Looking around, pending the imminent replacement of our bathroom (the
one where I gave up boxing in pipework in July last year)

Screwfix do a shower mixer that looks like an unfeasibly high flow
rate compared to most other bar mixers, their ref: 31693 - apparently
does 8L/min at 0.1bar - does this sound "right" to those who've fitted
them ?

While i'm at it, how the hell do you figure out what your pressure is?

We're on a combi-boiler (WB28SIii), have good cold water pressure, and
at the moment, c/o the kitchen sink, it chucks out 9 litres of hot
water per minute - although I believe from a previous thread you can't
equate throughput with pressure.

I've tried to play safe when ordering taps for the bath / sink by
going for 0.2bar jobbies, but I don't know how low I should be setting
my sights at for a shower mixer valve (i.e. would a 0.5bar be ok ?)

Any advice would be much appreciated :-}

While i'm at it, because of the difference in pressures, would you fit
a pressure equalising valve / pressure reducing valve on the cold ?

Last but not least, and this might sound really dumb :-} but as a
shower fed by a combi won't get any hot water for several seconds, how
does the temperature setting work - does it let the hot flow until it
gets to temp, then let cold through ?

We're on a combi-boiler (WB28SIii), have good cold water pressure, and
at the moment, c/o the kitchen sink, it chucks out 9 litres of hot
water per minute - although I believe from a previous thread you can't
equate throughput with pressure.
I've tried to play safe when ordering taps for the bath / sink by
going for 0.2bar jobbies

FWIW our current bath taps are these - screwfix ref 10532 - says
suitable for high or low pressure systems, but doesn't specify flow /
pressure rates.

The shower off it is certainly better than some electric showers i've
been in (where you can pi$$ faster than the shower dunks water over
you) - but would I be right in thinking that the shower head adds
quite a lot of restriction to the flow, so something like screwfix ref
89618 might be more of a sad dribble than an actual shower ?

Colin Wilson wrote:

Screwfix do a shower mixer that looks like an unfeasibly high flow
rate compared to most other bar mixers, their ref: 31693 - apparently
does 8L/min at 0.1bar - does this sound "right" to those who've fitted
them ?

THe 8L/min I can believe, but at 0.1 bar sounds a bit unlikely. At 1 bar
it may be right.

While i'm at it, how the hell do you figure out what your pressure is?

If you really want to know, something like:

http://www.screwfix.com/prods/82412/...ure-Test-Gauge

Not that it tells you much of interest usually.

We're on a combi-boiler (WB28SIii), have good cold water pressure, and
at the moment, c/o the kitchen sink, it chucks out 9 litres of hot
water per minute - although I believe from a previous thread you can't
equate throughput with pressure.

Not really. There is also a difference in the flow rate the main will
supply (probably well in excess of 9 lpm), and that which the boiler can
heat to a reasonable level.


I've tried to play safe when ordering taps for the bath / sink by
going for 0.2bar jobbies, but I don't know how low I should be setting
my sights at for a shower mixer valve (i.e. would a 0.5bar be ok ?)

Chances are you have mains pressure of at least 1 bar.

Any advice would be much appreciated :-}

While i'm at it, because of the difference in pressures, would you fit
a pressure equalising valve / pressure reducing valve on the cold ?

With a combi the pressure should be equal (roughly) anyway.

Last but not least, and this might sound really dumb :-} but as a
shower fed by a combi won't get any hot water for several seconds, how
does the temperature setting work - does it let the hot flow until it
gets to temp, then let cold through ?

Some wax capsule types allow hot to flow until the output mix reaches a
limit temperature, and then they progressively constrict the flow of the
hot. The valve normally being such that you can't select just hot - you
must also have some cold. Hence throttling the hot supply cools the mix.
Depending on the flow rate you are demanding from the combi, you can get
a self defeating mechanism that comes into play here and slows the
reaction time of the mixer. Say you boiler can heat 10 lpm to 45
degrees, but you have the boiler limit stat set to 60 degrees, you may
find the combi responds to the reduction in flow imposed by the mixer by
simply allowing it to get hotter, just negating the change made by the
mixer. A mixer that responds to a rise in temp by increasing the cold
flow rate works better in these cases.


--
Cheers,

John.

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

The 8L/min I can believe, but at 0.1 bar sounds a bit unlikely. At 1 bar
it may be right.

Thought it sounded a little suspect...

While i'm at it, how the hell do you figure out what your pressure is?
If you really want to know, something like:
http://www.screwfix.com/prods/82412/...ure-Test-Gauge
Not that it tells you much of interest usually.

Given that i'm only likely to be using it the once, hardly seems worth
it :-}

Not really. There is also a difference in the flow rate the main will
supply (probably well in excess of 9 lpm), and that which the boiler can
heat to a reasonable level.

I'd hazard a wild guess at saying the cold water can deliver at LEAST
twice as much per minute than the hot, perhaps as much as four times
(without getting jugs out of the cupboard to measure it)

I've tried to play safe when ordering taps for the bath / sink by
going for 0.2bar jobbies, but I don't know how low I should be setting
my sights at for a shower mixer valve (i.e. would a 0.5bar be ok ?)
Chances are you have mains pressure of at least 1 bar.

....wonder how well one of the large shower heads will cope... missus
likes them...

With a combi the pressure should be equal (roughly) anyway.

Nah, nowhere near - either that, or i'm confusing pressure with flow
rate again - it's a downstairs bathroom, and the main comes in about 6
feet from the bath taps, and the cold tap is plumbed direct to it
(give or take a tee for basin and bog) - the boiler is almost directly
above, but on the first floor.

Some wax capsule types allow hot to flow until the output mix reaches a
limit temperature, and then they progressively constrict the flow of the
hot. The valve normally being such that you can't select just hot - you
must also have some cold. Hence throttling the hot supply cools the mix.

I would have thought it'd go the other way around - it'd let the hot
flow at full force, and the cold would be adjusted to keep within the
temperature range.

A mixer that responds to a rise in temp by increasing the cold flow rate
works better in these cases.

Aha - I think that's what I was babbling on about in the previous
sentence - any ideas how you can tell one from the other ?

Colin Wilson wrote:

I've tried to play safe when ordering taps for the bath / sink by
going for 0.2bar jobbies, but I don't know how low I should be setting
my sights at for a shower mixer valve (i.e. would a 0.5bar be ok ?)
Chances are you have mains pressure of at least 1 bar.

...wonder how well one of the large shower heads will cope... missus
likes them...

The large drencher heads can often swallow quite substantial flow rates
( 15 lpm) and can be out of reach for many basic combi boilers.

With a combi the pressure should be equal (roughly) anyway.

Nah, nowhere near - either that, or i'm confusing pressure with flow
rate again - it's a downstairs bathroom, and the main comes in about 6
feet from the bath taps, and the cold tap is plumbed direct to it
(give or take a tee for basin and bog) - the boiler is almost directly
above, but on the first floor.

It may be that either your combi is one of those that regulates its
output flow rate - which in in some ways is good since it limits the
temperature swing, or, it has been fitted with a pressure reduction
valve on its inlet.

With my one for example, it places no restriction on the water flow rate
through the boiler (apart from the extra flow resistance introduced
through the plate heat exchanger) - you can run the "hot" as fast as the
cold, however the water won't be very hot at that flow rate.


A mixer that responds to a rise in temp by increasing the cold flow rate
works better in these cases.

Aha - I think that's what I was babbling on about in the previous
sentence - any ideas how you can tell one from the other ?

Some explicitly state they are suitable for combis and multipoint heaters.

--
Cheers,

John.

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

It may be that either your combi is one of those that regulates its
output flow rate - which in in some ways is good since it limits the
temperature swing, or, it has been fitted with a pressure reduction
valve on its inlet.

I think it has both from the manual

I remembered I scanned them for Ed's collection a while ago, and at
best, it looks like it might do 12.9L/min at a rise of 30 degrees or
7L/min at 57 degrees rise

A mixer that responds to a rise in temp by increasing the cold flow rate
works better in these cases.
Aha - I think that's what I was babbling on about in the previous
sentence - any ideas how you can tell one from the other ?
Some explicitly state they are suitable for combis and multipoint heaters.

Thanks for the replies John, appreciated :-)

I think we might be going for the 89618 :-} (if the flow really is
crap, we can just use the handset over the winter months, or just sit
in the bath !)

Colin Wilson wrote:

I think we might be going for the 89618 :-} (if the flow really is
crap, we can just use the handset over the winter months, or just sit
in the bath !)

That can swallow 16 lpm at 0.5 bar[1], which your boiler will not cope
with (well not unless you like cold showers). It might be that you can
turn off the body jets which will reduce the demand a bit.

[1] Note that you may well have more than that and hence it will take
even more flow.

If you like that style, then something like:

http://www.screwfix.com/prods/91420/...ower-Fixed-Kit

sounds more like it. If you look at its manual it also comes with flow
limiter discs that can be used to adjust the flow rate.

--
Cheers,

John.

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