it's unlikely it'll work as you want.
i'm in a similar position and have done a fair bit of reading,
so i may be wrong on some points but to summise..
6m of head equates to roughly 0.6bar best case scenario
mains pressure is ?? (assuming 4 bar..)
would mean the mains is nearly 8 times more pressurised than the hot water,
worst case scenario, the mains cold will force its way up the hot pipe,
out the expansion pipe above the tank and you'll have an overflowing cold water cistern in the loft.
best case, you'll have a temperamental mixer that will be on a razors edge between hot and cold.
The simplest solution you'll get is to take a new cold feed from your cold water cistern and use this along side the hot,
this means the two would be at the same pressure (as they both have the same head)
My cold water tank isn't big enough to do this, so i'm doing it a different way to cure my problem, using a few things.
1) a PRV (pressure reducing valve) on the incoming mains set to about 3 bar, limits incoming pressure so the differental between hot and cold isn't too great.
2) 2 PEV's (pressure equalising valve), one for my basin mixer and one for the feed to the shower, which should bring the cold pressure down to match the hot, it also includes check valves / non return valves to stop the overflowing cold water cistern, these need to be cleaned out periodically so isolation before the PEV as well.
3) a thermostatic shower mixer, just because i don't like being shocked by cold water
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