Worcester Greenstar 24i. Junior. Hot then cold.

[JohnHuson]

If the bath top shower is used by turning on the cold tap first, then the hot supply, it will give a consistent short shower, but that is getting shorter day by day before it goes cold.
If the hot is turned on first, then the cold, that will result in domestic supply going cold within a few seconds.
If the supply of both to the shower (and bath taps) is shut down down then opened again it will be hot for
a short time then goes cold.

If just the normal domestic hot supply is turned on it gets very hot before going cold.

I suspect it is the plate heat exchanger but this is not a bad limescale area. Am I going in the wrong direction?
Or is this a sensor issue? Both temperature controls up high, system pressure good.

If it is the hex, I have read they are sods to replace but if the cold supply pipe is cut off and removed the exchanger can be brought
downwards out of the way and reasonably easy? My Gas Safe Engineer £70 plus part, fitted a high temp sensor but that has not made a difference.
I am curious in what is needed so he can return to the job with me being aware of the answer to the problem. I have worked on many boilers over the years in property management prior to the very sensible regulations we have now.

Thank you.

 

[ShaunCorbs]

are all the taps doing the same eg kitchen tap?

 

[JohnHuson]

are all the taps doing the same eg kitchen tap?

Hi Shaun, yes, so I have discounted a problem with the shower head delivery. Thanks.

 

[Stu-B]

Get another gas engineer in, the first one clearly didn’t know how to test a thermistor.

 

[JohnHuson]

You are correct Stu-B, he fitted the Thermistor which I paid for, and labour costing £70 with no improvement.
The problem was a partial plate hex circulation fault and faulty High Temperature stat (120 degrees) on top of the primary heat exchanger. (Do not forget to add Thermal Paste to the contact surface).
It was easy to get out the plate heat exchanger by removing the hot feed pipe, isolating all other feeds to the boiler and the unit will be removable to the rear downwards without removing the pump or anything else.
It is my experience that many Gas Safe skilled nice people are not also boiler engineers.
I hope my experience helps others.

 

[Last Plumber]

This could be any of a number of things, all of which can be tested to identify the cause.
A good heating engineer will be able to get to the bottom of it for you.