Split the problem before troubleshooting it: is it every tap, or just one? Weak flow at a single tap is almost always local — a clogged aerator or a kinked supply line. Weak flow throughout the whole house points upstream, to the main stopcock, a pressure-reducing valve, or the incoming supply itself. This guide covers the whole-house case.
Whole house or one tap?
Test two or three taps in different parts of the house, including one on each floor if you have more than one storey. If only one is weak, the cause is local to that fixture — check its aerator and supply line individually. If they are all weak, something upstream of the whole system is restricting flow, and that is what this guide covers.
Common causes
| Cause | How likely | Typical sign |
|---|---|---|
| Main stopcock not fully open | Very common | Often after other plumbing work nearby |
| Partially closed isolation valve somewhere in the run | Common | Especially after any recent plumbing work |
| Failing or poorly set pressure-reducing valve | Common | Gradual decline, or pressure that fluctuates |
| Scale build-up inside old pipework | Common | Gradual decline over years in a hard water area |
| Water heater restricting flow (hot taps only) | Occasional | Cold taps normal, hot taps weak |
| Utility-side low pressure | Occasional | Neighbours report the same issue |
| Leak elsewhere in the system | Occasional | Pressure drop plus an unexplained rise in water usage |
| Undersized supply pipe for the property | Uncommon | Long-standing issue, present since move-in |
How to work through it
- Check the main stopcock is fully open
Find it, usually under the kitchen sink or where the supply enters the property, and confirm it is turned fully counter-clockwise (open). A stopcock nudged partly shut during other work is a surprisingly common and completely free fix.
- Check any accessible isolation valves
If a plumber, or you, worked on any part of the system recently, check the valves involved in that work are fully open. This is the first thing to suspect after any plumbing job.
- Test hot and cold separately
If cold water pressure is fine but hot water is weak throughout the house, the restriction is at or after the water heater rather than in the main supply — see our water heater guides for that specific case.
- Check for a leak
Turn off every tap and appliance, then watch your water meter for a few minutes. If it is still moving, water is escaping somewhere, and the resulting pressure drop plus wasted water both point at a leak needing to be traced.
- Ask a neighbour
If a neighbour on the same supply line has normal pressure, the problem is specific to your property’s plumbing. If they have the same issue, it is likely a utility-side supply problem, and worth reporting to your water provider rather than working on your own pipework.
- Consider the age of the pipework
In older properties with galvanised steel pipes, decades of mineral scale build-up narrows the internal diameter gradually until flow is meaningfully restricted. This shows as a slow decline over years rather than a sudden drop, and the fix is replacing the affected sections rather than anything you can clear from the outside.
The pressure-reducing valve
Many homes, particularly on higher-pressure municipal supplies, have a pressure-reducing valve (PRV) fitted where the main supply enters the property. These fail gradually rather than suddenly, and a failing PRV is a common, easily overlooked cause of whole-house pressure that has quietly declined over months. Some PRVs have an adjustment screw, but altering it without a pressure gauge to check the result is guesswork — this is one of the few plumbing jobs genuinely better handed to a professional with the right equipment, since setting it too high risks damaging appliances and fittings throughout the house.
When to call a plumber
Call a plumber if the stopcock and any recently touched valves are confirmed open but pressure is still low, if you suspect a hidden leak after the meter test, if the pressure-reducing valve needs adjusting or has failed, or if old pipework is the likely cause — tracing and replacing scaled sections is not a straightforward DIY job. Also contact your water utility directly if a neighbour reports the same low pressure, since that points outside your own property altogether.
Stop and call a qualified appliance technician or electrician if you find scorched or melted wiring, smell burning, see water that has reached electrical components, or if the fault returns immediately after a repair. Never bypass a safety device — thermal fuses, door interlocks and flood switches fail for a reason, and defeating one converts a broken appliance into a fire or flood risk.
