I have an old (10 years+ ?) inherited boiler - a Vaillant COMBICompact
VCW-GB 242 EH.

It generally functions well but when I run a bath in cold weather, at
a constant medium tap flow rate, the water temperature fluctuates
greatly. Scalding hot for a minute or so, freezing cold for another
minute and so on. This makes it very hard to fill a warm bath as the
cold outweighs the hot over a few minutes.

The bathroom is upstairs and the boiler downstairs and mounted on the
outside wall of the main house, in a kitchen extension/garage type
thing we have.

I imagine that the boiler is struggling, after all it is old and it is
a cold time of year. But I would have thought at a certain flow rate
the temperature would be constant, even if lukewarm.

A British Gas chap told me "all combis are like that"/"yours is too
old"/"in the wrong place" etc etc. Is he right and it is time to fork
out for a new one ? Or is this something that can be fixed ?

thanks
Rik

It generally functions well but when I run a bath in cold weather, at
a constant medium tap flow rate, the water temperature fluctuates
greatly. Scalding hot for a minute or so, freezing cold for another
minute and so on.

This gets mentioned regularly, so no doubt one of the regular central=20
heating wizards will be along shortly to work their magic :-)

A British Gas chap told me "all combis are like that"/"yours is too
old"/"in the wrong place" etc etc. Is he right and it is time to fork
out for a new one ? Or is this something that can be fixed ?

From what I remember of previous answers, British Gas are simply trying=20
to apply their "perhaps we can sell a muppet a combo" logic !

No nastiness intended there - it was a general comment about people who=20
buy from BG in the first place, like my ma-in-law who just spent over =A34k=
=20
on a standard *combi* CH system from them... The same could be had for=20
=A32.5k maximum almost anywhere else

--=20
Please add "[newsgroup]" in the subject of any personal replies via email
* old email address "btiruseless" abandoned due to worm-generated spam *
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On Sat, 27 Dec 2003 20:15:59 +0000, Rik Smithies wrote:

I have an old (10 years+ ?) inherited boiler - a Vaillant COMBICompact
VCW-GB 242 EH.

It generally functions well but when I run a bath in cold weather, at
a constant medium tap flow rate, the water temperature fluctuates
greatly. Scalding hot for a minute or so, freezing cold for another
minute and so on. This makes it very hard to fill a warm bath as the
cold outweighs the hot over a few minutes.

The bathroom is upstairs and the boiler downstairs and mounted on the
outside wall of the main house, in a kitchen extension/garage type
thing we have.

I imagine that the boiler is struggling, after all it is old and it is
a cold time of year. But I would have thought at a certain flow rate
the temperature would be constant, even if lukewarm.

A British Gas chap told me "all combis are like that"/"yours is too
old"/"in the wrong place" etc etc. Is he right and it is time to fork
out for a new one ? Or is this something that can be fixed ?

Almost certinly the secondary heat exchanger has scaled up. Replacement or
descaling is called for.

A possible work around which is tedious is to set flow rate quite low and turn
down the thermostat to the point where the flames do not go off and on.
Since there is only one thermostat this may mean turning it up for heating,
that's inconvenient.

This happens on most combis sooner or later (VCWs are fairly robust and
have a substantially secondary heat exchanger so they tend to take a long
time (10 years) to get furred up.
I would think properly maintained your VCW has some more years in it.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at 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

thanks for replies, that secondary heat exchanger tip might end up
saving me a few quid.
Rik

Another thought, I live in a soft water area (Birmingham), so is this
scaling still a likely explanation ?

Rik Smithies wrote:
Another thought, I live in a soft water area (Birmingham), so is this
scaling still a likely explanation ?

I'd be surprised. The soft water is more likely to eat away the heat
exchanger than scale it up.

On Sun, 28 Dec 2003 13:45:53 +0000, Rik Smithies wrote:

Another thought, I live in a soft water area (Birmingham), so is this
scaling still a likely explanation ?

OK so we are not looking at limescale but the same effect could be had
by debris build up on either the primary or secondary side of the heat
exchanger.
Or you might have a normally working boiler but the temperature sensor has
drifted out of spec.
Or the thermostat board inside the die-cast box - middle horizontal board
has a problem.

To progress further you will need to see if the gas is cutting because the
primary flow temperature has gotten to 80-90C (bad 2ndary HE) or because
the sensor/PCB is over enthusiatic.

Another possibility is that the modulator on the gas valve has failed, you
can check to see if that might be the case by looking closely at the
flames to see if they reduce before the gas gets cut.

HTH

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at 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

"Ed Sirett" wrote in message
OK so we are not looking at limescale but the same effect could be had
by debris build up on either the primary or secondary side of the heat
exchanger.
Or you might have a normally working boiler but the temperature sensor has
drifted out of spec.
Or the thermostat board inside the die-cast box - middle horizontal board
has a problem.

To progress further you will need to see if the gas is cutting because the
primary flow temperature has gotten to 80-90C (bad 2ndary HE) or because
the sensor/PCB is over enthusiatic.

Another possibility is that the modulator on the gas valve has failed, you
can check to see if that might be the case by looking closely at the
flames to see if they reduce before the gas gets cut.

Apologies for hijacking this thread but my Ravenheat RSF100 is exhibiting
similar problems except that the DHW output never gets above lukewarm. When
the water temp. control on the boiler control panel is turned up from
minimum the flames increase momentarily then drop down to previous level &
then gas valve cuts the supply - is this indicative of a failed modulator ?
Boiler then fires up again then cuts out after 15 seconds and on and on. DHW
temp. sensor has been changed but this has not solved the problem. CH works
fine.


Neil

On Sun, 28 Dec 2003 21:49:10 +0000, Neil wrote:


"Ed Sirett" wrote in message
OK so we are not looking at limescale but the same effect could be had
by debris build up on either the primary or secondary side of the heat
exchanger.
Or you might have a normally working boiler but the temperature sensor has
drifted out of spec.
Or the thermostat board inside the die-cast box - middle horizontal board
has a problem.

To progress further you will need to see if the gas is cutting because the
primary flow temperature has gotten to 80-90C (bad 2ndary HE) or because
the sensor/PCB is over enthusiatic.

Another possibility is that the modulator on the gas valve has failed, you
can check to see if that might be the case by looking closely at the
flames to see if they reduce before the gas gets cut.

Apologies for hijacking this thread but my Ravenheat RSF100 is exhibiting
similar problems except that the DHW output never gets above lukewarm. When
the water temp. control on the boiler control panel is turned up from
minimum the flames increase momentarily then drop down to previous level &
then gas valve cuts the supply - is this indicative of a failed modulator ?
Boiler then fires up again then cuts out after 15 seconds and on and on. DHW
temp. sensor has been changed but this has not solved the problem. CH works
fine.

Sorry, the way I worded it could seem to say that the flames going up and
down means a failed modulator (or PCB control). No, the job of the
modulator is to match the gas flame to the demand so when the flames go
down before going out the modulator is doing its job and has not failed.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at 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 message .uk, Ed
Sirett writes
On Sat, 27 Dec 2003 20:15:59 +0000, Rik Smithies wrote:

I have an old (10 years+ ?) inherited boiler - a Vaillant COMBICompact
VCW-GB 242 EH.

Almost certinly the secondary heat exchanger has scaled up. Replacement or
descaling is called for.

A possible work around which is tedious is to set flow rate quite low and turn
down the thermostat to the point where the flames do not go off and on.
Since there is only one thermostat this may mean turning it up for heating,
that's inconvenient.

This happens on most combis sooner or later (VCWs are fairly robust and
have a substantially secondary heat exchanger so they tend to take a long
time (10 years) to get furred up.
I would think properly maintained your VCW has some more years in it.


A few years ago, Vaillant changed the design of the secondary heat
exchanger from one which looked like three black plastic tubes to one
which looks like a sardine can.

So, if you do buy one and it doesn't look anything like your old one,
that's why.

"thanks for replies, that secondary heat exchanger tip might end up
saving me a few quid."

yes, but prepare yourself for a bit of a shock though (£150?)



--
geoff

In message , BillR
writes
Rik Smithies wrote:
Another thought, I live in a soft water area (Birmingham), so is this
scaling still a likely explanation ?

I'd be surprised. The soft water is more likely to eat away the heat
exchanger than scale it up.

I doubt it, the old style ones are made of plastic


--
geoff