Suspected dodgy diverter valve or heat exchanger - your thoughts?

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
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On Wed, 30 Apr 2008 11:03:47 -0700, Mathew Newton wrote:

I'd go for the DHW heat exchnanger being full of chalk. If the diverter valve was leaving a significant flow to the CH circuit then the burner would not cut out so quickly.
Also there is a tendency when the DHW exchanger gets partially blocked for water to be forced past the diverter valve in the CH circuit.
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Ed Sirett - Property maintainer and registered gas fitter.
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That logic is sounding good to me...
Do you think I am best off with replacement as opposed to trying to clean it?
Mathew
P.S. I had a snipe set on a new heat exchanger off eBay... just my luck though that my Internet connection has been pi55 poor tonight so it failed to bid.... serves me right I guess.
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On Wed, 30 Apr 2008 14:52:11 -0700, Mathew Newton wrote:

My approach would be to buy the spare and fit it. I'd then leave the old unit covered with dilute HCl for a couple of days and see what gunk comes out.
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I forgot to ask - given you think it might be a partially blocked heat exchanger causing water to be forced past the diverter are you therefore thinking it is the primary side of the exchanger that's too blame? Is that still likely given my use of inhibitor? I was initially thinking it'd be the 'unprotected' HW side of it... but then that wouldn't explain the associated diverter problems (but might explain the overheat due to poor heat transfer).
Mathew
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On Wed, 30 Apr 2008 20:38:14 +0000, Ed Sirett wrote:

Not sure when you say chalk, Ed, whether you mean scale on the secondary (DHW) side or blocked on the primary. I'd suspect the primary: I had one like that - it ran hot & cold, the diverter operated OK, and when I took off the heat exchanger I got bits of rusty flakes out of the primary side.
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John Stumbles

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This is sounding very reassuring - thanks. The boiler is 11 years old and as we're in a medium-hard water area I'm hoping that a blocked/ scaled/whatever heat exchanger is an entirely reasonable problem to be facing. That, and the fact that swapping out the heat exchanger is a little easier than the diverter!
Mathew
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On Thu, 01 May 2008 19:49:22 +0000, John Stumbles wrote:

Well the blockage to heat transfer and flow can be on either side or a combination (ahem...) of both. The primary side seems to form a chalk/black oxide compound which comes out in flakes.
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On Fri, 02 May 2008 23:07:56 +0000, Ed Sirett wrote:

OK, that's what I meant. Where do you get the bit about chalk being part of it? I assumed it was oxide flaking off the inside of a radiator somwhere in the system.
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John Stumbles

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On Sat, 03 May 2008 09:34:48 +0000, John Stumbles wrote:

It froths and forms a disgusting gunge when treated with HCl. There may be other chemistry at work but I assumed it was CO3 to CO2 in the presence of an acid.
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Ed Sirett - Property maintainer and registered gas fitter.
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