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

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

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

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.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

On Apr 30, 9:38 pm, Ed Sirett 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.


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.

On Apr 30, 9:38 pm, Ed Sirett 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.

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

On Wed, 30 Apr 2008 14:52:11 -0700, Mathew Newton wrote:

On Apr 30, 9:38 pm, Ed Sirett 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.


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.

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.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

On Wed, 30 Apr 2008 20:38:14 +0000, Ed Sirett 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.

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.

--
John Stumbles

I used to think the brain was the most interesting part of the body
- until I realised what was telling me that

On 1 May, 20:49, John Stumbles wrote:
On Wed, 30 Apr 2008 20:38:14 +0000, Ed Sirett 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.

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.

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

On Thu, 01 May 2008 19:49:22 +0000, John Stumbles wrote:

On Wed, 30 Apr 2008 20:38:14 +0000, Ed Sirett 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.

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.

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.



--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

On Fri, 02 May 2008 23:07:56 +0000, Ed Sirett wrote:

The primary side seems to form a chalk/black oxide compound which comes
out in flakes.

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.

--
John Stumbles

Question Authority

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

On Fri, 02 May 2008 23:07:56 +0000, Ed Sirett wrote:

The primary side seems to form a chalk/black oxide compound which comes
out in flakes.

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.

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.



--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

On Sat, 03 May 2008 16:53:10 +0000, Ed Sirett 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.

Aah! I don't think I tried acid on it.

--
John Stumbles

Xenophobia? Sounds a bit foreign to me.