DHW problems - suspected diverter / heat-exchanger issue?

[MJN]

Hi everyone,

I would be very grateful for some expert advice - I'll try and be concise whilst not missing out any necessary details...

Boiler: Worcester Bosch 24CDI RSF combi, approx 11 years old

Problem: DHW operation is somewhat intermittent insofar as we are experiencing fluctuations of hot water temperature (hot - cold - hot - ...) be it at a tap or shower. As this is occurring the boiler operation is as follows: DHW demand correctly detected, boiler ignites, full flames for 30 seconds or so, flame gradually modulated down, flame gets extinguished, 15 second or so pause, flames reignite, and repeat...

Other symptoms/information:

- No fault indicator lights, nor permanent failure, just poor operation as described
- CH assumed to be working fine (not directly observed given the weather/temperature)
- *** CH flow pipe heats up even during DHW-only demand (I initially though it might be conducted heat through the pipes but have now decided it's more than that - proper heating of the water within). I am assuming this fact is the relevent biggy here? ***
- Diaphragm was replaced ~4 years ago. I am not suspecting this as it is operating the microswitch fully/correctly/consistently.
- Medium-hard water area (Bristol), mains pressure ~65PSI
- Inhibitor in the primary circuit for at least the last 6 years, no softener in the mains water feed
- Gas valve replaced ~5 months ago due to regular ignition lockouts
- PCB fault ruled out same symptoms with a spare fitted

My thoughts:

The heating-up of the CH flow pipe is ringing the alarm bells for me... I thought perhaps the diverter valve might be sticking.. However the pin is making it as far as the microswitch but could the divert valve still not be moving sufficiently? Am I right in thinking that it the diaphragm serves only to activate the microswitch, and doesn't actually move the diverter mechanism itself? The latter being done by DHW water flow (against the actions of a return spring)?

Given the age of the boiler (~11yrs) and the medium-hard water area I was wondering if the HW path in the secondary heat exchanger might be scaled up hence causing insufficient flow to fully open the diverter? At the same time this would restrict heat transfer to the HW and hence cause the primary circuit to overheat thus cutting the flame ...?

I hope I've not wittered on too much but still giving you enough to go on... Your help/thoughts would be very much appreciated.

Regards,

Mathew

 

[gas4you]

the DV can be stuck mid position whilst the pin still moves and operates the boiler correctly.

The tempature fluctuation is indicative of the plate HE being partially blocked, not necessarily scaled though, may be on the system side.

 

[MJN]

Thanks for the clarification.

Would your gut feeling be that I've got two faults - scaled/blocked heat exchanger and sticking diverter, or just one of them? As mentioned I'm thinking that if I've got a scaled/blocked exchanger (I'm thinking more on the HW side) then it could cause the diverter not to open fully? What do you reckon?

Mathew

 

[gas4you]

Without being there to see it then yes I do think you have 2 faults, but not directly connected.

I suppose though that if the DV was stuck more over to the heating side, then the restriction in circulation when on HW could give the same symptons as a blocked plate HE.

Get the DV sorted first.

 

[MJN]

Damn... I was hoping you say heat exchanger first as there's a new one going cheap on eBay...

 

[kildareking72]

On some old Worcester boilers when you have run off some hot-water the heating pump will kick in(pump over-run t/stat) and dissipate any heat from the heat exchanger in turn stopping the boiler from over-heating.
You might find that one of your rads has 2 x lockshield rad valves acting as a by-pass.

 

[MJN]

I'm not sure what you mean there. My overheat is occuring during DHW demand, not after.

I do have a radiator with no TRVs, but then the boiler has an integral bypass fitted anyway.

Or have I misunderstood what you're saying?

Mathew

 

[MJN]

Without being there to see it then yes I do think you have 2 faults, but not directly connected.

I suppose though that if the DV was stuck more over to the heating side, then the restriction in circulation when on HW could give the same symptons as a blocked plate HE.

Get the DV sorted first.

Well Dave... I guess this is what separates you as a pro to me as a DIYer!

I took the secondary heat exchanger off today expecting (hoping?) it to be partially blocked... However the secondary side of it was as clean as a whistle and the primary, whilst containing a fair few black flakes, wasn't too bad either.

I hosed it out and pumped some DS3 through it for a while and out came quite a lot of black flakes - it's hard to quantify how much but not enough to really make me worry... although I suppose if the water channels are narrow (anyone know how narrow?) then it certainly wasn't helping. By the end of my cleaning I'm sure the flow (through a hose) was better...

But... back on the boiler whilst I'm not getting the flame being extinguished (although I haven't the past couple of days anyway - typical!) the CH flow pipe is still heating up.

So... I think you were spot on - two faults being a less-then-perfect secondary heat exchanger but more importantly a partially sticking diverter.

I've contemplated removing the diverter for cleaning - are they 'repairable'? - but might just buy a new one... I don't fancy not curing it (again) and having to reassemble the bl00dy boiler with all the threats of potential leaks that the design of this thing almost guarantees! That said, this time time was no problem, and I had to re-use a couple of o-rings... I was holding my breath whilst re-pressurising!

Mathew

 

[gavinda]

However the secondary side of it was as clean as a whistle and the primary, whilst containing a fair few black flakes, wasn't too bad either.

you'de be srprised how much trouble thos few flakes can cause. try replacing your diverter, desludge that hex and maybe fit a magnaclean, because if those flakes arent a sgn of problems to come, i dont know what is.

 

[MJN]

Just to follow up on this for the sake of the archives I finally managed to get a cheap diverter off eBay (£40 new) and fitted it this morning. Job's a good 'un now - no DV leakage to the CH circuit with DHW demand and the DHW temperature fluctuations are a thing of the past (I suppose even a partial DV bypass would be allowing an equal amount of cold return in to the primary loop hence the boiler will have had to work to overcome this whilst trying to maintain an even temperature for the DHW draw).

Thanks for all the suggestions, and indeed the forum in general - it was most helpful in narrowing down the problem (and saved me a packet in doing so!).

Cheers,

Mathew

 

[Agile]

But did you ever clean out the plate HE?

Its not so much the flakes you can see but those stuck inside it out of sight.

Tony

 

[MJN]

But did you ever clean out the plate HE?

Yes, see above (pumped hot sulphamic acid through it for an hour).

It was pretty clean, indeed surprisingly so given it's been in 11 years. After this amount of time in a relatively hard water area (~350ppm) I don't think the plate HE owes me anything anyway so if it does ever give up the ghost (if it ever does given its current apparent condition) I'll have no qualms about sticking a new one in.

Mathew