Pressure testing isolates a section of pipework, pressurises it, then watches the gauge over time. A steady gauge means that section holds; a falling gauge proves it is losing water. Testing section by section narrows a whole property down to one specific run of pipe, the groundwork that makes tracer gas or acoustic detection quick and targeted.
Before anyone can pinpoint the exact hole in a pipe, they need to know which pipe to look at. That is the job pressure testing does. Rather than guessing which run of pipework is losing water, isolating sections and watching how each one behaves under pressure proves, with a gauge reading rather than a hunch, exactly where the problem sits. This guide walks through the isolation logic, the difference between wet and dry tests, and how to read what the gauge is telling you. See our guide to water leak detection for how this fits into the wider process.
On this page
The isolation logic: why you test in sections
Testing an entire property’s pipework as one system only ever tells you one thing: that a leak exists somewhere within it. That is rarely useful on its own, especially in a larger or older property with multiple bathrooms, an outbuilding supply, or heating pipework running through several zones. Pressure testing becomes genuinely useful once the system is broken into sections using existing stop valves, isolation valves, or temporary end caps, and each section is tested in turn.
Each isolated section that holds pressure can be ruled out. Each one that loses pressure becomes the focus. Working through a property this way turns “there’s a leak somewhere in the house” into “the leak is in the run feeding the upstairs bathroom”, which is a completely different, and much more manageable, starting point for the next stage of detection.
Wet tests vs dry tests
Pressure tests come in two main forms, and the choice depends on the pipework, its condition, and what is practical for the property:
- Wet testing pressurises the isolated section with water, close to how the pipe normally operates. It is common for testing systems that are already in use and already full of water, such as an existing central heating circuit.
- Dry testing uses air or an inert gas instead of water to pressurise the section. It is often preferred for new installations before they are commissioned, or where introducing water would complicate the reading or risk further damage if a leak is confirmed.
Both approaches follow the same underlying principle, isolate, pressurise, then watch the gauge, but the practicalities and the equipment differ.
Reading a gauge drop correctly
A pressure gauge on a freshly pressurised system rarely behaves in a perfectly flat line even when there is no leak. Temperature changes in the pipe and the surrounding air cause small, expected fluctuations as the system settles. Reading a test properly means giving it time to stabilise before drawing conclusions, then watching for a genuine, sustained drop rather than reacting to the first few minutes of natural settling.
There is no single number that applies to every system, since the expected operating pressure and the relevant test standard vary by pipe material and installation type. What matters consistently is the pattern: steady after stabilising means the section holds, continuing to fall means it does not.
Wet test vs dry test at a glance
| Feature | Wet test | Dry test |
|---|---|---|
| Medium | Water | Air or inert gas |
| Typical use | Existing systems already in use | New installations, pre-commissioning |
| Reading style | Gauge pressure over time | Gauge pressure over time |
| Practical consideration | Straightforward on systems already full of water | Avoids introducing water where a leak is suspected |
What a pressure test involves, step by step
- Isolate a section: existing stop valves, isolation valves or temporary caps separate one run of pipe from the rest of the system.
- Pressurise: the isolated section is brought up to an appropriate test pressure using water (wet test) or air/inert gas (dry test).
- Allow stabilisation: the gauge is left to settle briefly, since temperature and minor air pockets can cause an initial, non-leak-related dip.
- Monitor over time: the gauge is watched over an appropriate period to distinguish normal settling from a genuine, continuing drop.
- Compare sections: results across each isolated section are compared, ruling out the ones that hold and focusing on the one that does not.
After testing: narrowing to tracer gas
Pressure testing answers “which section?”, not “which centimetre?”. Once a section is confirmed as the source, the next step is usually tracer gas leak detection, using a safe mix of 5% hydrogen and 95% nitrogen introduced into the empty, isolated pipe and picked up at the surface with a sensitive probe, pinpointing the exact spot. Acoustic detection can also be used at this stage, listening for the leak’s sound signature within the already-narrowed section. Testing first, then pinpointing, is far quicker than trying to run acoustic or tracer gas equipment across an entire property’s pipework at once.
Older systems in Cornwall & Devon
Across Cornwall and Devon, older cottages and farm conversions often have pipework added in stages over decades, sometimes with isolation valves missing or seized in places where a modern system would have them as standard. On these properties, part of the pressure testing groundwork is establishing where sections can practically be isolated at all, sometimes fitting temporary caps or working with what is available, before the testing itself begins. We see this most weeks on older stone and cob properties, where a system installed as one continuous run decades ago has to be worked back into logical, testable sections before a gauge reading means anything useful.
Frequently asked questions
What is pressure testing on a pipe system?
Pressure testing isolates a section of pipework, pressurises it, then watches a gauge over time to see whether the pressure holds or falls. A falling gauge shows that section is losing water somewhere, proving which run of pipe is affected before any invasive detection work begins.
What is the difference between a wet test and a dry test?
A wet test pressurises the pipe with water, close to how it normally operates, and is common for checking a system already in use. A dry test uses air or an inert gas instead of water, often used on new installations or when water in the pipe would complicate the reading.
How much pressure drop indicates a leak?
There is no single universal figure, since it depends on the system, the test standard being followed and the pipe’s normal operating pressure. What matters is whether the gauge holds steady over the test period once temperature has stabilised, or whether it continues falling, which points to an active leak in that isolated section.
Does pressure testing tell you exactly where the leak is?
No. Pressure testing tells you which isolated section is losing pressure, narrowing a whole property down to one run of pipe. Finding the precise point within that run typically needs a further step, such as tracer gas or acoustic detection, once the affected section is identified.
Why isolate sections of pipe before testing?
Testing a whole property at once only tells you that a leak exists somewhere. Isolating sections, one at a time, using stop valves or temporary end caps, means each test result points to a specific run, turning a vague problem into a narrowed-down search area.
Is pressure testing safe for old pipework?
Pressure testing is carried out at controlled levels appropriate to the pipe material and its normal operating pressure, typically not far above what the system already experiences day to day. An experienced tester adjusts the approach for older or more fragile pipework rather than applying a blanket pressure regardless of condition.
What happens after pressure testing confirms a leaking section?
Once a section is confirmed as the source, tracer gas, a safe mix of 5% hydrogen and 95% nitrogen, or acoustic detection is typically used to pinpoint the exact location within that run, so a repair can target the precise spot rather than opening up the whole section.
Need to prove which pipe run is leaking? We’ll test it properly
We isolate and pressure test pipework across Cornwall & Devon, then use tracer gas and acoustic detection to pinpoint the exact spot, all with fixed pricing agreed upfront.
Call Dickie on 07822 025 911 No Find, No Fee on residential leak detection (subject to terms)