In a private e-mail to , on Thurs, 18 Nov 2004
20:43:28 -0000, Tania wrote:

HELLO

I have recently had a combi boiler put in, and whenever the hot tap is put
on there is a really hot burst of water (too painful to use). It then goes
cold for a while, then heats up to the temerature I set on the boiler
thermostat. I have two very young children and worry about them turning on
the hot tap in the loo or bathroom. Has anyone else found a solution to
this. I asked the engineer who came out to sort out a lighting problem
today, and he said 'other people don't moan about it' which left me feeling
fed up and no wiser. He did explain to me how it happens several times,
each more slowly than the last, he was a very rude man! he had no will to
try and reassure me, but when pushed did say it would be 64 degrees
maximum - so not scalding. Any ideas will be gratefully received.

Regards

Tania

Water temperature fluctuations seems a common problem with combis in
houses I have visited. Can anyone help with a solution to this problem
that I received via email from the FAQ website?

Thanks

Phil
The uk.d-i-y FAQ is at http://www.diyfaq.org.uk/
Remove NOSPAM from address to email me

On Thu, 18 Nov 2004 22:47:21 GMT, Phil Addison
wrote:

In a private e-mail to , on Thurs, 18 Nov 2004
20:43:28 -0000, Tania wrote:

HELLO

I have recently had a combi boiler put in, and whenever the hot tap is put
on there is a really hot burst of water (too painful to use). It then goes
cold for a while, then heats up to the temerature I set on the boiler
thermostat. I have two very young children and worry about them turning on
the hot tap in the loo or bathroom. Has anyone else found a solution to
this. I asked the engineer who came out to sort out a lighting problem
today, and he said 'other people don't moan about it' which left me feeling
fed up and no wiser. He did explain to me how it happens several times,
each more slowly than the last, he was a very rude man! he had no will to
try and reassure me, but when pushed did say it would be 64 degrees
maximum - so not scalding. Any ideas will be gratefully received.

Regards

Tania

Water temperature fluctuations seems a common problem with combis in
houses I have visited. Can anyone help with a solution to this problem
that I received via email from the FAQ website?

Yes,why oh why do combi installers chuck out a perfectly usable
cylinder,especially in large family houses?. My view for family
properties is to keep the cylinder,feed it from the combi,obviously
make sure things are well insulated/controlled.
Thanks

Phil
The uk.d-i-y FAQ is at http://www.diyfaq.org.uk/
Remove NOSPAM from address to email me

"tarquinlinbin" wrote in message
...

Yes,why oh why do combi installers chuck out a perfectly usable
cylinder,especially in large family houses?. My view for family
properties is to keep the cylinder,feed it from the combi,obviously
make sure things are well insulated/controlled.

This that is your view you should change it, as the cylinder will burst.
Find how water systems work. here are high flow combi's around, that will
do a large family.

An advantage of a combi is that it liberates space tanks and cylinders take
up.

IMM wrote:

This that is your view you should change it, as the cylinder will burst.
Find how water systems work. here are high flow combi's around, that will
do a large family.

Have you ever though about taking remedial English lessons?

--
Andy

IMM wrote:

This that is your view you should change it, as the cylinder will burst.
Find how water systems work. here are high flow combi's around, that will
do a large family.

Have you ever thought about taking remedial English lessons?

--
Andy

"Andy Wade" wrote in message
...
IMM wrote:

This that is your view you should change it, as the cylinder will burst.
Find how water systems work. There are high flow combi's around, that
will
do a large family.

Have you ever thought about taking remedial English lessons?

No need to as I am brilliant at English and life in general.

IMM wrote:

No need to as I am brilliant at English and life in general.

The evidence for that seems to be a little thin on the ground...

--
Andy

IMM wrote:

"Andy Wade" wrote in message
...

IMM wrote:


This that is your view you should change it, as the cylinder will burst.
Find how water systems work. There are high flow combi's around, that

will

do a large family.

Have you ever thought about taking remedial English lessons?


No need to as I am brilliant at English and life in general.

Should we award the same respect to your other posts as we do to the one
above?


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

"IMM" wrote in message
...



No need to as I am brilliant at English and life in general.

IMM , every post a gem. long may you reign.

The retort puts me in mind of Tom Baker narrating "Little Britain"
fab.

Rich

IMM wrote:

"Andy Wade" wrote in message
...

IMM wrote:


This that is your view you should change it, as the cylinder will burst.
Find how water systems work. There are high flow combi's around, that

will

do a large family.

Have you ever thought about taking remedial English lessons?


No need to as I am brilliant at English and life in general.





"You left out a 'there is', a second 'at', and there is a superfluous
'to' and three commas are missing."

3/10.


EXCERPT FROM IMM'S SCHOOL PSYCHITATSIC CAREER REPORT
----------------------------------------------------
Must try harder.

This child is held back by conviction of his own correctness.

I strongly doubt he would be capable of being even a plumber.

Incapable of coming to terms with reality, he is gullible and defensive.
Mild sociopathic tendencies.

John works best in a situation in which he is given clear simple
instructions, and there is no ambiguity, and some else is doing the
thinking for him.

Left to hs own devices he grabs for any piece of printed matter that has
one or two longer words on than he can understand, and uses it as an
effective Bible. His deep supsicion that other people are laughing at
him and trying to manipulate his (very limited) intellect does not
extend to the printed word.

He would hae made an ideal Camp guard at Buchenwald, or Roman Catholic
Priest, but today is likely to never find satisfaction for his
socipathic tendencies, and will probably end up stalking, becoming an
Internet geek, or join the political party that tells the biggest lies,
in the fewest words.

In article ,
IMM wrote:
Yes,why oh why do combi installers chuck out a perfectly usable
cylinder,especially in large family houses?. My view for family
properties is to keep the cylinder,feed it from the combi,obviously
make sure things are well insulated/controlled.

This that is your view you should change it, as the cylinder will burst.

Strange. I know of one system where the combi heats a tank in the normal
way, and the water heating side is used just for the kitchen. Not sure
what the benefits are, as the owner is as mad as you.

Find how water systems work. here are high flow combi's around, that
will do a large family.

And the cost to achieve this, assuming a good storage system already
exists?

An advantage of a combi is that it liberates space tanks and cylinders
take up.

Not everyone lives in a rabbit hutch.

--
*Few women admit their age; fewer men act it.

Dave Plowman London SW
To e-mail, change noise into sound.

Strange. I know of one system where the combi heats a tank in the normal
way, and the water heating side is used just for the kitchen. Not sure
what the benefits are, as the owner is as mad as you.

It can be good in a large house to reduce water lead times, particularly if
the boiler is in the kitchen. The hot water cylinder is placed near the
bathroom to provide lots of hot water quickly, whilst the kitchen runs off
the combi, because you'd have to draw off 10 litres of water before anything
came through from the distant bathroom. It also provides drinking hot water
in the kitchen, which is very useful when cooking. I use my hot tap all the
time to fill kettles and pans. (It's a heat bank, not a combi, but still
drinking water).

Christian.

In article ,
Christian McArdle wrote:
Strange. I know of one system where the combi heats a tank in the normal
way, and the water heating side is used just for the kitchen. Not sure
what the benefits are, as the owner is as mad as you.

It can be good in a large house to reduce water lead times, particularly
if the boiler is in the kitchen. The hot water cylinder is placed near
the bathroom to provide lots of hot water quickly, whilst the kitchen
runs off the combi, because you'd have to draw off 10 litres of water
before anything came through from the distant bathroom. It also provides
drinking hot water in the kitchen, which is very useful when cooking. I
use my hot tap all the time to fill kettles and pans. (It's a heat bank,
not a combi, but still drinking water).

Right. Suppose it makes some sense. Any combi should be ok for just the
kitchen tap. Could it still be used if the central heating side was
drained down for any reason?

--
*The more people I meet, the more I like my dog.

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" wrote in message
...
In article ,
IMM wrote:
Yes,why oh why do combi installers chuck out a perfectly usable
cylinder,especially in large family houses?. My view for family
properties is to keep the cylinder,feed it from the combi,obviously
make sure things are well insulated/controlled.

This that is your view you should change it, as the cylinder will burst.

Strange. I know of one system

snip tripe

Please do not comment on fields you have no experience of.

On Fri, 19 Nov 2004 15:05:35 -0000, "IMM" wrote:


"Dave Plowman (News)" wrote in message
...
In article ,
IMM wrote:
Yes,why oh why do combi installers chuck out a perfectly usable
cylinder,especially in large family houses?. My view for family
properties is to keep the cylinder,feed it from the combi,obviously
make sure things are well insulated/controlled.

This that is your view you should change it, as the cylinder will burst.

Strange. I know of one system

snip tripe

Please do not comment on fields you have no experience of.

How about attempting top answer the actual original question?

Phil
The uk.d-i-y FAQ is at http://www.diyfaq.org.uk/
Remove NOSPAM from address to email me

On Fri, 19 Nov 2004 14:33:48 +0000, Dave Plowman (News) wrote:

In article ,
IMM wrote:
Yes,why oh why do combi installers chuck out a perfectly usable
cylinder,especially in large family houses?. My view for family
properties is to keep the cylinder,feed it from the combi,obviously
make sure things are well insulated/controlled.

This that is your view you should change it, as the cylinder will burst.

Strange. I know of one system where the combi heats a tank in the normal
way, and the water heating side is used just for the kitchen. Not sure
what the benefits are, as the owner is as mad as you.

I installed a similar system for my nieghbours directly opposite my
house. S-plan for the house, combi HW for a sink in the garage below the
house. They like baths and it would have been hard to gain access to the
HW pipes under the kitchen floor to supply the garage sink.



Find how water systems work. here are high flow combi's around, that
will do a large family.

And the cost to achieve this, assuming a good storage system already
exists?

The cost IMHO is less significant the space taken up in the kitchen
(usually). The space to be lost in the kitchen would usually be more
precious than the space gain in the loft and/or airing cupboard.


An advantage of a combi is that it liberates space tanks and cylinders
take up.

Not everyone lives in a rabbit hutch.

Round here a lot do.

--
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
news
On Fri, 19 Nov 2004 14:33:48 +0000, Dave Plowman (News) wrote:

In article ,
IMM wrote:
Yes,why oh why do combi installers chuck out a perfectly usable
cylinder,especially in large family houses?. My view for family
properties is to keep the cylinder,feed it from the combi,obviously
make sure things are well insulated/controlled.

This that is your view you should change it, as the cylinder will
burst.

Strange. I know of one system where the combi heats a tank in the normal
way, and the water heating side is used just for the kitchen. Not sure
what the benefits are, as the owner is as mad as you.

I installed a similar system for my nieghbours directly opposite my
house. S-plan for the house, combi HW for a sink in the garage below the
house. They like baths and it would have been hard to gain access to the
HW pipes under the kitchen floor to supply the garage sink.



Find how water systems work. here are high flow combi's around, that
will do a large family.

And the cost to achieve this, assuming a good storage system already
exists?

The cost IMHO is less significant the space taken up in the kitchen
(usually). The space to be lost in the kitchen would usually be more
precious than the space gain in the loft and/or airing cupboard.

????

An advantage of a combi is that it liberates space tanks and cylinders
take up.

Not everyone lives in a rabbit hutch.

Round here a lot do.

Most do. You find that a cylinder removed from the airing cupboard and a
small rad installed is greatly appreciated. The space it releases is
phenomenal. Using Elson tanks can make a difference. The square shapes take
up far less space. Gledhill now make the XStream unvented cylinder in a
square shape, with all connections on one side.

Water temperature fluctuations seems a common problem with combis in
houses I have visited. Can anyone help with a solution to this problem
that I received via email from the FAQ website?

As well as weird effects due to the combi design, another possibility if the
combi DHW output and the CH pipes run in very close proximity (particularly
if boxed in) is that the DHW is being heated by the CH pipes whilst sitting
adjacent to them, to a much higher temp that the actual DHW output. When you
turn on the tap, this slug of hot water comes out.

It is only a suggestion, and probably isn't the problem, but could be fixed
by separating the pipes with a bit of insulation.

Christian.

On Fri, 19 Nov 2004 19:18:27 +0000, "Ed Sirett"
wrote:

In a private e-mail to , on Thurs, 18 Nov 2004
20:43:28 -0000, Tania wrote:

HELLO

I have recently had a combi boiler put in, and whenever the hot tap is put
on there is a really hot burst of water (too painful to use). It then goes
cold for a while, then heats up to the temerature I set on the boiler
thermostat. I have two very young children and worry about them turning on
the hot tap in the loo or bathroom. Has anyone else found a solution to
this. I asked the engineer who came out to sort out a lighting problem
today, and he said 'other people don't moan about it' which left me feeling
fed up and no wiser. He did explain to me how it happens several times,
each more slowly than the last, he was a very rude man! he had no will to
try and reassure me, but when pushed did say it would be 64 degrees
maximum - so not scalding. Any ideas will be gratefully received.

Regards

Tania

On Fri, 19 Nov 2004 15:20:17 +0000, Phil Addison wrote:

Water temperature fluctuations seems a common problem with combis in
houses I have visited. Can anyone help with a solution to this problem
that I received via email from the FAQ website?


IME experience there are several reasons why (ordinary) combi boilers have poor
regulation of DHW temperature.

1) Drawing off water from elsewhere especially in the same dwelling.
2) Thermostatic mixing valves fighting the boiler.

but mainly:
3) When the DHW exchanger becomes scaled up the heat transfer rate becomes
limited, that's bad enough. Eventually the heat transfer rate drops below
the lowest modulated setting of the burner (typically around 10-12kW on a
popular 24kW unit), at this point the control system, in order to modulate
the burner further has to start switching it on an off. Given the
considerable thermal inertia of the components a fluctuation of the output
temperature begins.

1. Can be mitigated by good design and practice. Namely don't fit a combi
where the supply is inadequate. Take it's supply upstream of other
draw offs (even consider putting a slight restriction on the feed to
everything except the combi.

2. This is not a common problem and shoudl be mitigated by getting 1 and 3
right.

3. Descale or replace secondary heat exchanger.

Hi Ed, Thanks for getting us back on topic. Sorry if my post was
confusing, but I am passing on a problem I got from a reader of the FAQ
site, who does not seem able to access the ng.

However, she says the combi was recently installed, so scaling (3)
should not be the cause, and her description appears to rule out 1 & 2.

The problem seems to be that from when turned on the water initially
comes out very hot, then goes cold, then warms up to the correct stat
temperature. I wonder if it could be because she has the tap turned on
only a small amount initially, so it overheats and shuts down, then she
turns the tap on more in order to make it re-light. Could this last
action could produce cold until the over-heat resets, and then the
greater flow enables it to modulate properly and warm up to the right
temperature?

Phil
The uk.d-i-y FAQ is at http://www.diyfaq.org.uk/
Remove NOSPAM from address to email me

On Fri, 19 Nov 2004 19:51:52 +0000, Phil Addison wrote:

On Fri, 19 Nov 2004 19:18:27 +0000, "Ed Sirett"
wrote:

In a private e-mail to , on Thurs, 18 Nov 2004
20:43:28 -0000, Tania wrote:

HELLO

I have recently had a combi boiler put in, and whenever the hot tap is put
on there is a really hot burst of water (too painful to use). It then goes
cold for a while, then heats up to the temerature I set on the boiler
thermostat. I have two very young children and worry about them turning on
the hot tap in the loo or bathroom. Has anyone else found a solution to
this. I asked the engineer who came out to sort out a lighting problem
today, and he said 'other people don't moan about it' which left me feeling
fed up and no wiser. He did explain to me how it happens several times,
each more slowly than the last, he was a very rude man! he had no will to
try and reassure me, but when pushed did say it would be 64 degrees
maximum - so not scalding. Any ideas will be gratefully received.

Regards

Tania

On Fri, 19 Nov 2004 15:20:17 +0000, Phil Addison wrote:

Water temperature fluctuations seems a common problem with combis in
houses I have visited. Can anyone help with a solution to this problem
that I received via email from the FAQ website?


IME experience there are several reasons why (ordinary) combi boilers have poor
regulation of DHW temperature.

1) Drawing off water from elsewhere especially in the same dwelling.
2) Thermostatic mixing valves fighting the boiler.

but mainly:
3) When the DHW exchanger becomes scaled up the heat transfer rate becomes
limited, that's bad enough. Eventually the heat transfer rate drops below
the lowest modulated setting of the burner (typically around 10-12kW on a
popular 24kW unit), at this point the control system, in order to modulate
the burner further has to start switching it on an off. Given the
considerable thermal inertia of the components a fluctuation of the output
temperature begins.

1. Can be mitigated by good design and practice. Namely don't fit a combi
where the supply is inadequate. Take it's supply upstream of other
draw offs (even consider putting a slight restriction on the feed to
everything except the combi.

2. This is not a common problem and shoudl be mitigated by getting 1 and 3
right.

3. Descale or replace secondary heat exchanger.

Hi Ed, Thanks for getting us back on topic. Sorry if my post was
confusing, but I am passing on a problem I got from a reader of the FAQ
site, who does not seem able to access the ng.

However, she says the combi was recently installed, so scaling (3)
should not be the cause, and her description appears to rule out 1 & 2.


Sorry I have now read the OP properly.
The combi is working correctly and the 'feature' does occur on many
models.

I'm familiar with several models although some models might not exhibit
the problem though most do I would guess. A model name would help here.
In fact the only ones which I reckon are ones with a sperate internal
bypass pipework.

On many models the secondary heat exchanger also serves as a part of the
bypass ciurcuit. This is easily implemented by arranging for the
diverter valve to bleed primary water through the secondary heat exchnager
when in CH mode. So in winter the 2ndry HE gets to the temperature of the
primary flow. This means that the contents of the 2ndry HE are hot, this
slug of very hot water will comes out first in DHW mode.

To mitigate the problem try turning the radiator tempertaure down.
Of course if the rads are not over sized then the house won't get
heated. 8-(.

All the boilers which have double primary heat exchnagers (eg Ferolli)
or integral tube-in-tube type like (Pott. Lynx, Saunier Duval)
will also have this problem and likely a worse version.

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

On Sat, 20 Nov 2004 21:12:25 +0000, "Ed Sirett"
wrote:

Sorry I have now read the OP properly.
The combi is working correctly and the 'feature' does occur on many
models.

I'm familiar with several models although some models might not exhibit
the problem though most do I would guess. A model name would help here.
In fact the only ones which I reckon are ones with a sperate internal
bypass pipework.

On many models the secondary heat exchanger also serves as a part of the
bypass ciurcuit. This is easily implemented by arranging for the
diverter valve to bleed primary water through the secondary heat exchnager
when in CH mode. So in winter the 2ndry HE gets to the temperature of the
primary flow. This means that the contents of the 2ndry HE are hot, this
slug of very hot water will comes out first in DHW mode.

To mitigate the problem try turning the radiator tempertaure down.
Of course if the rads are not over sized then the house won't get
heated. 8-(.

All the boilers which have double primary heat exchnagers (eg Ferolli)
or integral tube-in-tube type like (Pott. Lynx, Saunier Duval)
will also have this problem and likely a worse version.

Hi,

Does it bleed water regardless of the flow in the primary and can it
be blocked somehow? If there is a differential pressure valve in there
or it can be blocked it may be possible to use a lower pressure one
elsewhere.

cheers,
Pete.

On Sun, 21 Nov 2004 13:29:07 +0000, Pete C wrote:

On Sat, 20 Nov 2004 21:12:25 +0000, "Ed Sirett"
wrote:

Sorry I have now read the OP properly.
The combi is working correctly and the 'feature' does occur on many
models.

I'm familiar with several models although some models might not exhibit
the problem though most do I would guess. A model name would help here.
In fact the only ones which I reckon are ones with a sperate internal
bypass pipework.

On many models the secondary heat exchanger also serves as a part of the
bypass ciurcuit. This is easily implemented by arranging for the
diverter valve to bleed primary water through the secondary heat exchnager
when in CH mode. So in winter the 2ndry HE gets to the temperature of the
primary flow. This means that the contents of the 2ndry HE are hot, this
slug of very hot water will comes out first in DHW mode.

To mitigate the problem try turning the radiator tempertaure down.
Of course if the rads are not over sized then the house won't get
heated. 8-(.

All the boilers which have double primary heat exchnagers (eg Ferolli)
or integral tube-in-tube type like (Pott. Lynx, Saunier Duval)
will also have this problem and likely a worse version.

Hi,

Does it bleed water regardless of the flow in the primary and can it
be blocked somehow? If there is a differential pressure valve in there
or it can be blocked it may be possible to use a lower pressure one
elsewhere.

I'm fairly sure that modern Vaillants and Vokeras work as I describe.
The diverter-valve does not entirely block the path through the primary
side of the secondary heat exchanger. There is nothing smart about this
it is simply a deliberate hole in the valve. The effect is to slightly
reduce the output of the boiler in CH mode, since the output in CH mode is
usually far in exces of requirements this is not a problem.

The only way to prevent the primary water doing this wouod be to modify
the diverte valve. The effect of doing this would be disasterous.
1) The boiler would have no bypass.
2) On Vaillants there would be no sensing of primary water temperture
leading to a catastrophe.









--
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

"Phil Addison" wrote in message
...
On Fri, 19 Nov 2004 19:18:27 +0000, "Ed Sirett"
wrote:

In a private e-mail to , on Thurs, 18 Nov 2004
20:43:28 -0000, Tania wrote:

HELLO

I have recently had a combi boiler put in, and whenever the hot tap is
put
on there is a really hot burst of water (too painful to use). It then
goes
cold for a while, then heats up to the temerature I set on the boiler
thermostat. I have two very young children and worry about them
turning on
the hot tap in the loo or bathroom. Has anyone else found a solution
to
this. I asked the engineer who came out to sort out a lighting problem
today, and he said 'other people don't moan about it' which left me
feeling
fed up and no wiser. He did explain to me how it happens several
times,
each more slowly than the last, he was a very rude man! he had no will
to
try and reassure me, but when pushed did say it would be 64 degrees
maximum - so not scalding. Any ideas will be gratefully received.


This is a recognised problem with some combis which are the tube within tube
heat exchanger type. When the heating is running the heat exchanger attains
a high temperature and thus the bit of tapwater within this also reaches
this temperature. When the hot tap is opened this "slug" of very hot water
travels along and passes out of the tap. The boiler runs through its
sequence and fires at the hot water rate which is designed to provide hot
water at the "normal" temperature. The net result is just as described i.e.
a brief pulse of scalding water followed by cooler then normal temperature
hot water. A long length of pipe from boiler to tap will mitigate the
problem but creates an annoying lag in delivery times.
C'est la vie

On Mon, 22 Nov 2004 18:33:03 +0000 (UTC), "John"
wrote:


"Phil Addison" wrote in message
...
On Fri, 19 Nov 2004 19:18:27 +0000, "Ed Sirett"
wrote:

In a private e-mail to , on Thurs, 18 Nov 2004
20:43:28 -0000, Tania wrote:

HELLO

I have recently had a combi boiler put in, and whenever the hot tap is
put
on there is a really hot burst of water (too painful to use). It then
goes
cold for a while, then heats up to the temerature I set on the boiler
thermostat. I have two very young children and worry about them
turning on
the hot tap in the loo or bathroom. Has anyone else found a solution
to
this. I asked the engineer who came out to sort out a lighting problem
today, and he said 'other people don't moan about it' which left me
feeling
fed up and no wiser. He did explain to me how it happens several
times,
each more slowly than the last, he was a very rude man! he had no will
to
try and reassure me, but when pushed did say it would be 64 degrees
maximum - so not scalding. Any ideas will be gratefully received.


This is a recognised problem with some combis which are the tube within tube
heat exchanger type. When the heating is running the heat exchanger attains
a high temperature and thus the bit of tapwater within this also reaches
this temperature. When the hot tap is opened this "slug" of very hot water
travels along and passes out of the tap. The boiler runs through its
sequence and fires at the hot water rate which is designed to provide hot
water at the "normal" temperature. The net result is just as described i.e.
a brief pulse of scalding water followed by cooler then normal temperature
hot water. A long length of pipe from boiler to tap will mitigate the
problem but creates an annoying lag in delivery times.
C'est la vie

Thanks Ed and John. So summarizing your replies, Tania should try
turning down the C/H temp if the weather is not too cold. Apart from
that I'm sure telling her it is 'normal' will be of some comfort, so she
can just expect the hot slug and wait for it to drop back to the proper
HW temperature.

I appreciate the explanation of what causes it - and wonder if there is
a safety regulation covering max temp of DHW. Is there a list, or some
easy way of recognising combis that do or don't display this
mis-feature?

I'll be passing the advice onto Tania.

Phil
The uk.d-i-y FAQ is at http://www.diyfaq.org.uk/
Remove NOSPAM from address to email me

"Phil Addison" wrote in message
...
On Mon, 22 Nov 2004 18:33:03 +0000 (UTC), "John"
wrote:


"Phil Addison" wrote in message
...
On Fri, 19 Nov 2004 19:18:27 +0000, "Ed Sirett"
wrote:

In a private e-mail to , on Thurs, 18 Nov 2004
20:43:28 -0000, Tania wrote:

HELLO

I have recently had a combi boiler put in, and whenever the hot tap
is
put
on there is a really hot burst of water (too painful to use). It
then
goes
cold for a while, then heats up to the temerature I set on the
boiler
thermostat. I have two very young children and worry about them
turning on
the hot tap in the loo or bathroom. Has anyone else found a
solution
to
this. I asked the engineer who came out to sort out a lighting
problem
today, and he said 'other people don't moan about it' which left me
feeling
fed up and no wiser. He did explain to me how it happens several
times,
each more slowly than the last, he was a very rude man! he had no
will
to
try and reassure me, but when pushed did say it would be 64 degrees
maximum - so not scalding. Any ideas will be gratefully received.


This is a recognised problem with some combis which are the tube within
tube
heat exchanger type. When the heating is running the heat exchanger
attains
a high temperature and thus the bit of tapwater within this also reaches
this temperature. When the hot tap is opened this "slug" of very hot
water
travels along and passes out of the tap. The boiler runs through its
sequence and fires at the hot water rate which is designed to provide
hot
water at the "normal" temperature. The net result is just as described
i.e.
a brief pulse of scalding water followed by cooler then normal
temperature
hot water. A long length of pipe from boiler to tap will mitigate the
problem but creates an annoying lag in delivery times.
C'est la vie

Thanks Ed and John. So summarizing your replies, Tania should try
turning down the C/H temp if the weather is not too cold. Apart from
that I'm sure telling her it is 'normal' will be of some comfort, so she
can just expect the hot slug and wait for it to drop back to the proper
HW temperature.

I appreciate the explanation of what causes it - and wonder if there is
a safety regulation covering max temp of DHW. Is there a list, or some
easy way of recognising combis that do or don't display this
mis-feature?

I'll be passing the advice onto Tania.

You could put in a secondary circulation loop on the combi and guarantee hot
water instantly at the taps. This would for most of the draw-off time not be
scalding as the pump would slowly circulate the water in the loop and combi
heat exchanger. Have a pipe stat to keep the draw-off temp to around 40C,
so it will not be extracting heat from the combi continuously.

Phil Addison wrote:

I appreciate the explanation of what causes it - and wonder if there is
a safety regulation covering max temp of DHW. Is there a list, or some
easy way of recognising combis that do or don't display this
mis-feature?

My ideal isar has a plate heat exchanger for the HW side, and it does
not demonstrate this behaviour. It also maintains a small (i.e. couple
of litres) store of water that it keeps tempered at the selected HW
output temp to try and provide a more instant and stable temp during
initial demand. Presumably others of similar design would also be free
from the problem.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Mon, 22 Nov 2004 19:30:53 +0000, Phil Addison wrote:

On Mon, 22 Nov 2004 18:33:03 +0000 (UTC), "John"
wrote:


"Phil Addison" wrote in message
...
On Fri, 19 Nov 2004 19:18:27 +0000, "Ed Sirett"
wrote:

In a private e-mail to , on Thurs, 18 Nov 2004
20:43:28 -0000, Tania wrote:

HELLO

I have recently had a combi boiler put in, and whenever the hot tap is
put
on there is a really hot burst of water (too painful to use). It then
goes
cold for a while, then heats up to the temerature I set on the boiler
thermostat. I have two very young children and worry about them
turning on
the hot tap in the loo or bathroom. Has anyone else found a solution
to
this. I asked the engineer who came out to sort out a lighting problem
today, and he said 'other people don't moan about it' which left me
feeling
fed up and no wiser. He did explain to me how it happens several
times,
each more slowly than the last, he was a very rude man! he had no will
to
try and reassure me, but when pushed did say it would be 64 degrees
maximum - so not scalding. Any ideas will be gratefully received.


This is a recognised problem with some combis which are the tube within tube
heat exchanger type. When the heating is running the heat exchanger attains
a high temperature and thus the bit of tapwater within this also reaches
this temperature. When the hot tap is opened this "slug" of very hot water
travels along and passes out of the tap. The boiler runs through its
sequence and fires at the hot water rate which is designed to provide hot
water at the "normal" temperature. The net result is just as described i.e.
a brief pulse of scalding water followed by cooler then normal temperature
hot water. A long length of pipe from boiler to tap will mitigate the
problem but creates an annoying lag in delivery times.
C'est la vie

Thanks Ed and John. So summarizing your replies, Tania should try
turning down the C/H temp if the weather is not too cold. Apart from
that I'm sure telling her it is 'normal' will be of some comfort, so she
can just expect the hot slug and wait for it to drop back to the proper
HW temperature.

I appreciate the explanation of what causes it - and wonder if there is
a safety regulation covering max temp of DHW. Is there a list, or some
easy way of recognising combis that do or don't display this
mis-feature?


There are regulation covering maximum HW temperature but they don't (yet)
apply to normal domestic dwellings.

I doubt there has been a list of suspects made with this problem.
Pott. Lynx
Saunier Duval
Ferroli
Fer
Vaillant Turbomax+

It can also occur where the 2ndary heater exchanger is also used a CH
bypass circuit.

--
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