A central heating system is a closed loop of pipes, radiators, valves and a boiler, and the water in it goes round and round. When some of it escapes, the clues depend on where the escape is. A weeping radiator valve leaves a crust. A leak under a floor leaves a warm patch. A leak on an older, tank-fed system may leave almost nothing at all.
This guide goes through the parts of a heating system that most often leak and the signs that go with each one. If your main symptom is a boiler that keeps losing pressure, our guide to finding where the pressure is going covers the gauge readings, the relief valve and the expansion vessel in detail, so they are not repeated here.
What are the general signs of a central heating leak?
Whatever the location, heating leaks tend to produce some of these:
- a pressure gauge on a sealed system that falls and needs topping up;
- dark, rusty or black staining, because heating water carries corrosion residue;
- white or green crusting on valves and joints, where small drips have evaporated;
- damp patches that feel warm when the heating is on;
- radiators that go cold at the top and need bleeding more often than before;
- a slightly metallic or chemical smell from damp carpet or boards, where inhibitor is present.
The sections below explain where each of these usually comes from.
Where do central heating systems usually leak?
| Part | Typical sign | Usually visible? |
|---|---|---|
| Radiator valves and tails | Crust or drip under the valve, stained carpet or floorboard | Yes, if you look underneath |
| Radiator body | Rust spots or a pinhole weep, often along the bottom edge | Often hidden behind furniture |
| Towel rails | Drip at the valves, rust at the lower joints, water at an element fitting | Usually |
| Pump and pump valves | Drips or crust at the union nuts, damp airing cupboard floor | Yes, if the pump is separate from the boiler |
| Boiler connections | Water on the pipes or floor below the boiler, crust on fittings | Usually, below the casing |
| Joints under floors and in walls | Warm patch, damp skirting, stain on the ceiling below, pressure loss | No |
How do you spot a leak at a radiator valve or tail?
Radiator valves are the most common visible heating leak. Water can escape at the spindle where the valve head turns, at the union nut that joins the valve to the radiator tail, at the thread where the tail screws into the radiator, or at the compression nut on the pipe below. Run a dry tissue round each of those points with the heating on. A tissue picks up moisture that you cannot see.
A slow weep on a hot valve often evaporates before it drips, so the only sign may be a white or green crust, or a rusty stain on the floor directly below. Each point has its own fix, and our guide to radiator valve leaks explains which ones a homeowner can safely tighten.
What about the radiator itself?
Steel radiators corrode from the inside when the system water has not been properly treated. The corrosion tends to settle as sludge along the bottom, and eventually a pinhole can form there or along a seam. Look for small rust spots, a bubble in the paint or a damp line along the lower edge. A radiator that needs bleeding far more often than the others can also point to corrosion inside it. A corroded radiator is usually replaced rather than repaired.
Why do towel rails leak?
Heated towel rails sit in the wettest room in the house, and their valves and joints are often at floor level where splashes and cleaning products add to the wear. Common signs are a drip at a valve, rust at the lower joints and a damp patch in the flooring that does not dry. Towel rails fitted with an electric element as well as a heating connection have an extra fitting where water can escape, and any leak near an electrical element needs the power isolating before anything else.
In a bathroom, it is easy to blame a towel rail leak on the shower or bath. A damp patch that is warm, or that stays wet when nobody has used the bathroom, points more towards the heating.
Can the heating pump leak?
On many older systems with a regular, heat-only boiler, the pump sits outside the boiler, often in an airing cupboard next to the hot water cylinder. It is fitted with a valve on each side so it can be changed without draining the system, and those union nuts and valves are frequent sources of drips. Look for crusting, rust marks or a damp cupboard floor. On combi and most modern system boilers the pump is inside the casing, and any water there is a job for a Gas Safe registered engineer.
What are the signs of a leak at the boiler connections?
Under most wall-hung boilers there is a set of pipes: heating flow and return, hot and cold water on a combi, the gas pipe, and the filling loop. Water on those pipes, a stain on the wall below or drips on the floor can come from one of those connections or from the filling loop fittings. Do not remove the boiler casing. Note what you see, check the pressure gauge, and have a heating engineer look at it. The pressure relief pipe that runs outside is a separate case, covered in our guide to boiler pressure loss linked above.
How do you tell if a heating pipe is leaking under the floor?
Most of the pipework in a heating system is out of sight: under floorboards, inside solid floors and chased into walls. A leak there shows itself indirectly.
- A warm patch on the floor when the heating is on, in a place with no heating pipe you know of.
- Damp or stained skirting near a radiator, where water is soaking out of the floor.
- A stain on the ceiling below that is darker than a clean water stain, and appears or spreads when the heating is on.
- Lifting or swelling in laminate or engineered flooring.
- Pressure loss on a sealed system with no visible leak anywhere.
In solid floors, older systems were often laid with copper pipe directly in the screed, which can corrode over time. Our guide to heating leaks under concrete floors explains how those are located.
Are the signs different on an older tank-fed system?
Yes, and they are easier to miss. A sealed system, the type used with combi and most modern system boilers, has a pressure gauge that falls when water escapes. An older open vented system is fed from a small feed and expansion tank in the loft, which tops the system up automatically. There is no gauge to watch, so a leak can continue for a long time.
Signs on an open vented system are the feed tank valve refilling when nobody has touched the system, a trickling sound from the loft, rust or sludge building up quickly, and radiators that keep needing bleeding. Because the system keeps drawing in fresh water, the inhibitor is diluted and corrosion speeds up, so a small leak can make the system age faster.
When should you call a leak detection specialist?
A visibly dripping valve or a weeping pump union is a job for a heating engineer or plumber. Call a leak detection specialist when:
- a sealed system keeps losing pressure and no leak can be seen;
- there is a warm damp patch in a floor or wall;
- a stain on a ceiling below appears when the heating runs;
- the feed tank on an older system keeps refilling;
- you need the leak located before a floor is lifted, or a written report for an insurer.
Heating leaks respond well to thermal imaging, because the escaping water is warm, and to pressure testing and tracer gas when the pipe is deep in a floor. Our central heating leak detection surveys start at £150 for thermal imaging, and under-floor leak detection is £300 to £700. Call 020 3912 0200 to talk it through.
Frequently asked questions
How do I know if my central heating is leaking?
On a sealed system, the most reliable sign is a pressure gauge that falls and keeps needing topping up. Other signs are crusting or stains under valves, rusty spots on radiators, and warm damp patches in floors or walls.
What colour is water from a central heating leak?
Often darker than tap water, from brown to black, because it carries corrosion residue from the radiators and pipes. Stains from heating leaks tend to be darker and rustier than stains from clean water leaks.
Is a central heating leak dangerous?
The water itself is not usually a hazard, but it can be hot, it can reach electrical fittings, and a system that keeps losing water can lead to boiler faults. Isolate any electrics that are wet and get the leak looked at promptly.
Can I keep using the heating if it is leaking?
If the leak is small and the pressure is stable after a top-up, many people carry on in the short term. If the pressure keeps falling, water is reaching ceilings or electrics, or the boiler locks out, switch it off and get it checked.
Where are central heating leaks most commonly found?
At radiator valves and their tails, on towel rails, at pump valves and around boiler connections. Leaks at joints under floors and inside walls are less common but harder to find, and they cause the most damage.
