Why Does My Shower Lose Pressure When Someone Turns on a Tap?

There's an important distinction here. A shower that is always weak is one problem. A shower that is perfectly acceptable until someone turns on a tap, flushes a toilet or starts the washing machine is a different one — and the fix is usually different too.

What that specific symptom tells you is that your property doesn't have enough flow to satisfy two outlets at once. Not pressure exactly — flow rate. The two get conflated constantly, and the distinction is the whole diagnosis.

Pressure is the force behind the water, measured in bar. Flow rate is how many litres per minute actually arrive, and it's limited by the narrowest, most restrictive part of the path.

You can have good pressure and poor flow — a pinched or undersized pipe will happily hold pressure with everything closed, then collapse the moment two outlets open. That's exactly what you're experiencing when the shower drops away as the kitchen tap runs.

Measuring it is simple and worth doing: put a bucket under the fully open cold kitchen tap for six seconds, measure what you caught, and multiply by ten for litres per minute. Under about 12 litres per minute, your incoming supply is on the low side for running two outlets comfortably.

If your home is fed directly off the mains — as most combi-boiler and unvented-cylinder properties are — every outlet shares one incoming pipe. Older properties in Kingston and the surrounding area often still have a narrow supply pipe from the street, and a 15mm or older lead supply simply can't deliver enough litres per minute for a shower plus a tap.

Partially closed valves are worth ruling out first, and frequently are the answer. Check your internal stopcock is fully open (they're often left half-shut after work), and check the external stop tap in the pavement box. Old gate-valve stopcocks also seize partially open and restrict flow badly.

A shower fed by a long run of 15mm pipe that also serves the basin and the toilet will always suffer when another outlet opens. Good practice is a larger-bore distribution run with branches taken off close to each outlet; many older or DIY-extended systems instead daisy-chain outlets off one small pipe.

Scale narrows things further. In a hard-water area, decades of limescale can meaningfully reduce the bore of old copper and, more commonly, block the shower's own inlet filters and the flow restrictor in the head. Cleaning those costs nothing and occasionally solves a surprising amount.

Combi boiler: the boiler heats water on demand from the mains, and its output is rated in litres per minute. Open a second outlet and the flow through the boiler drops, so it modulates down — the shower goes weak and can run cool. This is a characteristic of combis, not a fault, but it can be improved with correct sizing or a change of system.

Unvented cylinder (Megaflo and similar): mains-pressure stored hot water, which generally handles simultaneous demand much better — unless the incoming supply itself is the bottleneck, or the cylinder's pressure-reducing valve has drifted or failed.

Gravity-fed (tank in the loft, cylinder in the airing cupboard): pressure depends on the height of the tank above the shower. On a first-floor shower with a loft tank just above it, there's very little head, so any additional draw-off is immediately felt. This is the classic case for a shower pump.

Thermostatic mixer valves are designed to shut down the hot side if the cold pressure drops, to prevent scalding. So a cold-side pressure drop when the kitchen tap opens can cause the shower to reduce dramatically or cut back — the valve is doing its job, and the underlying cause is still the flow imbalance.

Also in this category: partially closed service valves on the shower feeds, failed pressure-reducing valves, and cheap mixer cartridges that scale up and throttle the flow.

The diagnosis is systematic rather than guesswork. We gauge the static and dynamic pressure at the stopcock, measure flow rate at several outlets both alone and simultaneously, check the supply pipe material and bore where it enters the property, inspect the stopcock and any PRVs, and then isolate whether the restriction is upstream of the house, in the distribution pipework, or at the shower itself.

That determines the remedy — which might be as small as opening a valve properly or descaling a cartridge, or as significant as upsizing the incoming supply, re-running the shower feed in larger bore, fitting an accumulator, or installing a pump on a gravity system. Our water pressure problems service exists precisely because the wrong fix here is expensive.

Maxfix diagnoses shower and water pressure problems across Kingston upon Thames, Surbiton, New Malden and Richmond. See our shower repairs page, call 020 3488 0912, or request a free quote for honest plumbing services in Kingston.