best way is to wait and see you never know what might leak.
but i always get them to read my terms then sign before i start, warning them that any rads,valves,pipework etc that has corrosion may well cause problems during powerflushing.
so if any rads etc do go during powerflushing thats down...
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Cavitation is a general term used to describe the behavior of voids or bubbles in a liquid. Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation and non-inertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid...
http://www.salamanderpumps.co.uk/
you can fit a shower pump your hot is gravity fed so your cold feed needs to be tank fed.
you may need to fit a larger cws tank as well.
if you read they say balanced pressures, a min of 1 bar.
if you have gravity hot at 0.2bar and mains cold at 2.0bar and you fit a pressure equalising valve you will end up with 0.2bar on both supplys.
not enough
http://www.screwfix.com/prods/76597/Plumbing/Brassware/Comap-Full-Bore-Isolating-Valve-Chrome-15mm
or
http://www.screwfix.com/prods/98263/Plumbing/Compression-Fittings/Stop-End-15mm-2Pk
or
http://www.screwfix.com/prods/94176/Plumbing/Push-Fit-Fittings/JG-Speedfit-Stop-End-15mm-2Pk