As per posts above - they want a mini EV, to take up any expansion / water hammer effects on the cold mains/DHW circuit, likely due to customer being on water meter.
Water meter installs usually (always?) include a check valve in the circuit - therefore WB dont want high static pressure caused by suddenly closing the tap to blast out a seal from the plastic waterblock.
Water meter installs usually (always?) include a check valve in the circuit - therefore WB dont want high static pressure caused by suddenly closing the tap to blast out a seal from the plastic waterblock.
From Greenstar CDI manual
Where the mains water supply has a non-return, back flow prevention valve fitted, a mini expansion vessel must be connected to the mains water inlet pipe between the non-return valve and the boiler.