OAPs with HW problems on Greenstar 24i junior combi condensing boiler

[RonL]

Trying to help my elderly in-laws with 5-year-old Greenstar 24i junior combi condensing boiler. It has run OK until now, and no plumbing work undertaken recently, except for me replacing washer in cold tap last week.
The story so far:
On Monday night, they report major HW problems: boiler responds to tap demand, runs warm-to-hot for 20-30 seconds, then cycles down again (with some gurgles and perhaps a muffled clunk) and water runs cold.
I notice pressure gauge reads zero, so re-pressurised using fitted hose to get it up to 1.5 bar -- seemed OK.
Sadly, HW is no better Tuesday, so investigated further: pressure still at 1.5; diverter valve solenoid definitely lifts up on tap demand (and stays up as water runs cold again). Tried "service" function and found that CH is fine, radiators began to warm up with minutes. Have also tried turning mains electric supply off & on a few times.
Any ideas please -- heat exchanger (£££) perhaps...?
Thanks to anyone who can help me to help my ageing in-laws.
RonL

 

[Puddle]

Does sound like plate heat exchanger blocked up ,plenty of info on here about what happens,however if boiler on old system could all need cleaning out

imho

 

[stevetheplumber]

id agree with puddle take the plate heat exchanger out and clean it out id try this first as its virtually free to try just soak it in some cleaner and flush out with a hose shaking and tapping to loosen any crud

 

[diamondgas]

It certainly sounds like the plate heatx as long as the hot water sensor is okay

 

[arran197]

Does the water run REALLY hot before going cold?

 

[EPH]

Could still be under warranty if it's around 5 years old

 

[arran197]

Only 2 years Worcester isn't it?

 

[RonL]

First of all: THANKS to everyone who has replied (puddle, stevetheplumber, diamondgas, arran197, EPH).

The DHW sensor (thermostat?) did seem a likely culprit, so I tried by-passing it last night (unplugged it, connected 2 contacts on plug with bit of wire); now, the boiler does not cycle down on an HW demand, but water is only lukewarm (esp. in their bath, sadly, which is their main issue!) -- during this time, it is very noticeable that CH output pipe feels much hotter than HW output pipe, despite diverter valve seeming to operate OK (solenoid lifts it up).

Cleaning DHW heat exchanger sounds like big job, involving complete draining first (no detailed instructions on that in book left by installer) and dismantling hydraulic block etc...
Could it possibly be that the diverter valve is only partially diverting to DHW (given that CH output feels hotter than HW)? I don't know how high up the solenoid should lift it...

RonL

I realise that the 24i is not much of a boiler (Worcester-Bosch themselves call it their "entry-level" choice!), but that's what is installed -- by an Australian who has since gone back down under.

 

[armourer]

It still sounds like the diverter valve letting by.

You should be able to buy an internal seal kit for the diverter. (ie: the diverter is operating but the water is escaping past the seals and going into the central heating flow).

Think of your diverter valve as a tee junction which directs the heated water either into the radiators or through the plate heat exchanger to heat your domestic hot water.

If your CH flow pipe is getting hot on hot water demand it has to be the diverter unless it is going through the bypass valve.

 

[stevetheplumber]

usually two allen screws hold plate heat exchanger no need to remove block screws are left and right on front of block