[poleman]

Whats the difference in the above membranes  looking to buy a static system and its £100 more for the HF5 is there anything to be gained in having the HF5 

[Nick Wareham]

HF5 is suitable for lower water pressure compared to the HF4.  I think with HF4 you generally need a booster pump, but with a HF5 if your tap pressure is about 50psi or above, it will be ok.

I could be wrong, but thats what I think anyway.