Thermographic Diagnostics

Central Heating Leak Detection London

A sealed heating system that loses pressure is a symptom with several candidate causes, and only some of them involve a pipe. A waterlogged expansion vessel, a pressure relief valve discharging quietly outside, a weeping auto air vent and a fill loop left fractionally open all produce the same reading on the gauge as a buried leak, with no water escaping into the building at all.

Book an engineer See pricing

Quick answer

A sealed system losing pressure is a symptom with several causes and only some of them are pipes. A waterlogged expansion vessel and a relief valve discharging outside both lose pressure with nothing visibly wet indoors. The boiler side is therefore eliminated first, then the circuits are isolated and decay tested zone by zone before any instrument goes near a floor.

What builds the fee on a heating system survey

Stage of the survey	What sets the fee	What lengthens it
Eliminating the appliance and fill side first	Whether the vessel, relief valve, air vents and fill loop check out	Faults inside the appliance that mimic a pipe leak
Isolating and decay testing each heating zone	Number of zones and whether valves exist to separate them	Single circuit systems with nothing to isolate against
Reading the pipe route and floor build	Whether runs are documented or must be traced from scratch	Successive refits that left mixed materials in one circuit
Locating within the failed zone	Whether the construction suits thermal or forces a drain and gas	Deep screed, boarded voids and immovable fitted furniture
Post repair pressure test and moisture baseline	Whether drying has to be measurable against a starting figure	Saturated party structures that need readings in more than one demise

We do not publish a price list, because the figure is built from the property rather than picked from a menu. The detection fee is confirmed in writing before an engineer travels, and repair is quoted separately once there is a located point to quote against.

That is why a heating survey here begins at the boiler, not the floor. Vessel charge pressure is checked against the system, the relief discharge is inspected, and the appliance is isolated from the circuits so the two can be tested independently. If the circuits hold and the appliance does not, nothing under the floor is going to help, and lifting it would have been an expensive way to find that out.

Where a circuit genuinely fails, the floor construction and the pipe material decide what happens next. Suspended timber voids read well acoustically on copper. Screeded underfloor loops in PEX carry almost no leak noise, so those go to tracer gas with thermal used to narrow the loop first. Every stage is recorded, because a landlord or agent authorising floor access needs to see why the opening is where it is.

Heating surveys are priced on the number of zones to isolate and what the floor construction forces us to use afterwards. Where a job reaches tracer gas, the drain down, refill and inhibitor dose are set out before that stage begins rather than added later.

What you get

Method-Specific Equipment: We deploy Trotec thermal imaging cameras for radiant heat mapping of underfloor systems and high-sensitivity acoustic listening devices for pressurised pipework.
System Pressure Analysis: Initial assessment of pressure decay rate determines leak severity and guides whether tracer gas injection or acoustic methods are most viable.
Pipework Material Consideration: Diagnostic sensitivity is adjusted for copper, PEX, or obsolete lead piping common in London's housing stock.
Concealed Space Diagnostics: We specialise in leaks within floor voids, behind stud walls, and under screed, where visual inspection fails.
Boiler & Component Isolation: Testing includes strategic isolation of radiators, manifolds, and the boiler heat exchanger to localise the fault.
Detailed Technical Report: Findings are documented with method rationale, leak coordinates, and recommended repair scope for landlords and insurers.

How it works

A method, not a guess

Boiler side eliminated first

Expansion vessel charge pressure tested against system pressure, relief valve discharge checked for flow, auto air vents and the fill loop inspected. These components imitate a pipe leak precisely and are the cheapest thing in the chain to rule out.

Zone by zone pressure decay

Each heating zone and the underfloor manifold loops are isolated and decay tested separately, with the appliance valved off. We record which circuit falls and how quickly, which distinguishes a pinhole from a fitting and tells us how much migration to expect through the floor.

Method matched to the floor build-up

Acoustic correlation on accessible copper in timber voids. Thermal imaging over a controlled heat cycle to identify the failing loop under screed. Tracer gas into the drained circuit where the pipe is plastic or the run is cast in. Usually two of the three, so the finding is corroborated.

Point marked, then re-tested after repair

The location is marked on the building and recorded with the method that established it. Once the repair is made the circuit is re-pressurised and held, because a system that passes its own decay test is the only evidence that the right thing was fixed.

Before you book anyone

7 things to know before you book central heating leak detection in London

Run the free overnight isolation test before you call anyone

You can narrow this yourself for nothing. Repressurise the system, close the flow and return valves under the boiler, and read the gauge again eight to twelve hours later. If pressure holds with the valves shut and falls when they are open, water is escaping out in the pipework or radiators and detection is the right call. If it falls with the valves shut, the fault is inside the appliance and no amount of floor lifting will find it. Do this before you pay anybody, and tell whoever you book what the result was.

Do not buy parts before someone has pressure tested the circuits

The familiar sequence is a new pressure relief valve, then a recharged or replaced expansion vessel, then talk of a heat exchanger, with the gauge still falling at the end of it. Every one of those is a plausible cause and none of them is a diagnosis until the circuits have been isolated and decay tested. Component replacement without a test is a paid guess, and it is the most common way householders and landlords spend real money on this problem without moving any closer to the answer.

Check whether the guarantee excludes heating circuits

Heating systems are the most frequently carved out category in no find no fee terms in this trade, often on the grounds of corrosion and historic leak sealer residue. That is precisely the system you are trying to have surveyed. Read the terms page rather than the homepage banner, and ask directly whether a sealed heating circuit is inside or outside the guarantee. The answer is a reasonable proxy for how confident a firm is in its own method.

Ask specifically how they will handle underfloor loops

Underfloor heating in screed is a different job from radiator pipework in a timber void, and it is where one method surveys come apart. Thermal imaging over a controlled heat cycle will usually identify which loop is behaving differently, which is genuine progress. It will not reliably give you the breach point, because the pipe reads warm along its whole length and escaped water travels sideways through screed before it surfaces. Getting a point means draining that loop and putting tracer gas into it. If a firm does not describe that two step sequence, ask what their plan is when thermal alone is inconclusive.

Treat leak sealer as a last resort and check your warranty first

Dosing a chemical sealer into the system is a quick intervention that can hold a genuine pinhole weep for a while. It is also an additive circulating through the fine waterways of a modern condensing heat exchanger, and some boiler manufacturers set conditions on additives in their warranty terms. Read what your manufacturer actually requires before anyone pours anything in, and treat a trader who reaches for sealer before pressure testing as someone managing a symptom rather than finding a fault.

Understand what every top-up is costing the system

Refilling a sealed system feels like a workaround rather than a problem. Each refill introduces fresh oxygenated water and dilutes the corrosion inhibitor that protects radiators, pipework and the boiler, which over months encourages sludge and corrosion in a system that was previously protected. A heating circuit that needs topping up weekly is a circuit being slowly degraded while the search continues, which is the practical argument for testing properly early rather than living with it.

Ask what the report says about the circuits that passed

On heating systems the negative findings matter disproportionately, because more than one zone can fail on an older installation and because the boiler and the pipework are often two different contractors' responsibility. A report that records each circuit, its decay rate and whether it held lets a landlord or managing agent see the whole picture, including whether they are looking at one leak or two, and whether the appliance has been cleared or merely not tested.

Compare like for like

Most pressure loss is not a pipe, and it is cheap to prove

A sealed system dropping pressure has several plausible causes and only some of them involve a pipe under a floor. The difference between an hour of elimination and a day of scanning is almost entirely a matter of sequence. Here is how the same complaint is handled three different ways.

What to check	A visit that starts by scanning	A firm that finds and opens on one price	London Leak Detection 247
Where the investigation starts	At the floor, with an instrument, hunting for the leak that has been assumed to exist.	At the scope of works, with a view to lifting a section of floor to look.	At the appliance. Expansion vessel charge, relief valve discharge, automatic air vents and the fill loop are eliminated before a single board is considered, because each of them loses pressure with nothing visibly wet indoors.
The overnight isolation test	Not offered, because it produces a result without a visit.	Not offered, for the same reason.	Closing the flow and return valves under the boiler and watching the gauge is the cheapest diagnostic in the sequence. If pressure still falls with the appliance isolated, the fault is inside it and detection equipment will find nothing because there is nothing to find.
Zone by zone testing	Skipped. The system is treated as one object and the search area stays the size of the property.	Skipped, since a broad area supports a broad opening.	Each zone is isolated and decay tested separately with the rate recorded, so the failed circuit is named and the search reduces from a building to a run.
Sealant additives	Sometimes poured in as a cheap fix, masking the fault and complicating every later diagnosis.	Sometimes recommended alongside works.	Not added during a survey. A survey that changes the thing it is measuring stops being a survey, and a coated system no longer behaves in a way that can be interpreted.
Proving the repair	Pressure is topped up and the job is considered done.	The opening is reinstated and the account closed.	The circuit is brought back to pressure and held under a timed test, and a moisture baseline is recorded by room so that drying afterwards is measured against a figure rather than asserted.
What the report says about what was not found	Nothing.	Nothing, because it does not help.	The circuits that held, the rates that were recorded, and the parts of the system that could not be isolated are all stated. A heating system proved sound while damp continues is a result worth paying for.

Recurring questions

The arguments that come up again and again

Search any UK homeowner public discussion for a boiler losing pressure and the threads follow a recognisable arc: months of topping up, a sequence of replaced components, a chemical sealer somewhere in the middle, and a gauge still falling at the end. The recurring lesson in those threads is about the order things were done in, not about who did them.

Homeowners stuck in the component replacement cycle

The archetypal thread runs for months: a new pressure relief valve, then an expansion vessel, sometimes talk of a heat exchanger, with the gauge still dropping and the boiler engineer and the detection firm each pointing at the other. What is almost never mentioned in these accounts is a circuit by circuit pressure test. Our take: swapping components is a paid guess until someone has isolated each zone and measured its decay rate. The test is quick, it is cheap relative to parts, and it is the thing that tells you whether to look inside the appliance or out in the pipework.

Trade discussion about whether chemical leak sealer is a legitimate fix

Opinion among plumbers online is divided, and the caveats are consistent even among those who use it: it only ever addresses a genuine pinhole weep, it is a temporary measure, and there is concern about additives circulating through the narrow waterways of modern condensing heat exchangers. Our take: read what your own boiler manufacturer requires regarding additives before anyone doses the system, and treat a trader who reaches for sealer before pressure testing as managing a symptom. Sealer is sometimes a reasonable stopgap and it is never a diagnosis.

Owners asking whether weekly topping up actually matters

People living with a slow pressure loss often ask whether repressurising once a week is simply an acceptable maintenance habit. The answers point in one direction: every refill introduces fresh oxygenated water and dilutes the corrosion inhibitor protecting the radiators, pipework and boiler. Our take: a circuit that needs weekly topping up is degrading while the search continues, which turns a diagnostic decision into a time sensitive one. It is the practical argument for testing properly early rather than tolerating the symptom for another winter.

Confusion about what a thermal camera can actually show on underfloor heating

Owners of screeded underfloor systems often expect thermal imaging to produce a cross on the floor, and are frustrated when a survey returns a warm region instead. The explanation that eventually appears in these threads is that a pressurised pipe reads warm along its entire length and escaped water spreads sideways through screed before it surfaces. Our take: thermal is genuinely useful for identifying which loop is behaving differently, and that alone saves hours. Getting a point to core to means draining that loop and running tracer gas through it.

Leaseholders working out who is responsible for heating pipework in a flat

The general pattern is that pipework serving a single flat is the leaseholder's while communal risers and heating mains belong to the freeholder or management company, but the threads make clear the lease wording governs and it varies more than people expect. Our take: read the lease rather than the general rule, and get the survey to state which side of the boundary the evidence puts the fault on. A report that isolates and tests each side is worth considerably more in that conversation than an opinion about where the pipe probably runs.

Questions

Asked before every booking

The boiler keeps losing pressure but there is no damp anywhere. Is there really a leak?

Often there is not. A failed expansion vessel absorbs pressure with no water leaving the system, and a relief valve discharging to an external pipe loses water outdoors where nobody sees it. Both look identical on the gauge to a buried pipe. If a firm quotes you for whole property detection without first asking what the vessel and the relief discharge are doing, that is a reasonable moment to pause.

Why test the expansion vessel before the pipework?

Sequence follows cost and probability. The vessel takes minutes to test, is a common failure on systems past their first decade, and produces exactly this symptom. Pipework detection takes hours and may involve draining circuits and lifting floors. Testing the cheap, likely thing first is not cutting corners, it is the order any diagnostic discipline would use.

Does the heating have to be drained for the survey?

Not for the elimination stage, and not for thermal work, which needs the system hot and running to show anything. Draining is only required if the job reaches tracer gas, because the gas has to occupy the pipe rather than share it with water. We tell you before that point is reached, since draining and refilling a large system has its own time and inhibitor implications.

Underfloor heating is leaking. Why not just use thermal imaging?

Thermal is excellent at showing which loop is behaving differently, and that alone can save hours. What it will not reliably give you is the breach point, because a warm pipe reads warm along its length and escaped water spreads laterally through screed before it reaches the surface. Thermal narrows it to the loop, tracer gas on that drained loop gives the point you core to.

What if more than one circuit fails the pressure test?

Then we say so and treat them separately rather than assuming one cause. Two failing zones on an older system is usually two independent faults, commonly corrosion at similar age points, and reporting it as a single leak would understate the repair scope that a landlord is about to authorise. Each failing circuit gets its own location, method and entry in the report.

Can a boiler lose pressure with no leak in the pipework at all?

Yes, and it is common. A failed expansion vessel, a pressure relief valve passing to an outside discharge, a filling loop left slightly open or an internal appliance fault all drop system pressure without water entering the building. These are the cheapest things in the chain to test and should be eliminated before anyone surveys the floors. A detection firm that quotes for whole property work without asking what the vessel and the relief discharge are doing has skipped a step.

What is the overnight isolation test and how do I run it?

Repressurise the system to its normal cold pressure, close the flow and return isolation valves under the boiler, note the gauge reading and check it again eight to twelve hours later. If the pressure holds while the valves are closed, the loss is out in the pipework or radiators. If it falls with the valves closed, the loss is inside the appliance and is work for a Gas Safe engineer rather than a detection survey. Either answer saves money, which is why it is worth doing first.

How long does a central heating leak survey take?

Two to five hours is typical for a domestic system. The first part is circuit isolation and pressure decay testing, which on most properties takes well under an hour and frequently decides the job. Locating the point afterwards depends on the floor: accessible timber voids with copper resolve quickly on acoustic, screeded underfloor loops need a thermal pass to find the loop and then tracer gas on the drained circuit, which is where the longer visits come from.

Does the heating system have to be drained?

Not for pressure testing, and not for thermal imaging, which needs the system hot and circulating to show anything at all. Draining is only necessary if the job reaches tracer gas, because the gas has to occupy the pipe rather than share it with water. On a large system that brings a refill, an air purge and a fresh inhibitor dose with it, so it should be discussed and priced before it happens rather than after.

Two zones both fail the pressure test. Is that one leak or two?

Usually two, and they should be treated as two. Independent faults at similar age points are common on older installations, particularly where the pipework was installed at the same time in the same material. Reporting two failing circuits as a single leak understates the repair scope that a landlord or agent is about to authorise, so each failing circuit should get its own location, its own method and its own entry in the report.

Who is responsible for a heating leak in a leasehold flat?

As a general rule pipework serving only your flat is yours and communal risers and heating mains fall to the freeholder or management company, but the lease wording governs and it varies, so read it rather than assume. Where a leak has damaged the flat below, the buildings policy usually deals with the resulting damage. What resolves these situations fastest is an independent survey that states which side of the boundary the evidence puts the fault on.

Water going somewhere it shouldn’t?

Tell us the symptoms and your postcode. We’ll confirm the visit, the fixed detection fee and the arrival window before you commit to anything.

Book a detection visit