Is a combi boiler supposed to modulate its burners on and off every 15-20
secs, even when on a full hot water draw at the tap? High flow rate too.

Cheers
N.

In article ,
"nrh" writes:
Is a combi boiler supposed to modulate its burners on and off every 15-20
secs, even when on a full hot water draw at the tap? High flow rate too.

Might help if you gave the combi model.
"modulate" and "on and off" are rather opposite things.

A combi cycling on and off on full hot water draw can be
caused by the secondary (plate) exchanger becoming scaled
up, so it can't transfer the full power through the plate
exchanger. The primary circuit keeps climbing to max temp,
requiring burners to cut off. I recently descalled a friends
Baxi which had this problem. (It got left like this long
enough that the frequent cycling of the gas valve was
probably a contributory cause of the gas valve burning out.)

--
Andrew Gabriel

In ,
Andrew Gabriel scribed:
In article ,

Might help if you gave the combi model.
"modulate" and "on and off" are rather opposite things.

A combi cycling on and off on full hot water draw can be
caused by the secondary (plate) exchanger becoming scaled
up, so it can't transfer the full power through the plate
exchanger. The primary circuit keeps climbing to max temp,
requiring burners to cut off. I recently descalled a friends
Baxi which had this problem. (It got left like this long
enough that the frequent cycling of the gas valve was
probably a contributory cause of the gas valve burning out.)

Thanks for the reply Andrew, and sorry for the confusion re modulate. I do
mean that the burners turn off completely, not just decrease/increase in
strength.

I have a Vaillant Combi (Model VCW GB 282 EH) which was installed in Nov
1997.

The suggestion you have made about the secondary exchanger becoming furred
up sounds plausible to me, although as I live in a fairly soft water area,
it does surprise me a little. That said, in Feb 2001, I had to replace the
'Water Section' unit on the DHW side due to the seizing up of the flow rate
adjuster control, and there being a small leak

Although I have had a combi-care cannister fitted since
new, there was a period owing to family commitments in which I had forgotten
to replace the cartridge, and I think this is what contributed to the early
demise of the DHW water section. I have noticed the hot water efficiency
decline slowly over last couple of years, to the point where it is barely
possible to run a good hot bath. You certainly do not need to add any cold
water to the mix now, that's for sure.

Wondering if it might be a faulty temperature sensor of some kind, I located
the Overheat limiter sensor located on the DHW outlet pipe, and
saw there are 3 wires connected to it. One goes to an earth. The other two
are connected to the same spade/lug on the sensor. According to the wiring
diagram, one of these goes to something called an NTC Resistor and the other
goes into the electronic control box.

As an experiment, I removed just the latter wire (leading to the control
box), and
found that the gas burners remained lit, and a good continouous flow of hot
water was obtained at full draw, albeit accompanied by quite audible boiling
noises similar to a kettle. However, after a couple of minutes drawing hot
water, the flame lockout reset button popped out on the electronic control
box, and the boiler shut down. It was then I noticed that the temperature
gauge on the Central Heating circuit had shot up to around 100 deg/c, which
presumably caused an overheat on the CH circuit.

This then tends to confirm your diagnosis, I guess. If so, what would be the
remedy here? Should I attempt a descale, and if so how?

My very grateful thanks to you.

N.

In article ,
"nrh" writes:

This then tends to confirm your diagnosis, I guess. If so, what would be the
remedy here? Should I attempt a descale, and if so how?

Don't know that model, but the instructions should say how to
remove and reinstall the secondary heat exchanger. Descaling
can be done with kettle descaler or plumbing descaler such as
Furnox DS-3 (much cheaper and better than kettle descaler,
although it might only be available in large size).

--
Andrew Gabriel

In ,
Andrew Gabriel scribed:
In article ,
"nrh" writes:

This then tends to confirm your diagnosis, I guess. If so, what would be
the remedy here? Should I attempt a descale, and if so how?

Don't know that model, but the instructions should say how to
remove and reinstall the secondary heat exchanger. Descaling
can be done with kettle descaler or plumbing descaler such as
Furnox DS-3 (much cheaper and better than kettle descaler,
although it might only be available in large size).

Ah right. Yes, the manual does include instructions on how to drain down and
remove the exchanger. I was just unsure whether or not the descaling could
be done with the exchanger still in situ.

Not to be pedantic, but do you mean Furnox or Fernox? I haven't heard of the
former. Can one buy it 'off the shelf' at a plumbers merchants.

Many thanks for this.
N.

In k,
nrh scribed:

snipped

Not to be pedantic, but do you mean Furnox or Fernox? I haven't heard of
the former. Can one buy it 'off the shelf' at a plumbers merchants.



As you were. I have Googled it! Fernox it is.

N.

In k,
nrh scribed:

Update!

I have managed to complete a repair, but ended up having to buy a new
secondary heat exchanger for £96 inc VAT.

When I stripped down the old one to descale it, I noticed that the internal
pathways seemed crystal clean on the DHW side. However, the CH connection
points were full of black particles, similar to chips of rust! There was
also quite a bit of 'scale' build up around the venturi where the pipes from
the boiler connect. However, when I introduced a very strong solution of
DS-3 in boiling water, it had absolutely no effect whatsoever. No bubbles of
CO2 gas, nothing. Tried it in the DHW side as well, and same result. No
descaling action at all. (and I know the stuff's good, because I descaled my
kettle at the same time!) I tried force flushing through with cold water
from the mains, but the sludge was stubborn, and water could be heard
sloshing about inside, even though I had shaken it 'dry' and no more water
could be seen coming out. So, that was it, and I didn't want to take any
chances with it.

I am now getting a full draw of hot water with no intermittent cycling of
the burners (hurrah!!!), but also wondering if the sludge that had collected
inside the exchanger had come from the main CH exchanger instead? I think I
may be needing to replace the boiler sooner than I had thought, if the main
heat exchanger is silting up. I then gave it a force back flush through
with mains water, and a little black deposit came out, but nothing to really
shout about. That said, the burners don't cycle when the CH is working, and
temps are good to the rads etc. So who knows. Probably my water pump
disintegrating now! :-((

Anyway, fingers x'd. Just want to say thanks again for the help, and hope
this long ramble helps someone else.

N.

In article ,
"nrh" writes:

I am now getting a full draw of hot water with no intermittent cycling of
the burners (hurrah!!!), but also wondering if the sludge that had collected
inside the exchanger had come from the main CH exchanger instead? I think I
may be needing to replace the boiler sooner than I had thought, if the main
heat exchanger is silting up. I then gave it a force back flush through
with mains water, and a little black deposit came out, but nothing to really
shout about. That said, the burners don't cycle when the CH is working, and
temps are good to the rads etc. So who knows. Probably my water pump
disintegrating now! :-((

Most likely coming from the radiators, due to lack of inhibitor
in the system. The plate exchanger, being the thinnest part of
the system, has been acting as a filter. Really you should flush
the system out, and then refill using appropriate amount of
inhibitor. You can get in-line filters to trap these particles,
but washing them out of the system and preventing further
corrosion should be done anyway.

--
Andrew Gabriel

In ,
Andrew Gabriel scribed:

Most likely coming from the radiators, due to lack of inhibitor
in the system. The plate exchanger, being the thinnest part of
the system, has been acting as a filter. Really you should flush
the system out, and then refill using appropriate amount of
inhibitor. You can get in-line filters to trap these particles,
but washing them out of the system and preventing further
corrosion should be done anyway.

That's a thought. I've had some Fernox Copal in it because of a couple of
fancy aluminium rads in the circuit, but that was since new around 8 years
ago! Should have been changed at 5 years IIRC. Naughty me. I'll get right on
to it, and thanks again for that heads up Andrew.

N.

On Tue, 06 Dec 2005 18:37:20 +0000, nrh wrote:

In k,
nrh scribed:

Update!

I have managed to complete a repair, but ended up having to buy a new
secondary heat exchanger for £96 inc VAT.

When I stripped down the old one to descale it, I noticed that the internal
pathways seemed crystal clean on the DHW side. However, the CH connection
points were full of black particles, similar to chips of rust! There was
also quite a bit of 'scale' build up around the venturi where the pipes from
the boiler connect. However, when I introduced a very strong solution of
DS-3 in boiling water, it had absolutely no effect whatsoever. No bubbles of
CO2 gas, nothing. Tried it in the DHW side as well, and same result. No
descaling action at all. (and I know the stuff's good, because I descaled my
kettle at the same time!) I tried force flushing through with cold water
from the mains, but the sludge was stubborn, and water could be heard
sloshing about inside, even though I had shaken it 'dry' and no more water
could be seen coming out. So, that was it, and I didn't want to take any
chances with it.

I am now getting a full draw of hot water with no intermittent cycling of
the burners (hurrah!!!), but also wondering if the sludge that had collected
inside the exchanger had come from the main CH exchanger instead? I think I
may be needing to replace the boiler sooner than I had thought, if the main
heat exchanger is silting up. I then gave it a force back flush through
with mains water, and a little black deposit came out, but nothing to really
shout about. That said, the burners don't cycle when the CH is working, and
temps are good to the rads etc. So who knows. Probably my water pump
disintegrating now! :-((

Anyway, fingers x'd. Just want to say thanks again for the help, and hope
this long ramble helps someone else.

The black bits are general corrosion form the heating circuit. If there
are so many that you can get to the point where the heatXer is blocked
then it's probably time to have a thorough clean out of the heating system.
Running a desludging chemical for a time would be a good start.



--
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

In news Ed Sirett scribed:


The black bits are general corrosion form the heating circuit. If there
are so many that you can get to the point where the heatXer is blocked
then it's probably time to have a thorough clean out of the heating
system.

Yes, I did that when I had the new boiler put in, but admittedly, that was
around 8 yrs ago!
And look what time of the year it is. Not looking good for a spring clean
for at least a couple of months, assuming we don't get the harsh winter the
forecasters are predicting. Brrrr! :-(

Running a desludging chemical for a time would be a good start.

I think that is what I shall do immediately. Can you recommend one, please?
My system holds about 50 litres of water in a 3 bed two-storey maisonette
with 8 rads, two of which are aluminium.


Cheers and thanks again for your interest.
N.

On Wed, 07 Dec 2005 10:25:44 +0000, nrh wrote:

In news Ed Sirett scribed:


The black bits are general corrosion form the heating circuit. If there
are so many that you can get to the point where the heatXer is blocked
then it's probably time to have a thorough clean out of the heating
system.

Yes, I did that when I had the new boiler put in, but admittedly, that was
around 8 yrs ago!
And look what time of the year it is. Not looking good for a spring clean
for at least a couple of months, assuming we don't get the harsh winter the
forecasters are predicting. Brrrr! :-(

Running a desludging chemical for a time would be a good start.

I think that is what I shall do immediately. Can you recommend one, please?
My system holds about 50 litres of water in a 3 bed two-storey maisonette
with 8 rads, two of which are aluminium.


Cheers and thanks again for your interest.
N.
Sentinel X400 would be a 'standard' product.
I've not tried ferroquest but others have used it ok.



--
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

In news Ed Sirett scribed:

Sentinel X400 would be a 'standard' product.
I've not tried ferroquest but others have used it ok.

Muchos Gracias, Ed ;-)

N.