It becomes a problem when the pressure arriving upstairs is close to the minimum a fitting needs to work properly. Some fixtures, particularly certain instantaneous hot water units and some mixer designs, have a minimum operating pressure. Below it they behave erratically rather than simply weakly.
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For a burst pipe or a serious leak, turn the water off at the mains tap before you call. If water is anywhere near power points, the switchboard or light fittings, stay clear and treat it as an electrical hazard. Do not enter standing water in a room with live power.
