Two tests that split the problem in half
Test one: the container test
The Consumer Council for Water sets out the benchmark plainly. Ofwat measures the number of properties at risk of pressure below 10 metres head, which is 1 bar at the outside stop valve, at a flow of 9 litres per minute, and CCW notes that should be enough to fill a one gallon container, 4.5 litres, in 30 seconds.
So take a bucket you can measure, fill it from the kitchen cold tap with everything else off, and time it. Under 30 seconds means the supply arriving at the property is not the problem and you should stop thinking about the water company. Well over 30 seconds means something upstream of the tap is restricting it, and that something may be the main, may be your supply pipe, and may be a leak.
Test two: one tap or all of them
Thames Water make the same cut in one sentence: if water is coming out of one tap but nowhere else in the home, it is very likely a problem with the internal plumbing. A single bad tap is a blocked aerator, a perished washer or a partly closed isolation valve under the sink, and none of that needs a leak specialist. Poor pressure everywhere, including the kitchen cold tap, is a supply question.
Before going further, check both stop valves are fully open. The inside stop valve is usually under the kitchen sink and opens anti-clockwise, and the outside stop valve needs to be open too. A stop valve that has been closed a quarter turn after a repair years ago is a surprisingly common cause. If yours is seized or weeping, read stopcock leaking first.
Pressure and flow are not the same thing
People use the two words interchangeably and it causes a lot of misdiagnosis. Pressure is the force pushing the water, measured in bar or in metres of static head. Flow is how much water actually arrives, measured in litres per minute. A property can have perfectly adequate pressure at the boundary and poor flow at the tap, and that combination is the signature of a restriction: a narrow old supply pipe, a partly closed valve, a furred-up section, or a leak bleeding off volume before the water reaches you.
It matters because the two are fixed differently. Genuinely low pressure at the boundary is the water company's to answer for. Adequate pressure with poor flow is almost always something on your side of the outside stop valve, and no amount of complaining to the supplier will change it. The container test measures flow, which is why it is the first thing to do: it tells you which of the two conversations you are about to have.
The same distinction explains the appliance complaints that follow. CCW note that although pressure is normally above the level set by the guaranteed standards, some new heating appliances and showers will not work below a certain pressure, and suggest asking your water company what the normal pressure in your area is and consulting the installer before an appliance is fitted, since it may be possible to adjust it or to fit a booster pump. A combi boiler that will not fire at low flow is not evidence of a leak on its own.
Where your water company stops being responsible
This is the part that decides who pays. The Guaranteed Standards Scheme requires your supplier to maintain seven metres static head, which is 0.7 bar, in the communication pipe, normally measured at the outside stop valve. The communication pipe comes off the main and belongs to the company, and it ends at that stop valve. CCW states the position that follows with no ambiguity: the company has no obligation to meet pressure requirements inside your external private supply pipe, whether shared or not, or inside your property.
Everything from the outside stop valve to your kitchen tap is yours. If the pressure is fine at the boundary and poor at the tap, the loss is happening on pipework you own, and a leak on that pipe is one of the ways it happens. There is more on that boundary in our Thames Water section.
The causes that are not a leak
Most low pressure is mundane. CCW lists the common external causes, and the internal ones are equally ordinary.
| Cause | How it behaves |
|---|
| A burst on the main, repairs or scheduled works | Sudden, affects the street, resolves without you doing anything |
| Your supply has been re-zoned from another source | Permanent step change, neighbours affected too |
| High demand | A clear daily pattern, typically mornings and early evenings |
| A shared supply | Drops when the other property draws water |
| A valve shut down slightly | Dates from the last time anyone worked on the system |
| Frozen external pipework | Cold snap only, returns when it thaws |
| Original half-inch pipework in an older property | Always been like it, worse when a power shower or dishwasher runs |
| An extension added to old pipework | Started when the building work finished |
| Limescale in the aerator, shower head or a flexible hose | One outlet only, improves when the part is descaled or replaced |
| A failed pressure reducing valve or a clogged combi filter | Affects the hot side, or everything downstream of the boiler |
The pattern that is a leak
A leak on a private supply pipe is on CCW's own list of causes, with the note that you will need to get it repaired, though in some cases the water company will do it for you free of charge. It does not look like any of the rows above. It looks like this:
- The pressure has fallen gradually over months rather than changing overnight.
- Your neighbours on the same main are unaffected.
- Your water meter keeps turning with everything off.
- There is a permanently damp, mossy or unusually green strip across a lawn, path or driveway between the boundary and the house.
- Water appears at a manhole, an air brick or the bottom of a wall in dry weather.
- You can hear a faint continuous hiss at the stop valve when nothing is running.
- Your bill has risen without a change in how much water you use, which is covered under water bills.
An underground supply pipe leak rarely surfaces where the leak is. London clay moves water sideways along the line of the trench and it emerges at the first soft spot, which is why guessing costs more than measuring.
What you are owed if it is the company
If pressure falls below 0.7 bar on two occasions, each lasting more than one hour, within a 28-day period, the company must automatically make a Guaranteed Standards Scheme payment. CCW recommends recording the times of each incident over a period, for example two weeks, and reporting them, and you can ask the company to visit and test the pressure at the property.
There are exceptions. CCW lists them: a payment already made to the same customer in the same financial year, cases where it was impractical to identify you as affected and you did not claim within three months of the most recent incident, industrial action, and pressure falling below the standard because of necessary works or a drought.
What we do not do
We do not improve mains pressure, fit booster pumps, or argue with a water company about a re-zoned supply. If the container test says the supply is fine and one tap is slow, you need an hour of an ordinary plumber's time, not a leak survey. Saying that early is cheaper for everyone.
What we do is the case where the pressure loss is happening on your own buried supply pipe. That is located with acoustic listening on the pipe run, correlation between two points where the run allows it, and tracer gas once the pipe is isolated and drained, which will find a pinhole in MDPE under a driveway without excavating the driveway to look for it. Then it is repaired or, where the pipe is old lead or has failed in more than one place, replaced. That is mains and supply pipe repair.
What it costs
Our rate is £150 per hour, fixed before we attend, and if we do not find the leak you do not pay for the time spent looking. Excavation and repair are quoted once we know where the fault is and what the surface above it is. See pricing for how the hours are counted.
