We've just had a WB 40 CDi combi installed into a house that was
previously a standard boiler/tank/cylinder affair. The short story is
that we can't run a bath and shower off it at the same time (I don't
mean that the pressure drops off a bit on the shower - it actually
stops). Various plumbers and even the WB site itself would indicate
that you can't expect to do everything at once on a combi, but I did
expect more when just two outlets were opened.

Any ideas why this might be and/or what can we do?


[big detailed description]

Of course there was no existing pipework for the hot water from the
boiler position, but luckily there was both a hot and a cold 15mm pipe
right under it as that's where the washing machine is.

The plumber has used these (thus reversing the old hot flow -
presumably the cold is the same as it always was as the washing
machine would have been plumber off the mains). The 22mm pipe that
used to come out of the cylinder has been capped above the airing
cupboard floor (we'll be taking it into the loft later to drop down as
a shower). The 15mm mains feed to the old cylinder has been spliced
into the remains of the 22mm pipe from the loft tank to the taps and
likewise a spur capped above the floor for the later shower. We
haven't had the taps/mixers replaced as we're doing the bathrooms
later this year, so the bath/shower mixers are still the old 22mm-feed
ones. The plan is to have a shower in the ensuite and a both/shower
in the bathroom.

The cold water pressure is excellent, even through though those bath/
shower mixers. So I can run the cold into the bath and a cold shower
at a good rate for both.

The hot pressure is a lot less than expected. We did expect a drop in
pressure if you were running both, but the current situation is that
running the hot into the bath leads to not enough pressure to keep the
shower on in the ensuite.

If I run the sink tap in the bathroom instead of the bath mixer (my
attempt to simulate drawing off water through a 15mm bath pipe), we do
still get some pressure out of the shower. It's reasonable, although
right at the bottom of my expectations/hopes pre boiler install.

Another plumber that we've had around to quote for kitchen work
expressed surprise at the flow when he saw just the bath turned on.

Any general comments would be appreciated, but I've a few specific
questions (which may or may not be pertinent):

1. 22mm pipes. I'm not sure as chipboard floors are preventing me
easily checking, but I believe that the 15mm hot out of the boiler
probably becomes a 22mm pipe under the 1st floor and then into the
bath mixers. Might that be the problem?

2. Length. I'm pretty sure that the cold supply to the boiler is
fine as the cold tap in the cloakroom (just before the boiler in the
scheme of things) belts out water. The total length of the hot supply
from the boiler to the bath mixers is probably 15m+ (all on ground/
first floors). To give you an idea, the kitchen tap (which is
probably closer to 20m from the boiler in pipe length) takes ~60
seconds to heat up if it hasn't been used for a whilee. Forgetting
it's bore for the moment, might the length of the pipes be an issue
pressure/flow rate wise?

3. Kitchen tap. Whilst we're doing the kitchen, we're having the
ceiling down. We thought it would make sense to run a hot feed from
the boiler the ~5m to the kitchen tap, which will presumably reduce
the time-till-hot to a perfectly fine 10-20 seconds. Might it also
make sense to carry that 15mm feed up into the bathrooms to try to
solve our pressure/flow problem?

It's worth noting that although some plumbers expressed concern at
switching to a combi for the house, British Gas were quite happy to
install effectively the same boiler rebadged. Presumably that
wouldn't be the case if it was a total no-brainer bad idea .

[/bdd]

wrote in message
oups.com...
We've just had a WB 40 CDi combi installed into a house that was
previously a standard boiler/tank/cylinder affair. The short story is
that we can't run a bath and shower off it at the same time (I don't
mean that the pressure drops off a bit on the shower - it actually
stops). Various plumbers and even the WB site itself would indicate
that you can't expect to do everything at once on a combi, but I did
expect more when just two outlets were opened.

Any ideas why this might be and/or what can we do?

Not *that* surprised. Combi's are really designed to run showers. Unless
it's a massive combi, a typical bath flow would take water at the maximum
rate the combi could heat it, leaving no flow left over for the [higher
resistance] shower. The usually reasonable assumption is that baths are
infrequent these days and as long as the combi can run one, the fact that
you can't take a shower at the same time is irrelevant as it won't often be
a problem. Stored water systems have no maximum flow but run out, combis are
flow limited but never run out. For perfect results you need a massive
installation of either! FWIW if you take a lot of deep baths, a combi isn't
for you. If several inhabitants in turn tend to take long showers (ie
teenagers) then a stored water system isn't for you! There are, of course
fancier hybrid systems available which you can look in the archives for.


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)

1. 22mm pipes. I'm not sure as chipboard floors are preventing me
easily checking, but I believe that the 15mm hot out of the boiler
probably becomes a 22mm pipe under the 1st floor and then into the
bath mixers. Might that be the problem?

The size of the pipework is irrelevant. The bigger bore pipes will hold a
greater volume of water (and cause a slightly longer dead-leg of cold water
to come through before hot) but offer less resistance than 15mm.

The problem is that the boiler can't deliver hot water at the rate you
require it, and the limited flow is going to the path of least resistance -
the bath tap.

If you restricted the bath tap a bit (try with it half open) you'd probably
get a bit of flow to the shower - but unlikely enough to mix with cold and
give a decent shower performace.

As Bob said - it's a side effect of a combi. The hot water won't run out but
the trade off is poorer flow than stored HW.

Alan.

On Thu, 26 Apr 2007 15:50:42 +0100, Bob Mannix wrote:

wrote in message
oups.com...
We've just had a WB 40 CDi combi installed into a house that was
previously a standard boiler/tank/cylinder affair. The short story is
that we can't run a bath and shower off it at the same time (I don't
mean that the pressure drops off a bit on the shower - it actually
stops). Various plumbers and even the WB site itself would indicate
that you can't expect to do everything at once on a combi, but I did
expect more when just two outlets were opened.

Any ideas why this might be and/or what can we do?

Not *that* surprised. Combi's are really designed to run showers. Unless
it's a massive combi, a typical bath flow would take water at the maximum
rate the combi could heat it, leaving no flow left over for the [higher
resistance] shower. The usually reasonable assumption is that baths are
infrequent these days and as long as the combi can run one, the fact that
you can't take a shower at the same time is irrelevant as it won't often be
a problem. Stored water systems have no maximum flow but run out, combis are
flow limited but never run out. For perfect results you need a massive
installation of either! FWIW if you take a lot of deep baths, a combi isn't
for you. If several inhabitants in turn tend to take long showers (ie
teenagers) then a stored water system isn't for you! There are, of course
fancier hybrid systems available which you can look in the archives for.

Quite so, at least these days we don't have to keep spelling out that "one
size does NOT fit all".


For a complete understanding you would need to measure a lot of things.

Essentially there is a restriction on the flow of HW which the bath tap
aggravates to a serious extent.

I and others can offer office chair speculations which can be
much more accurate if there are some numbers. Like ...
Standing pressure of mains cold.
Flow rate of cold and hot at kitchen sink.
Flow rate of cold and hot at bath.
Flow rate of cold and hot at shower.

With a 40kW combi you really should be able to run a shower and bath
together.

My best guess without numbers is that the DHW flow through the boiler is
limited (it may have an adjustment) so as to get a reasonable temperature
on the bath taps. The bath taps are probably something that looks pathetic
unless there is a fire hose behind them, (the mixer types with a elongated
oval bell mouth spout are dire at _looking_ effective).
Putting a restriction on the bath HW supply and remving all other
restrictions is the way forward. 15mm pipe is not a restriction unless
kinked.


--
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
Gas Fitting Standards Docs he http://www.makewrite.demon.co.uk/GasFittingStandards

wrote in message
oups.com...
We've just had a WB 40 CDi combi installed into a house that was
previously a standard boiler/tank/cylinder affair. The short story is
that we can't run a bath and shower off it at the same time (I don't
mean that the pressure drops off a bit on the shower - it actually
stops). Various plumbers and even the WB site itself would indicate
that you can't expect to do everything at once on a combi, but I did
expect more when just two outlets were opened.

Any ideas why this might be and/or what can we do?


Before you go any further check the gas rate with the hot water all on full
and compare with the specification for the boiler. Should be recorded on
the Benchmark. Its possible that the DHW cold feed has been restricted
and the gas rate is limiting. With hot water on full what is the indicated
temperature for the hot water?

The problems you are experiencing are typical for a combi unfortunately.

Jim A

Before you go any further check the gas rate with the hot water all on full
and compare with the specification for the boiler. Should be recorded on
the Benchmark.

I can't see a gas flow rate (or anything in m^3/h) on the Benchmark or
the Eurotron "receipt". The tech specs say it should be 4.2 m^3/h.
I've got a Corgi plumber (not the same one) coming around on Monday.
Is it a quick & easy test for him to perform if I give him advance
warning tomorrow?

I know that they did test the gas as Transco came out to replace the ?
regulator? (or something similar on the meter side - I remember some
explanation about how otherwise there was a chance that when the
boiler came on the gas to the hob might cut out).

Its possible that the DHW cold feed has been restricted
and the gas rate is limiting. With hot water on full what is the indicated
temperature for the hot water?

It rose to 62* after running it for a minute or so.

Thanks

On 26 Apr, 17:45, "Alan" wrote:

If you restricted the bath tap a bit (try with it half open) you'd probably
get a bit of flow to the shower - but unlikely enough to mix with cold and
give a decent shower performace.


Thanks for the other info. Might it help if we ran a the hot from the
boiler, via the kitchen sink (never in use at the same time as the
bath &/or shower) to the shower and then the bath? The old spur would
be used for the cloakroom only and capped off there.

The shower would be nearer the boiler (in pipe terms) than the bath
then. Or doesn't pressure work that way?

On 26 Apr, 15:50, "Bob Mannix" wrote:
wrote in message

There are, of course
fancier hybrid systems available which you can look in the archives for.

Without getting into the specifics yet (I've seen some other threads
on the subject ), are we talking about things like pressurised
cylinders and heat banks here?

On 2007-04-26 23:17:02 +0100, said:

On 26 Apr, 15:50, "Bob Mannix" wrote:
wrote in message

There are, of course
fancier hybrid systems available which you can look in the archives for.

Without getting into the specifics yet (I've seen some other threads
on the subject ), are we talking about things like pressurised
cylinders and heat banks here?

Not necessarily. There are storage combis which have an internal
cylinder storing heating water at 80 degrees. This is used to heat the
potable water using a heat exchanger.
The point of this is to be able to deliver a lot of heat quickly to
cold water for a period of time. That time will be short because the
amount of heat exceeds that being produced by the burning of gas in the
boiler.

In essence there are like a packaged boiler and heatbank. They will
deliver a greater flow rate for a period of time but then fall back to
the rate being produced natively by the boiler. Really you have to
work back from the flow rate and temperature you need and the
temperature of cold mains (can be 5 degrees in winter). These will
tell you how much running time you will have at the higher flow rate
before the performance drops off.

A packaged appliance may meet your needs. The challenge becomes the
size which of course is influenced by the amount of HW needed in time T.

Beyond a certain point it goes beyond what will fit into a box the size
of a washing machine and one can then go for a storage system of some
kind being conventional (pressurised or not), heat bank or thermal
store.

wrote in message
ups.com...
Before you go any further check the gas rate with the hot water all on
full
and compare with the specification for the boiler. Should be recorded on
the Benchmark.

I can't see a gas flow rate (or anything in m^3/h) on the Benchmark or
the Eurotron "receipt". The tech specs say it should be 4.2 m^3/h.
I've got a Corgi plumber (not the same one) coming around on Monday.
Is it a quick & easy test for him to perform if I give him advance
warning tomorrow?

I know that they did test the gas as Transco came out to replace the ?
regulator? (or something similar on the meter side - I remember some
explanation about how otherwise there was a chance that when the
boiler came on the gas to the hob might cut out).


Different test. A gas rate test is bread and butter only involves an
eyeball on the meter and a watch. Ask for it specifically.


Its possible that the DHW cold feed has been restricted
and the gas rate is limiting. With hot water on full what is the
indicated
temperature for the hot water?

It rose to 62* after running it for a minute or so.

That's a reasonable maximum but if its 62deg in April by late summer the gas
will *definately* be limiting as the cold feed warms. You could chose a
different setup which gives you more flow at the expense of temperature in
winter but would always be able to get full set temperature by turning down
the hot tap.

Jim A

I posted this last night but it seems to have got lost. Glad I saved
a copy!


On 26 Apr, 18:04, Ed Sirett wrote:

I and others can offer office chair speculations which can be
much more accurate if there are some numbers. Like ...
Standing pressure of mains cold.

Sorry, not sure how to do this one. "It belts out" is the best that I
can do .


Flow rate of cold and hot at kitchen sink. (Ground floor)

Bearing in mind I was using a bucket to measu
COLD 11.5 lts/min
HOT 13.2 lts/min
Benchmark says 14 lts/min, for what it's worth. Not sure where that
was tested.
The kitchen cold is nearest to the stopcock but has never been great;
the kitchen hot furthest from the boiler (and the pipe is kinked).
I'm guessing that someone has fallen onto the tap at some point which
may account for the wierd cold reading (and the hot kink).

Flow rate of cold and hot at bath. (1st floor)
COLD 37.5 lts/min
HOT 12.8 lts/min

Flow rate of cold and hot at shower. (1st floor)
COLD 24 lts/min
HOT 11.5 lts/min
Remember this is actually a bath mixer. The readings on the taps into
the bath we
COLD 33 lts/min
HOT 12 lts/min

I've added a few other measurements that might help:

* Flow rate of hot shower when cold bath running.
COLD BATH 30 lts/min (c.f. 37.5 by itself)
HOT SHOWER 8 lts/min

* Flow rate of hot shower in ensuite when hot sink in bathroom on (hot
sink by itself is 11.8 lts/min).
HOT SHOWER 2 lts/min (measured with a 1/2 lt jug as I couldn't be
bothered to wait the estimated 5 minutes to fill the bucket )

* Flow rate of cold in cloakroom (~2m before hits boiler). (Ground
floor)
COLD 27 lts/min (actually probably higher - I couldn't open the tap
fully without the water escaping the basin)


With a 40kW combi you really should be able to run a shower and bath
together.

Nice to know that we've not necessarily made a huge mistake and that
there's hopefully a solution .

Putting a restriction on the bath HW supply and remving all other
restrictions is the way forward. 15mm pipe is not a restriction unless
kinked.

I take it that a restriction reduces the flow rate to the bath (in
this case) if the unrestricted shower is turned on, leaving more for
the shower. Will a restriction reduce the flow rate to the bath when
no other hot taps are on?

If not then that definitely sounds good to me.


Thanks.

On Fri, 27 Apr 2007 01:16:02 -0700, Bromley86 wrote:

I posted this last night but it seems to have got lost. Glad I saved
a copy!


On 26 Apr, 18:04, Ed Sirett wrote:

I and others can offer office chair speculations which can be
much more accurate if there are some numbers. Like ...
Standing pressure of mains cold.

Sorry, not sure how to do this one. "It belts out" is the best that I
can do .


Flow rate of cold and hot at kitchen sink. (Ground floor)

Bearing in mind I was using a bucket to measu
COLD 11.5 lts/min
HOT 13.2 lts/min
Benchmark says 14 lts/min, for what it's worth. Not sure where that
was tested.
The kitchen cold is nearest to the stopcock but has never been great;
the kitchen hot furthest from the boiler (and the pipe is kinked).
I'm guessing that someone has fallen onto the tap at some point which
may account for the wierd cold reading (and the hot kink).

Flow rate of cold and hot at bath. (1st floor)
COLD 37.5 lts/min
HOT 12.8 lts/min

Flow rate of cold and hot at shower. (1st floor)
COLD 24 lts/min
HOT 11.5 lts/min
Remember this is actually a bath mixer. The readings on the taps into
the bath we
COLD 33 lts/min
HOT 12 lts/min

I've added a few other measurements that might help:

* Flow rate of hot shower when cold bath running.
COLD BATH 30 lts/min (c.f. 37.5 by itself)
HOT SHOWER 8 lts/min

* Flow rate of hot shower in ensuite when hot sink in bathroom on (hot
sink by itself is 11.8 lts/min).
HOT SHOWER 2 lts/min (measured with a 1/2 lt jug as I couldn't be
bothered to wait the estimated 5 minutes to fill the bucket )

* Flow rate of cold in cloakroom (~2m before hits boiler). (Ground
floor)
COLD 27 lts/min (actually probably higher - I couldn't open the tap
fully without the water escaping the basin)


With a 40kW combi you really should be able to run a shower and bath
together.

Nice to know that we've not necessarily made a huge mistake and that
there's hopefully a solution .

Putting a restriction on the bath HW supply and remving all other
restrictions is the way forward. 15mm pipe is not a restriction unless
kinked.

I take it that a restriction reduces the flow rate to the bath (in
this case) if the unrestricted shower is turned on, leaving more for
the shower. Will a restriction reduce the flow rate to the bath when
no other hot taps are on?

If not then that definitely sounds good to me.


Thanks.

Looks like you have an excellent mains supply, somewhat restricted at the
kitchen sink.

Cutting back the flows at the bath and basin cold taps looks like a useful
activity.

My guess is that there is some restriction on the HW supply to the
bath/shower mixer. Twisted flexible? Valve to the mixer not turned on
fully?



--
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
Gas Fitting Standards Docs he http://www.makewrite.demon.co.uk/GasFittingStandards

On Fri, 27 Apr 2007 01:16:02 -0700, Bromley86 wrote:

Flow rate of cold and hot at kitchen sink. (Ground floor)
....
COLD 11.5 lts/min
....
The kitchen cold is nearest to the stopcock but has never been great;

Flow rate of cold and hot at bath. (1st floor)
COLD 37.5 lts/min
HOT 12.8 lts/min

I've added a few other measurements that might help:
* Flow rate of hot shower when cold bath running.
COLD BATH 30 lts/min (c.f. 37.5 by itself)
HOT SHOWER 8 lts/min

Sounds as if the pressure of the cold feed to the boiler is dropping when
cold is drawn off elsewhere. Probably this is a result of taking the
boiler cold feed from whatever happened to be nearby. The low flow from
the kitchen cold tap suggests this too. Providing a better supply to the
boiler may help. Apart from that, as Ed suggests, restricting flow to the
bath hot tap will probably improve the shower's performance. Unfortunately
there's no way to do this without permanently reducing the bath's filling
rate, but there should be a point at which you can get a reasonable rate
for both. There should be a servicing isolation valve on the feed to the
bath taps which you can turn partly off to experiment.