its illegal to pump from the mains. Tank only
Generally, the regulations center around drawing more than 12LPM of water through a pipe rather than focusing on pressure. If it was illegal then all these Salamander pumps that are sold to boost water to shower heads and combi boilers would also be illegal. Salamander pumps and others advertise they boost household water pressure by 1.5 bar above current household water pressure so it appears some have mistaken this pressure as part of the regulations.
If you want to boost water to an open tap then it is very likely that the water limit would be exceeded and hence illegal if not used with a breaker tank. (I'm referring to the booster pump that Gardiners sell as well as the one Machine Mart sell.) But with a booster pump being used to boost the pressure for a 4040 setup correctly, I very much doubt that the 12LPM marker would be exceeded. (My 4040 produces 2LPM pure and virtual the same waste at 50psi. So I'm using around 4LPM of tap water. If I boost that I could get 4LPM of pure and 4LPM of waste, still well within the 12LPM limit.)
So when flushing the r/o it would be best practice to switch the booster pump off.
I guess its best to inquire directly from the water board that supplies water in that particular area as to how they enforce the regulations.
The only 'concern' I would envisage is noise. These booster pumps aren't silent. So it would be worth considering a suitable place to situate it so as not to attract attention or annoy the neighbours.
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