Central Heating Leak Detection Costs: Why a Sealed System Takes Longer to Trace

Why a heating leak is not priced like a mains leak

A leak on the incoming cold supply is, in pricing terms, the straightforward case. The pipe is under constant pressure from the street and there is a meter on it. Close every outlet, watch the dial over a fixed interval, and you know whether water is being lost before anyone opens a toolbox. That one test costs nothing but time.

A sealed central heating system offers none of that. It is a closed loop holding roughly 1 to 1.5 bar when cold, topped up through a filling loop that should be disconnected in normal use. There is no meter on it. The only reading available at the outset is the pressure gauge on the boiler, and a falling gauge is not a leak. It is a symptom with at least four candidate explanations, three of which involve no escape into the building at all. Work on a mains leak begins with a measurement. Work on a heating leak begins with an argument that has to be settled before any measurement means anything.

The closed loop gives no volume figure

On a mains supply the meter tells you roughly how much water is escaping, and that figure steers everything that follows. A few litres an hour points at a joint or a pinhole; tens of litres an hour points at a fractured run. A heating circuit gives no equivalent number. What you have is a rate of pressure decay, which depends on system volume, the health of the expansion vessel, the water temperature at the reading, and whether anybody bled a radiator that week.

Turning a pressure decay into a useful statement therefore needs a controlled test rather than an observation, and that means the system isolated, stabilised and watched over a known interval. It is the first chargeable hour on almost every heating survey, and it happens before any detection equipment comes out of the van.

The innocent explanations that have to go first

A boiler losing pressure over a week is as likely to have a failed expansion vessel as a leak. When the vessel loses its air charge, the water has nowhere to expand into as it heats, the relief valve lifts and a small volume discharges outside. Nothing is wet indoors, the gauge falls anyway, and a survey that goes straight to thermal imaging finds nothing and still has to be paid for.

The same applies to a relief valve that has picked up debris, to an automatic air vent that is passing, and to a filling loop left connected and quietly refilling the system. That sequence is set out in a combi boiler losing pressure with no leak found, the most common reason a heating job ends without a leak.

The elimination work that comes before detection

A competent heating survey is two jobs in sequence: prove there is a loss, then find it. The first is where a well run survey saves money, because a meaningful proportion of heating enquiries end there with a component fault rather than an escape.

Test	What it establishes	What it takes off the list
Meter read over a fixed interval, all outlets closed	Whether the cold supply is losing water	A mains leak presenting as a heating fault
Filling loop disconnected, system held at a test pressure	Whether the sealed side holds independently	A filling loop quietly topping the system up
Expansion vessel charge checked against system pressure	Whether the vessel can still absorb expansion	Discharge through the pressure relief valve
Pressure relief discharge pipe checked outside	Whether water is leaving the building	A leak that does not exist inside the property
Moisture readings at surface and at depth	Whether the wetting pattern fits a pressurised escape	Condensation and penetrating damp

Every line in that table is time, and time is what a detection fee buys. It is also why two quotes for one address can differ by a factor of two without either being dishonest. One has priced the elimination and the report; the other has priced an hour with a thermal camera.

Isolating the system and holding a test pressure

Isolation means closing the valves that separate the heating circuit from everything else, disconnecting the filling loop so the system cannot replenish itself, and bringing the circuit to a known pressure. It then sits, usually cold, and is read at intervals. A circuit that holds for an hour but falls over three days is saying something different from one that drops while you watch it, and that distinction changes which detection method is worth deploying.

Where the property has an older vented arrangement with a feed and expansion tank in the loft, the test changes shape. There is no gauge, so the tank level and float valve become the instruments and a loss shows as a valve that keeps admitting water. Vented layouts remain common enough in London that a survey must establish which it faces before scoping.

What makes a heating survey short and what makes it long

Once a genuine loss is established, the length of the job is decided almost entirely by where the pipework runs and what sits on top of it.

Variable	Short job	Long job
Circuit isolation	Zone valves allow the system to be split into halves and quarters	One undivided loop, so nothing narrows by elimination
Pipe run	Surface clipped, in a service void, or under floorboards that lift	Buried in screed, chased into walls, or under a poured slab
Floor construction	Suspended timber with a void that carries sound and heat	Deep insulated screed under stone, which flattens the thermal signal
System type	Sealed, with a working gauge and a clear decay rate	Vented, or sealed with a failed vessel confusing every reading
Occupancy and noise	Quiet property where acoustic work is usable	Tenanted flat, or plant noise carrying through the structure

The variable a client most often controls is the first. A system with working zone valves can be halved, then halved again, and a leak that would take a day on an undivided loop narrows to one zone inside an hour. Without isolation, the survey must treat every metre as a candidate, and the price reflects that.

Underfloor loops are a different job again

Underfloor heating changes the method rather than simply the difficulty. The pipe is continuous plastic buried in screed, with no joints along the loop to fail. Most underfloor failures are therefore mechanical damage from a fixing driven through the pipe during later work, or a failure at the manifold, which is where the joints are. Finding one takes two instruments, in a fixed order.

Thermal imaging maps the loop before anything else

Running the floor warm and imaging it gives you the pipe layout, which is the thing nobody has drawings for. Occasionally it shows the escape directly as a warm blur sitting off the pattern. More often it does the less glamorous job of establishing where the pipe is not, so the next stage is aimed rather than exploratory. Where the screed is deep, well insulated and covered with stone, the signal smears and the map is approximate, which is the case that adds hours.

Tracer gas finishes what thermal starts

Tracer gas is a hydrogen and nitrogen mixture introduced into the drained loop. It migrates up through screed and floor covering and is picked up at the surface with a probe. It converts a warm area into a marked point a repair can be aimed at. It also carries its own cost line, because the loop has to be drained, charged, surveyed, then refilled and recommissioned. Our underfloor heating leak detection page sets out that sequence.

Isolating loop by loop at the manifold

The cheapest thing anyone can do on an underfloor job is close the manifold ports one at a time and watch which loop loses pressure. A manifold with individual isolation on flow and return turns a whole floor survey into a single loop survey. One boxed in behind fitted joinery that nobody can open has the opposite effect. In practice this is the difference between a morning and a full day.

What the published market rates actually say

We do not publish our own detection fee on a blog page, because a figure quoted without the property in front of you is a guess dressed up as information. What can usefully be stated is what independent directories publish.

Checkatrade's water leak detection cost guide gives an average of £162, a typical range of £115 to £234, and a full spread across recorded jobs of £80 to £1,600. It puts hourly rates at £95 to £145 and an emergency call out at £100 to £120, and notes that most tests take around half a day but a leak under concrete can take longer. Its underfloor heating cost guide puts pipework leak repairs from around £300, rising where flooring is lifted and relaid.

Published figure	Source	What it is actually measuring
£162 average, £115 to £234 typical	Checkatrade leak detection guide	A national midpoint, weighted towards accessible jobs
£80 to £1,600 full spread	Checkatrade leak detection guide	The range once buried and sealed system jobs enter the sample
£95 to £145 per hour	Checkatrade leak detection guide	Time based work, which an undivided heating loop becomes
£100 to £120 emergency call out	Checkatrade leak detection guide	Attendance outside normal hours, before detection begins
Underfloor pipework repairs from around £300	Checkatrade underfloor heating guide	Repair and reinstatement, a separate line from detection

Two things follow. The widely repeated claim that leak detection costs about £500 on average in the UK does not match any published average we have been able to find, and should be treated as folklore rather than a benchmark. And a national average is the least useful figure in the set for a London heating job, because that dataset is dominated by accessible mains leaks in houses, which is exactly what a sealed circuit under a screed floor is not.

What the survey produces, and why that affects the price

The output of a heating survey is not a phone call saying the leak is somewhere under the hall. It is a document, and that document is much of what separates a cheap search from a survey worth paying for. At minimum it records:

• The system type, the pressure it was tested at, and the decay observed against the interval.
• Which components were eliminated, and on what reading. A vessel checked and found sound is a finding, not a blank.
• Which detection methods were used, in what order, and why the survey moved from one to the next.
• The location of the escape, stated against something measurable from a fixed datum rather than from furniture.
• The access required to reach it, and the reinstatement that follows, itemised separately from detection.

Where a third party is involved, the document must state cause as well as origin, a separate and harder finding set out in the leak report a landlord or managing agent can actually use. For a freeholder, an agent or an insurer this decides whether the fee is recoverable: a report documenting method and readings supports a trace and access claim, while an invoice saying leak detection does not.

Why repair is quoted separately from detection

Detection and repair are different jobs with different risk profiles, and quoting them as one figure means pricing an unknown. Until the escape is located, nobody knows whether the repair is a joint remade behind a lifted board or buried pipe in screed under stone. A combined number has to assume the worse case, so every client with the easy case pays for the possibility of the hard one. There is also a third line people forget: recommissioning.

Cost line	What it covers	When it can honestly be quoted
Detection	Elimination, testing, location and the written report	Before the visit, from system type, floor and access
Access	Lifting covering, cutting screed, removing boxing or ceiling	Once the escape has been located and marked
Repair	The pipe, joint or fitting itself	Once the pipe is exposed and the failure mode is visible
Recommissioning	Refilling, venting, rebalancing, dosing and repressurising	Once the repair is complete
Reinstatement	Screed, floor finish, plaster and decoration	Usually a separate trade, priced separately again

Recommissioning matters more than its size suggests. A system drained and refilled has lost its corrosion inhibitor, and running it unprotected is how the next generation of pinhole failures starts. Refilling also brings the filling loop back into use, a backflow risk point governed by the Water Supply (Water Fittings) Regulations 1999, which is why it should be disconnected again once the pressure is correct.

How to brief the job so the quote means something

Most of the variance between heating detection quotes comes from missing information rather than differing rates. Five facts, given on the phone, move a quote from a guess to a figure that will survive contact with the property.

• System type: sealed with a gauge, or vented with a loft tank. A photograph of the boiler and the loft settles it.
• How fast the pressure falls: from what figure, to what figure, over how many days, and whether anyone topped it up.
• Floor construction over the suspect area, and the covering laid over it.
• Whether there is underfloor heating, where the manifold sits, and whether each loop isolates individually.
• Whether the system has zone valves, and whether anything has already been isolated or drained.

Two further points are worth raising before anyone attends. Whether the property is tenanted, because notice periods change the shape and sometimes the number of visits. And whether a report is needed for an insurer or managing agent, because writing to evidential standard is work and should be priced rather than assumed.

Our own fee for a heating survey is fixed, agreed before we travel, and quoted separately from any repair. What we will not do is publish a number on a page that has never seen your property. If you want the basis rather than a figure, our pricing page explains how a survey is scoped and our central heating leak detection service sets out what a visit covers. If you would rather work through the diagnostics yourself first, the order the tests go in is the sequence we follow on site.