thru some of the waterways ,and mains cold water runs through the others the two obviously do not mix. The heat from the primary water is transferred to the cold as it passes thru ,and it comes out the taps hot. It's the primarys waterways that block up ,not with limescale ,but with system crud and mostly magnetite. Don't know what your putting into boiler ,or where you are putting it exactly ,but it's unlikely to do the job !