On 25/03/2013 15:01, David.WE.Roberts wrote:
Static pressure 3.0 bar.

Flow from cold outside tap ~17l/min.

Accepted that there will have to be flow restriction on the cold taps in
the house to prevent loss of hot water pressure.

Just looking at mixer valves.

http://www.screwfix.com/p/triton-tes...ower-flexible-
exposed-chrome/36111

"Water minimum flow rate
For best performance within the specified running pressure range a minimum
flow of 8 litres per minute should be available to both inlets."

In the instructions is says:

Check that the appliance is capable of delivering hot water at a minimum
switch-on flow rate of 3 litres per minute. At flow rates between 3 and
8 litres per minute, the appliance
MUst be capable of raising the water temperature to 52°C (minimum).

Now does this mean that you need a total flow of 16 litres per minute
evenly distributed between the hot and cold?

IME bar mixers are not that "thirsty"... If you look at the graph at the
end of the instructions it certainly suggests that an inlet pressure of
3 bar will yield 18 lpm of output - but that is qualified with "Flow
control fully open, Open outlet - no hose or showerhead connected".

So in reality I would expect it to need at most 10 lpm (and probably a
bit less) of total water flow hot and cold combined. That ought to be
doable on a 24kW combi.

Note you can get some odd interactions with combis, when they are at
their limit power wise - these can introduce some large "dead" zones
where the temperature control does not seem to do much. The boiler will
be able to do around 10 lpm of hot water mid winter when running flat
out - but that will be at about final use temperature, and not the 55
deg that the shower maker is recommending. However asking for less water
will allow it to do it at closer to the suggested 55. (a quick
calculation suggests that it should be able to provide nearly 7 lpm at
55 deg with the inlet water at 5 deg (assuming the 24kW is genuine
output power and not just inlet)

I suspect it will "just work". If you have any problems, then throttling
the shower slightly (or fitting a more restrictive shower head) will fix
em.



--
Cheers,

John.

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