DHW goes cold quickly

[mattylad]

Hi, I have a WB Greenstar 25SI combi boiler, its about 10 years old, not been serviced.

Recently (well over the past year) DHW in the bathroom goes cold quickly.
We get a burst of hot then it quickly goes luke warm to touch.
That is the taps, the bath and shower.

Flow seems to be nice & strong (Ill try a flow test when I get a chance).

The kitchen taps however seem to be fine.

The boiler seems to indicate that the flame is on OK at all times so its not the gas/flame going out.
Im thinking (after some searching) that its most likely the DHW heat exchanger is clagged up or the diverter valve.

On a previous WB boiler (now well obsolete) the diverter valve was easily fixed using a generic diaphragm repair kit but for this plastic thing in the 25si is that even possible or is that only a replaced the whole part job?

Re the heatx, I can see that it is a bugger of a job to remove but a dose of spirit of salts and some flushing through may clean it.

Before I see the gas guys at work can you suggest anything else that needs checking?

 

[imiller9]

You can check the diverter easily. Remove the green cover and turn power back on. Does the motor move the spindle up and down when powering up. Then with the heating having been off for a while, run the hot water and feel the far left hand copper pipe coming out of the bottom of the boiler, if that gets hot, the diverter paddle is dirty and letting water past it.

 

[mattylad]

If it turns out to be that, how hard is it to get to the paddle itself?
Can it be cleaned/replaced?
I see they are available as spares.
https://www.ebay.co.uk/i/2835563680...mKUYU3zN2nwvYX1i7vrt8OU-lmJ_xlLhoCKRAQAvD_BwE

 

[Stadan20]

Not been serviced in 10 years ..... you aught to be getting a gas safe registered guy in and ask for a full strip and service on the appliance before anything else . Just note that any of the work you are mentioning should be carried out by an rgi because as soon as that front cover is taken off the combustion seal would be broken !

 

[mattylad]

I have several RGI's at work can assist but I want to figure out whats likely wrong with it before I get one to come round so as to reduce the time they spend on it.
However a "service" doesn't cover cleaning the DHW heat exchange, not fixing any faults does it, AFAIK its generally a clean & test.

 

[Stadan20]

I have several RGI's at work can assist but I want to figure out whats likely wrong with it before I get one to come round so as to reduce the time they spend on it.
However a "service" doesn't cover cleaning the DHW heat exchange, not fixing any faults does it, AFAIK its generally a clean & test.

No but it will involve a replacement of the main burner seal which is well overdue ......

 

[mattylad]

OK so after speaking to one of the gas guys at work we have a plan.
I'm going to put some cleaner/descaler stuff in the central heating one weekend then borrow the powerflush gear from work and flush out the system one weekend prier to him coming round and giving it a good service and repair, the DHW heatex is going to be replaced and Ill check the diverter valve pending replacing the diaphragm.

I Have every confidence in their competence (and they have the certs to prove it lol) given that we make burners that are way larger and more complicated than a combi (he also does combi's too)

 

[ericmark]

I had kitchen taps running cold, and no others, found it was a kinked flex pipe, it was using the heat store from boiler but not enough flow to kick in the boiler.

 

[mattylad]

You can check the diverter easily. Remove the green cover and turn power back on. Does the motor move the spindle up and down when powering up. Then with the heating having been off for a while, run the hot water and feel the far left hand copper pipe coming out of the bottom of the boiler, if that gets hot, the diverter paddle is dirty and letting water past it.

So yes, the motor goes through a cycle of up and down, also goes up when DHW is drawn, however as suggested the far left pipe (CH flow) is getting warm so it looks like new paddle time.

 

[mattylad]

OK so I have cleaned the internals of the diverter valve and now its passing water better without the CH flow getting hot.
I have a repair kit on order and will fit that on at a later date when I have to drain down the system again to remove the X400 that's currently in it.

BTW, is there a defined distance that the motor spindle travels? I.e. how far up should it go for DHW?

Although - the hot water is still not getting as hot as it should for much more than a few seconds so that to me points to the plate exchanger being clagged up.
Ill see how well this cleaner works on that and if needs be replace the exchanger on its forthcoming service.

 

[mattylad]

So, what flow rate can be derived from filling 8 litres in 6 seconds from the bath tap?