Water pressure dropping when a tap runs: what it can mean
Quick answer
If pressure noticeably drops at one tap or fixture when another is turned on elsewhere, this is often about how flow is shared across your plumbing, rather than a fault with the mains supply itself. It’s especially common with showers. Check whether it happens with specific combinations of fixtures. If pressure drops severely or affects the whole house even with nothing else running, contact a plumber.
Safety first
- If you’re dealing with an electric shower, don’t attempt any work on it yourself.
- A sudden, dramatic pressure drop across the whole house is worth checking for signs of a leak rather than assuming it’s this issue.
Full checklist: Safety guidance
What you may notice
- Pressure drops at one tap specifically when another tap or the toilet is used — may suggest normal flow-sharing behaviour across your plumbing, especially common in older systems.
- The drop is significant and affects your shower’s comfort — this is a common and recognised pattern, particularly with non-thermostatic showers or combi boilers under high simultaneous demand.
- Pressure is low even with nothing else running anywhere in the house — this points to a different, more general pressure issue — see our guide on low water pressure in the house.
These are possible explanations, not a confirmed diagnosis. An engineer can confirm the exact cause on inspection.
Safe checks to make, in order
- Test the pattern deliberately. Run the affected tap or shower, then have someone else turn on another tap or flush the toilet, and note what happens.
- Check whether this happens with every combination of fixtures, or just specific ones. This can help identify whether it’s a general flow-sharing pattern or specific to certain pipework.
- Check whether the issue also happens when nothing else is running. If pressure is fine on its own but drops only when shared, that’s a different situation from pressure being consistently low.
What you can do safely
- Testing the pattern deliberately with two fixtures at once.
- Noting which specific combinations trigger the drop.
- Checking whether pressure is normal when nothing else is running.
- Sharing these observations with a plumber if the pattern is disruptive.
When to call a professional
- The drop is severe enough to make showering uncomfortable or impractical.
- It happens with every combination of fixtures, suggesting a wider flow limitation.
- Pressure is also low even without another fixture running.
- You’re considering a solution like a thermostatic shower valve or pressure improvements and want advice.
What to tell the engineer
- Which specific fixture combinations trigger the drop.
- How severe the drop is, and whether it’s a minor dip or a major change.
- Whether pressure is normal when only one fixture is running.
- Whether you have a combi boiler, system boiler, or another setup, if known.
FAQs
Is it normal for pressure to change when another tap is used?
Some degree of change is fairly common, especially in homes without a pressure-independent supply setup. A significant, disruptive drop — particularly affecting showers — is worth discussing with a plumber even if it’s a known pattern.
Would a thermostatic shower valve help with this?
A thermostatic valve helps maintain a stable temperature despite pressure changes, though it doesn’t necessarily prevent the pressure drop itself. A plumber can advise on whether this or another solution suits your setup.
Could my combi boiler be the cause if it’s specifically hot water that drops?
Yes, combi boilers have a maximum flow rate, so high simultaneous hot water demand can affect performance. This is different from a mains pressure issue and worth mentioning to an engineer.
Is this the same as low mains pressure?
Not quite — low mains pressure tends to affect things consistently, while this pattern is specifically about pressure changing when fixtures are used together. See our guide on low mains water pressure if your issue is more constant.
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