"John Rumm" wrote in message
...
IMM wrote:

The Mira 415 is the one to go for. Dedicated
to mains pressure systems. It is not thermostatic
and has an integral pressure balancing valve. The combi
provides the constant temperature, so no need for a
thermostat cartridge in

Why do you say the combi provides a
constant temperature?

It has an output thermostat attached that modulates the burner to suit.

Even modulating combis with a maximum
water temp thermostats will vary
temperature in inverse proportion to requested
flow rate (the max temp rising up to the cap
set by the stat at low flow rates)

The temperature stability is not that precise though, but ballpark enough.

So if you have the boiler stat set to limit
at 60 degrees and draw off 8
l/min then the temp will be 60 deg.
If the dishwasher then decides to
take the total hot water flow rate to
15 l/min the temperature will have
to fall.

It will not fall. The burner modulates up to cope. Although some combi's are
better than others in maintaining outlet temperature. Generally the more
expensive the better the control.

How does the pressure balance cope with that?

If you are drawing 8 l/min and then taps are turned on to ramp up to 14
l/min, then the pressure in the hot line will drop. The balancing valve will
drop the cold inlet to the mixer to the same as the hot, preventing
excessive cold being drawn off through the mixer, keeping the temperature
stable.

(I can see how it would work well on
a unvented HW cylinder for example,
because the HW flow rate would not
affect the temperature of the water)

Yes, an unvented cylinder is better in this respect. But, some thermostatic
blending valves will be mandatory on new installations. So, the speed of
reaction of the blending valve may perform like a combi. When the blenders
are mandatory, I would take the hot shower supply direct from cylinder
before any blending valve. Although to conform to the future regs you have
to install something like an Excel mixer with temperature limit setting.