JT expressed precisely :
Could there have been something else during the construction that may
have affected the water flow? Or what else can I try to find the
bottleneck?

Could the problem be outside your property?

Our pressure sometimes varies between two levels, we put it down to the
water authority turning pumps on and off at the local water tower.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org