How To Find A Leak In Central Heating System

Reviewed by the DCI Leak Detection team · Last updated June 2026

A central heating leak rarely announces itself. More often it is a slow drip behind a radiator, a weeping joint under a floorboard, or simply a boiler you keep having to repressurise. The good news is that a homeowner can find a lot of leaks with nothing more than a torch, some kitchen roll and a bit of patience. This guide walks through the warning signs, the spots where leaks hide, the checks you can safely do yourself, and the point at which it is worth bringing in a specialist.

Signs your central heating is leaking

Heating leaks tend to give themselves away gradually. The earlier you spot one, the less damage it does, and the cheaper it is to put right. Watch for any of these:

• Boiler pressure that keeps dropping. If you are using the filling loop to top up every few weeks, the system is losing water somewhere.
• Damp patches, staining or rust on walls, ceilings or floors, especially below pipework or near radiators.
• A musty, damp smell in a cupboard, under the stairs or near a radiator that never quite dries out.
• Pooling or drips beneath a radiator, valve or visible pipe.
• The boiler showing a low-pressure fault (often an F1 code or similar) and locking out the heating.

None of these on their own proves a leak, but two or three together usually do. A persistent drop in pressure with no obvious puddle is the classic signature of a hidden one.

Start with the pressure gauge

For most modern (sealed) systems, the pressure gauge is your single best diagnostic tool. When the heating is off, the needle should sit between 1 and 1.5 bar. When the system fires up and the water heats and expands, it is normal for the pressure to climb by around half a bar, so seeing it near 2 bar with the heating on is nothing to worry about.

What matters is whether it holds. A healthy sealed system should keep its pressure for months on end. If yours keeps slipping below 1 bar and you find yourself reaching for the filling loop again and again, that repeated loss of water is one of the most reliable signs of a leak in the system.

One honest caveat: not every pressure loss is a leak. A failed expansion vessel or a passing pressure-relief valve can mimic the same symptom, which is one reason a system that drips outside near the boiler's overflow pipe needs a heating engineer rather than a hunt for puddles. If your boiler is visibly weeping, our guide on why a boiler leaks water covers those causes, and the companion piece on a boiler that keeps losing pressure goes deeper on the gauge itself.

Where central heating leaks usually hide

Leaks are not random. They cluster in a handful of predictable places, so knowing where to look saves a lot of time.

The tricky part is that the most damaging leaks are often the least visible. On stored hot water systems it is also worth ruling out a leaking hot water cylinder, which can lose water without any obvious puddle. A small leak on a hot pipe under a floor can evaporate almost as fast as it escapes, so the surface looks dry while the pressure quietly drains away. That is exactly the kind of leak that needs equipment rather than eyes.

DIY checks you can do safely

Before you call anyone, there is a sensible sequence of checks you can run yourself. Work methodically, because leak-hunting rewards patience.

1. Read and log the pressure. Note the cold reading, then watch it over a few days as described above.
2. Walk every radiator. Wrap a piece of dry kitchen roll or tissue around each valve and the joints at both ends, then check it an hour or two later. If it comes away damp or crinkled, you have found a weep. This catches the small leaks that evaporate off a hot pipe before you would ever spot a drip.
3. Look low and look up. Run your eye along skirting boards and the floor beneath radiators for clean, dust-free patches or staining. Then check the ceilings below any upstairs pipework for tell-tale marks, especially where the ceiling meets the wall.
4. Feel the floor. With the heating on, a hidden under-floor leak can leave a warm or damp patch, though never assume an underfloor-heating leak is the culprit without proper testing.
5. Check around the boiler. Look underneath for drips and at the overflow pipe outside. Boiler-internal faults are not a DIY repair, but spotting them tells you who to call.

A word on safety: only ever work on parts you can reach without dismantling anything, never lift floors or open walls chasing a leak, and if water is escaping fast, turn the system off and isolate it. If your checks point to a hidden leak, stop there. Going further usually means damage you did not need to cause.

Cold radiators and other false alarms

Not everything that looks like a leak is one. The most common false alarm is a radiator that is hot at the top but cold at the bottom. That is almost always sludge (a build-up of corrosion debris that settles inside the radiator) rather than a leak. It needs a power flush or chemical clean, not a leak detective.

There is a genuine link worth understanding, though. Every time you top a sealed system up through the filling loop, you add fresh, oxygen-rich water and dilute the corrosion inhibitor that protects the metal inside. So a slow leak you keep topping up does more than waste water. It quietly accelerates the internal corrosion that causes sludge and cold spots in the first place. Fixing the leak early protects the whole system, which is one more reason not to live with a gauge you keep nudging back up.

When to call a professional

Plenty of heating leaks are within a confident homeowner's reach: a visibly weeping valve nut, an obvious drip on an accessible joint. But some signs mean it is time to bring in a specialist rather than keep guessing:

• The pressure keeps dropping but you cannot find the source after a thorough check.
• The leak appears to be under a solid floor, in screed or behind a wall.
• There is staining or damp with no obvious cause, or a warm patch in the floor.
• You would be lifting floors or opening walls just to investigate.

This is where non-invasive detection earns its keep. Rather than ripping up a floor to find a pinhole, a specialist uses thermal imaging to see the heat signature of the escaping water, acoustic sensors to hear it, and tracer gas to pinpoint the exact spot, then confirms it with a held-pressure test on the isolated circuit. The result is the leak found to within a few centimetres, with the smallest possible hole to repair it. If you would like the detail of how that detection work is carried out, our central heating leak detection service page explains the methods and what to expect.

There is an insurance angle, too. Escape of water is one of the most common home-insurance claims in the UK. The Association of British Insurers puts industry payouts at around £1.8 million a day, and most buildings policies include "trace and access" cover that pays to find a hidden leak and make good the damage caused by reaching it. A professional detection report is what supports that claim. Our guide to what trace and access cover is explains how it works.

Frequently asked questions