Water Pressure Problems and Their Causes
Complaints about water pressure usually concern flow — how much water arrives per minute — rather than pressure itself. The distinction matters because the causes differ.
Establishing what the problem is
Run a single tap and time how long it takes to fill a container of known volume. That gives flow rate.
Compare it at different outlets. If one is poor and others are fine, the problem is local. If all are poor, it is the incoming supply or the system.
Compare at different times of day, since mains pressure varies with local demand.
Common local causes
Blocked aerator on a tap, which is the most common and takes minutes to clean. Limescale and debris accumulate in the mesh.
Blocked shower head, similarly resolved by descaling.
Partially closed isolation valve under a basin or behind an appliance, frequently left that way after previous work.
A failed thermostatic cartridge in a shower valve.
System causes
In a gravity-fed system with a tank in the loft, pressure depends on the height difference between tank and outlet. Upstairs showers are consequently weak, and a pump is the usual remedy.
In a combi system, flow rate is limited by what the boiler can heat. A boiler with a modest hot water flow rate will not run two outlets simultaneously regardless of mains pressure.
In an unvented cylinder system, check the pressure reducing valve and expansion vessel, both of which fail over time.
Old galvanised pipework corrodes internally and progressively restricts flow, which develops gradually and is easy to miss.
Mains supply causes
A partially closed stopcock, either yours or the external one, is a frequent and simple cause.
A shared supply pipe serving several properties, or a small-diameter service pipe, limits what can be delivered.
Old lead or narrow-bore service pipes restrict flow and may warrant replacement, which the water supplier can advise on and which sometimes attracts assistance.
Leaks between the main and the property reduce flow and waste water. A sudden unexplained increase in usage suggests one.
Remedies
Clean aerators and shower heads first, since these resolve a large proportion of complaints at no cost.
Check all isolation valves and the stopcock are fully open.
For gravity systems, a shower pump or a change to a pressurised system.
For genuinely poor mains supply, the water supplier will test pressure at the boundary and is obliged to deliver a minimum standard.
Get a plumber to test before installing pumps or replacing appliances, since fitting a pump to a system with a restricted supply achieves nothing.
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