Three checks
Three checks beforeyou call anyone.
Three checks narrow a whole building down to one point.
01
One tap on its own
Close everything, then open one tap. A weak opening among strong ones is the fault.
02
Compare top with ground
Open the same tap on ground and top floor. Strong below, weak above, means the pump or riser.
03
Sudden or gradual
Remember if it started suddenly, or slid down slowly. That separates a forgotten valve from a real shortfall.
Where it comes from
Pressure is made inthe building.
Water fills a tank, and a pump lifts it and shares it between the floors. So what leaves your tap is pressure the building made, not pressure the street itself.
Anything eating into it sits on that route: the tank, the pump, the riser, the valves, then the tap last. So the question is not why it is weak, but where.
The first split
One tap, or thewhole house.
Open one tap at a time with everything else shut. One weak opening among strong ones is local: a filter, a half-shut angle valve, or a kinked tail behind it.
Weak everywhere points to a shared cause: the tank, the pump, the valve after them. Height eats pressure too, so the top floor shows a shortfall first when other floors run.
The last split
A forgotten valve,or a real shortfall.
A valve left half closed feels exactly like a narrowed pipe. It gets shut for heater work, a tank clean or a bathroom repair, then comes back open only halfway.
The story gives it away: sudden, soon after work on one branch, points to a valve. A real shortfall slides down slowly, with no event, on the same openings every time.
On weak flow
Questions that comewith weak flow.
Walk the valves from the tank to the branch. A lever valve is open along the pipe, closed across it.
Air has entered the line, usually from a low tank or a pump drawing it in. If it returns each start, check the suction side.
That is a different case, nothing failed. The installation never suited the points, or the run is long. The decision is pipework or pump.