daft questions - plumbing / showers etc

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 ?
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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 ?
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Colin Wilson wrote:

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

If you really want to know, something like:
http://www.screwfix.com/prods/82412/Hand-Tools/Plumbing-Tools/Pressure-Test-Equipment/Monument-Tools-Mains-Water-Pressure-Test-Gauge
Not that it tells you much of interest usually.

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.

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

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

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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Thought it sounded a little suspect...

http://www.screwfix.com/prods/82412/Hand-Tools/Plumbing-Tools/Pressure-Test-Equipment/Monument-Tools-Mains-Water-Pressure-Test-Gauge
Given that i'm only likely to be using it the once, hardly seems worth it :-}

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)

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

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.

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.

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 ?
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Colin Wilson wrote:

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.

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.

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

John.

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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

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 !)
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Colin Wilson wrote:

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/Bathrooms/Showers/Mixer-Showers/Thermostatic/Thermostatic-Dual-Control-Shower-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.

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...that's the only thing that worries me tbh, but if the worst comes to the worst, it would appear to be the shower head that most determines the flow, so swapping to a smaller head might sort any issues out - if not, it'll just dribble like an old guys' john thomas :-}
I think it's fair to say there must be a fair amount of pressure - I can't stop the flow by putting my finger over the end of a tap, it's just that it'll seriously struggle to deliver that quantity :-}

SWMBO doesn't like the olde worlde style mixers, but does like the large spray heads - go figure...
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Colin Wilson wrote:

Hmm, ok how about:
http://www.screwfix.com/prods/69520/Bathrooms/Showers/Mixer-Showers/Thermostatic/Triton-Dew-Thermostatic-Mixer-Shower #
--
Cheers,

John.

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http://www.screwfix.com/prods/69520/Bathrooms/Showers/Mixer-Showers/Thermostatic/Triton-Dew-Thermostatic-Mixer-Shower #
That was already on the shortlist ;-)
I think she's going for 89618 and if it doesn't work, tough on her !
Thanks John :-)
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Colin Wilson wrote:

Even so, you know it will be your fault if it does not work! ;-)
--
Cheers,

John.

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