Why does my water pressure drop when someone else runs a tap?
What simultaneous demand actually does to a plumbing system, why some houses cope and others don't, and the fixes that work at each point in the chain.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 4 min
Short answer
Every outlet running at once shares the same finite flow, and the restriction is usually the supply pipe from the street or an undersized internal run. It is a flow problem, not a pressure problem. Bigger pipework, an accumulator, or a thermostatic shower valve that holds temperature through the dip are the three real answers.
This is the most complained-about behaviour in UK plumbing: the shower dies when someone flushes, the kitchen tap fades when the washing machine fills. It is not a fault in the sense that something is broken. It is your system telling you how much water it can move, and where the bottleneck is.
Step by step
- Work out whether it is pressure or flow.If the water still comes out with the same force but less of it, that is flow. If it goes soft and weak, pressure has dropped too. On a mains-fed house both usually fall together because the pressure drop is caused by the flow through a restriction.
- Identify the shared restriction.Everything downstream of a restriction shares what gets through it. The usual candidates in order: the buried supply pipe from the street, the first length of internal pipe before it branches, and any single 15mm run feeding several outlets that should have been 22mm.
- Test which branch is the bottleneck.Run the kitchen cold tap and measure the flow. Then run it with a bath tap open as well and measure again. If the total of the two is roughly the same as the single figure, you are at the supply limit. If the total is much higher, the individual branches are the restriction.
- Check the obvious restrictions first.Stopcock not fully open, an isolating valve part closed, a scaled tap aerator, a partly blocked filter. These cost nothing to check and account for a surprising number of cases.
- Consider the shower valve rather than the plumbing.A thermostatic mixer holds the temperature when the cold side dips, which removes the scalding and the shock even though the flow still falls. On an old manual mixer that dip is felt as a burst of hot water and is genuinely dangerous with children in the house.
- Look at pipe sizing before you look at pumps.Running a 22mm main run through the house with 15mm branches to each outlet is what modern practice does, and it is why new houses cope better. Re-piping a run is often cheaper and always more effective than a pump.
- Understand what an accumulator does.It stores mains water under pressure and releases it during peak demand, evening out short simultaneous draws. It cannot increase your average flow but it can absorb the three minutes when two showers run. It needs space and it is a proper installation, not a gadget.
- Know the tank-fed exception.On a vented gravity system the cold taps and the hot cylinder both come from the same loft tank, so simultaneous demand shows differently — the tank cannot refill fast enough and everything fades. Here a pump genuinely does help, because pumping from a tank is permitted where pumping off the main is not.
Tips
- Measure the flow at the kitchen cold tap on its own and then with a second outlet open. That single comparison tells you whether to spend money on the supply or on internal pipework.
- A house with several bathrooms and a 15mm supply pipe from the street will never behave, whatever is fitted inside it.
- If the drop is severe and recent, check the stopcock has not been knocked partly closed.
- Water companies will test the pressure and flow at your boundary, often free. That splits the problem cleanly into theirs and yours.
If it doesn't work
Shower goes hot when a cold tap is opened
Cause: Cold side pressure drops so the mixer delivers proportionally more hot — Fix: A thermostatic shower valve solves this by design. It is the single best fix and it is a scald-safety improvement, not just a comfort one.
Shower goes cold when a hot tap is opened
Cause: A combi diverting flow, or hot side restriction — Fix: On a combi it is a flow limit at the boiler; reduce the shower flow rate or accept that two hot outlets is beyond the boiler. On stored hot water it points at hot side pipe sizing.
Everything fades when the washing machine fills
Cause: The machine's fill valve is taking a significant share of a limited supply — Fix: Nothing is broken. Running the machine outside shower times is the free answer; larger supply pipework is the real one.
Upstairs dies completely when a downstairs tap runs
Cause: Very low residual pressure upstairs, so any drop takes it below what the outlet needs — Fix: Common on a gravity system with little head. A pump on the tank-fed hot and cold, or converting the system, are the options.
Questions people ask
Is this the water company's fault?
Only if the pressure and flow at your boundary are below what they are required to supply. Ask them to test. In most cases the boundary figures are fine and the restriction is the buried supply pipe or the internal pipework, both of which are yours.
Will a pump fix it?
On a tank-fed system, often yes. On a mains-fed system, a pump directly on the incoming main is not permitted and will not help anyway — you cannot pump water that is not arriving. An accumulator is the mains-fed equivalent.
Why is it worse in the evening?
Neighbourhood demand. Everyone is home, and the main is delivering to more properties at once. Your worst-case flow is an evening figure, not a mid-morning one.