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