Mira Mode Instructions Manual page 12

Gravity Fed System - The shower MUST be fed from a cold water cistern and hot
water cylinder providing nominally equal pressure.
Gas Heated System - The shower MUST be installed with a gas water heater or
combination boiler of a fully modulating design.
A fully modulating boiler is one where the draw off rate is indirectly controlling the
gas flow to the boiler, producing a relatively constant hot water temperature.
12