Boiler DHW keeps going cold

booksandbikes

We have a Worcester Greenstar 28CDi Compact boiler that keeps going from hot to cold to hot to cold whenever we are running the hot water tap. I've tried changing between PreHeat and Eco, but it makes no difference. The temperature on the hot water display will be around 55C, then will drop to 30C-ish then heat back up to 55C. It does this maybe two times and then stays around 55C.

Any idea what this could be? It's due for it's service next month, but would like to have some idea beforehand. 🤷‍♀️

Many thanks.

ritz55

Sounds like the plate heat exchanger to me. Will need the part to be replaced by a professional. 

Heedtheadvice

If it is of significant age I would call in Worcester Bosch. 

They have a fixed price fix and that would probably include a boiler service as part of the recommissioned.Not the rest of the heating though. It is just something like £350/ £400 and includes the faulty part. Guaranteed too. Contact them for details though.

I did that on my heating engineers advice and was very pleased.

WIAWSNB

ritz55 said:

Sounds like the plate heat exchanger to me. Will need the part to be replaced by a professional. 

It's certainly the classic symptom for this.

How old is the boiler, B&B? Is there a magnetic filter fitted? If so, make sure it's checked for trapped sludge - this is what slowly builds up until the P2P exchanger is blocked. 

marcia_

 My boiler is currently doing this. The guy from the insurance emergency cover said it was the plate heat exchanger so I got someone out to repair it but he said it's best to replace. After replacing it still didn't work and after further examination it turned out to be the divertor. I need to switch off the the heating via my hive app to get the hot water to work, he will replace the divertor when he orders it in. 

WIAWSNB

marcia_ said:

 My boiler is currently doing this. The guy from the insurance emergency cover said it was the plate heat exchanger so I got someone out to repair it but he said it's best to replace. After replacing it still didn't work and after further examination it turned out to be the divertor. I need to switch off the the heating via my hive app to get the hot water to work, he will replace the divertor when he orders it in. 

Yup - it's also a classic symptom of a sticking diverter valve

(Walks away 😗... )

rob7475

If you turn your hot water flow temp down on the boiler to around 45-50, does the water stay hot? If so, that usually indicates that the DHW plate heat exchanger is blocked.

Albermarle

WIAWSNB said:

marcia_ said:

 My boiler is currently doing this. The guy from the insurance emergency cover said it was the plate heat exchanger so I got someone out to repair it but he said it's best to replace. After replacing it still didn't work and after further examination it turned out to be the divertor. I need to switch off the the heating via my hive app to get the hot water to work, he will replace the divertor when he orders it in. 

Yup - it's also a classic symptom of a sticking diverter valve

(Walks away 😗... )

So the obvious question to the OP is -

Does the problem occur when the heating is also on, or off?

booksandbikes

- The boiler was installed in 2020, so fairly new (or at least, not old!). It is still under warranty, so hopefully a heat exchanger replacement will be covered 🤞

- pretty sure there is a filter attached, from the images I have googled. Will make sure it is checked

- the problem still happens whether the heating is on or off. I have tried various combinations, and it still cycles hot/cold

- I've turned the boiler temp down to 45C, and it didn't cycle just now, but I will try to repeat it again to see if the same thing happens

I have booked it for a a service later this month - I will report back with what the issue was. 😊

WIAWSNB

Do you recall what type of system cleanse was carried out on install?
How old is the original system - rads and pipes?
Pic of the filter? That is usually a DIYable task to check, but fully understand why you might not want to.
I'd ask to see the filter contents - if there is notable sludge, it'll suggest the system wasn't cleaned thoroughly, and is likely behind the P2P failure ( assuming that is the problem). So, a repair should be covered.

Eldi_Dos

@booksandbikes

How long does this hot and cold cycle last for, is it just a few seconds or several minutes?
If it is only seconds could the initial run of water have been sitting in pipework and have came up to ambient temperature followed by cold water coming in from the main causing the variation in temperatures until the boiler stabilise's.