A pressure test is usually the first real measurement in a leak investigation. Before anyone points a thermal camera at a wall or listens at a floor, it helps to know for certain that a pipe is losing water, and which pipe it is. A pressure test answers that. It does not, on its own, tell you where the leak is, and knowing the difference saves a lot of wasted effort.
This guide explains what a drop test measures, how the cold, hot and heating circuits are tested separately, how isolating sections of pipe narrows the search, and the things a gauge simply cannot show.
What does a pressure test on water pipes actually show?
The principle is simple. A section of pipework is filled with water, brought up to a set pressure, then sealed off from any supply. A gauge is watched over a period of time. If the pipework is sound, the pressure holds. If water is escaping somewhere in that section, the pressure falls. That fall is why it is often called a drop test.
So a pressure test gives you a yes or no answer for one defined section:
- Pressure holds: that section is very unlikely to be the source, and the search moves elsewhere.
- Pressure falls: something in that section is leaking, whether it is a pipe, a joint, a valve or an appliance connected to it.
How fast the pressure falls gives a rough idea of how big the leak is. A gauge that drops noticeably in a few minutes suggests a significant escape. One that creeps down over an hour suggests a weep. Neither tells you the position of the leak along the pipe.
How is a pressure test done?
The details vary with the system, but a leak detection pressure test on domestic pipework generally follows these steps.
- Decide what is being tested. The engineer agrees which circuit is under suspicion: the cold mains, the hot water, the heating, or a single branch.
- Isolate it. The section is shut off from its supply and from anything that would mask a leak, such as a storage tank, a cylinder or an open tap.
- Connect the test equipment. A test pump and a calibrated gauge are connected at a convenient point, often a drain-off cock, an appliance valve or a disconnected fitting.
- Fill and bleed. Air trapped in the pipes compresses and makes readings unreliable, so the section is filled and air is released where possible.
- Pressurise and seal. The pump brings the section up to the chosen test pressure and is then isolated from it.
- Record. The starting pressure and time are noted, and the gauge is read again at intervals.
- Interpret. A stable reading clears the section. A falling reading marks it for further investigation.
On new installations, the water regulations guidance used by UK water suppliers describes a commissioning test for rigid pipework at 1.5 times the maximum working pressure, held for an hour without loss. Plastic pipe has its own procedure, because plastic stretches slightly under pressure and the gauge can fall for a while even when nothing is leaking. A leak investigation on an older system is a different job. The engineer chooses a pressure that tests the pipework fairly without putting ageing joints under more strain than they see in normal use.
How are cold, hot and heating circuits tested separately?
Most homes have at least three separate pipe systems, and a leak on one tells you nothing about the others. Testing them one at a time is what makes the result useful.
| Circuit | How it is usually isolated | Points to watch |
|---|---|---|
| Cold mains | Internal stopcock closed, test rig connected downstream | A meter test first shows whether the supply side is losing water at all. A stopcock that lets water past spoils the test. |
| Hot water from a combi boiler | Tested along with the cold mains, or split at isolation valves | The hot pipes share the mains pressure, so they may need separating from the cold side to see which is leaking. |
| Hot water from a cylinder | Valves either side of the cylinder, or the hot pipework isolated from it | An unvented cylinder has safety valves that must not be tampered with. Its controls are for a qualified installer. |
| Sealed central heating | Boiler off and cool, system held at its own pressure or tested with a rig | The boiler gauge already acts as a rough test. The pressure should be read with the system cold, as it rises when hot. |
| Open-vented heating | Feed and expansion tank isolated before any pressure is applied | The system is open to the loft tank, so it cannot be pressurised without isolating that tank first. |
| Underfloor heating loops | Loops closed at the manifold and tested one at a time | Each loop is judged on its own, as a slow fall on one loop is easy to miss when they are tested together. |
If you are dealing with a sealed heating system that keeps needing topping up, our guide to a boiler that keeps losing pressure explains the checks worth doing before anyone connects a test pump.
How does isolating sections narrow down the leak?
Once one circuit has failed a test, the next step is to split it. Every closed valve divides the pipework into a part that is still under test and a part that is not. If the pressure now holds, the leak is in the part you shut off. If it still falls, the leak is in the part that remains.
In practice this uses whatever the system offers:
- Service valves on toilets, basins, appliances and showers, which remove each outlet from the test in turn.
- Radiator valves, which allow individual radiators to be taken out of a heating test.
- Zone or floor valves in larger homes and flats, which separate whole areas.
- Caps or test plugs fitted at a disconnected joint, where there is no valve in a useful place.
Each split roughly halves the pipework still in question. After a few rounds, a leak somewhere in the house becomes a leak on one branch, perhaps a few metres long. That is a far easier target for the location methods that follow.
Isolation has one catch: valves themselves can leak internally. A valve that does not close fully lets pressure through, and the result points the wrong way. An experienced engineer confirms that each valve is actually holding before relying on it.
Can you pressure test pipes for leaks yourself?
You can do the two simplest versions without any equipment.
- A meter test checks the cold mains. Turn off every tap and appliance, make sure no cistern is refilling, read the meter, wait at least half an hour without using water and read it again. Movement means water is going somewhere. Our guide to unexplained high water usage covers this step by step.
- A boiler gauge check tests a sealed heating system. Note the pressure when the system is cold, then check it at the same time each day. A steady fall with nothing visibly dripping points to a leak.
Hiring or buying a test pump is possible, but it is worth caution. Over-pressurising old pipework can open up a weak joint that was holding, which turns a weep into a leak. Heating systems contain inhibitor and may need a proper refill afterwards. And anything connected to an unvented cylinder should be left to someone qualified for that work.
What can a pressure test not tell you?
A pressure test is a strong tool with clear limits. It will not:
- Find the position of the leak. It confirms that a section is losing water, not where along that section.
- Test waste pipes in the usual way. Waste and soil pipes are not under pressure, so a pressure test is the wrong tool. They are checked with flow tests, dye or a camera instead.
- Catch leaks that only appear under different conditions. Some joints leak only when hot, when the pipe expands, or only when water is flowing through an outlet. A static cold test can miss them.
- Ignore temperature changes. Water warming or cooling during the test changes the pressure slightly, which is why readings are taken with care and over a sensible period.
- Stay accurate with air in the pipes. Trapped air makes the gauge slow to respond and can hide a small leak.
- Separate the leak from a passing valve. Unless each valve is checked, a valve letting water through can look like a leak.
- Tell you anything about rain, condensation or damp. If the section holds pressure and the wall is still wet, the water may not be coming from a pipe at all.
What happens after a pressure test shows a drop?
With the leaking section identified, the engineer chooses a method to pinpoint it:
- Thermal imaging traces hot and heating pipes and shows warm or cool patches where water is escaping.
- Acoustic listening picks up the sound of water escaping from a pressurised pipe, which works best in a quiet property.
- Tracer gas is pushed into the drained pipe and detected where it escapes through the floor or wall. Our guide to how tracer gas detection works explains the method.
- Moisture mapping shows how far the water has spread, which matters for the repair and for any insurance claim.
The pressure test is then often repeated after the repair, so everyone can see the section now holds.
When should you call a leak detection specialist?
A plumber can test and repair pipework that is exposed. A specialist is worth calling when:
- your meter test or boiler gauge confirms a loss but nothing is visibly leaking;
- the suspect pipework is buried in screed, chased into walls or under a fixed floor;
- you have a system with several circuits and no clear idea which one is at fault;
- you need the leak located precisely before anything is opened up;
- you need a written record of the tests for an insurer or landlord.
Pressure testing is part of our water leak detection surveys. A combined detection package is £250 to £500, and a trace and access survey is £350 to £450. Call 020 3912 0200 to talk it through.
Frequently asked questions
How long should a water pipe pressure test last?
For commissioning new rigid pipework, the guidance used by UK water suppliers describes holding the test pressure for an hour. In a leak investigation the engineer runs the test long enough to see a clear result, which depends on the size of the section and how fast any loss appears.
What pressure should water pipes be tested at?
New rigid pipework is commonly tested at 1.5 times the maximum working pressure. For an existing system being checked for a leak, the engineer picks a pressure suited to the age and type of pipework rather than applying a new-build figure to old joints.
Can a pressure test find where the leak is?
No. It confirms that a section is leaking. Splitting the system with valves narrows it to a branch, and methods such as acoustic listening, thermal imaging or tracer gas then find the exact point.
Why does the pressure drop on plastic pipes when there is no leak?
Plastic pipe expands slightly under pressure, so the reading can fall at first even on a sound system. Plastic pipework therefore has its own test procedure that allows for this before a result is judged.
Will a pressure test damage my pipes?
A properly chosen test pressure should not. The risk comes from testing old pipework at an excessive pressure, which can open up a weak joint. That is one reason to leave anything beyond a meter test or gauge check to someone with the right equipment.
