Combination confusion

I would opt for the Excel - but by a small margin in this case ;-)

I am not c "The Mira CombiForce 415 is a pressure balanced shower control which maintains a constant outlet temperature irrespective of changes in inlet pressures as long as the inlet water temperatures remain the same.", then later it says "Hot and cold water supply temperatures MUST remain relatively constant."

That last bits would seem to be critical. With a combi you can't rely on a constant output temperature. You can set its maximum temp with the hot water stat if it has one, but the minimum temp is dictated by flow rate.

So for example someone turning on a cold tap may drop the pressure on both hot and cold (and hence flow through both), the shower will see a reduction in pressure on both inputs and make the assumption that the pressure is still balanced. The boiler however will be providing hotter water (assuming it was not on the thermostatic limit).

A washing machine doing a hot fill will again drop the pressure on hot and cold (since the combi is sourcing its water to heat from the cold supply), so equal pressure drop again, however the output temp of the boiler may fall if you exceeded the flow rate that it can sustain the current temperature at.

In reality the 415 will obviously work better than an uncontrolled shower, it will work better with high power combis, and with combis where the hot water stat is set to a lowish value (e.g. 45 degrees), and better where the property is served by a nice fat incoming supply pipe.

Having said that I have an old Mira non thermostatic and non pressure balanced shower on a 35kW combi and it is usable. Adjustment takes a little bit of practice. SWMBO however opts to go use the new thermostatic shower because it requires less accommodation of quirky behaviour!