A central heating system that keeps needing topped up is losing water somewhere, even with nothing visibly wet. The usual places are pipes under a floor or in the screed, radiator valves and tails, and the boiler’s pressure relief valve discharging outside. A failed expansion vessel mimics a leak without one. Pressure testing and thermal imaging find which it is.
Every couple of weeks the boiler pressure is back down near zero and you are opening the filling loop again. You have looked under every radiator, run a hand along every pipe you can reach, checked under the boiler. Dry. And yet the water has gone somewhere, because a sealed system does not lose water without a way out. This guide is about the system rather than the boiler: the pipework, the radiators and the two components on the boiler that are the most common answers to “where is it going”. If the falling pressure itself is your main question, the existing guide to why a boiler keeps losing pressure covers that symptom; here we deal with finding the water. Our central heating leak detection service exists for exactly this problem.
Why nothing is wet
A heating leak of a few litres a week evaporates faster than it shows. Under a suspended timber floor the water drips onto the ground below and soaks away. In a screeded floor it wets the concrete and spreads sideways, warmed by the pipe it came from, so it dries from the top before it reaches the surface. At a radiator valve a weep that runs down the tail dries on the hot metal and leaves only a faint white or green crust. Because the water is warm and the volume is small, weeks can pass before a stain appears, and by then the pressure gauge has told you several times over. That is also why the water meter is no help: a sealed system is filled once through the loop and then closed, so a heating leak never turns the meter.
The suspects, in the order we check them
The pressure relief valve
Every sealed system has a safety valve on the boiler, set to open if pressure climbs too high. It discharges through a copper pipe to the outside wall. If the valve has been lifted once by over-pressure it often fails to reseat fully, and from then on it dribbles continuously. The pipe outside will be damp or have a green stain below it, and in a coastal wind you may never see a drip. This is the single most common cause of “losing pressure with no leak”, and it is worth checking before anything else. The valve itself is a boiler part and a heating engineer replaces it.
The expansion vessel
Inside or beside the boiler is a vessel with a rubber diaphragm and a charge of air on one side. It absorbs the expansion of the water as it heats. When the diaphragm fails or the air charge is lost, the system has nowhere to expand into, pressure climbs sharply when the heating runs, the relief valve opens and dumps water outside, and the pressure then reads low when the system cools. It looks exactly like a leak. The tell is the pattern: pressure high or the relief pipe running while hot, low when cold. A heating engineer can recharge or replace the vessel. No pipe has failed.
Radiator valves, tails and bleed points
The joints where a valve meets the radiator, where the tail screws into it and where the pipe enters the valve from below are the commonest points of a genuine small leak. Look for a crust, a dark ring in the carpet or a spot of rust on the floorboard. A bleed screw left fractionally open also weeps. Each of these can be sorted by a plumber once identified.
Pipes under the floor
This is where the water usually is when everything above the floor is dry. Copper heating pipes run under floorboards, in notches across joists, or buried in the screed of a solid floor, often without any protection from the concrete. Over decades the pipe corrodes where it touches the screed, at soldered joints, at bends made with a spring, and where a nail or screw went through it during a floor repair. Cornish houses add their own risks: pipes laid in the ground floor of a cottage extension with a damp slab, and heating systems in granite houses where the first-floor pipes cross uneven joists and rub at every crossing.
Underfloor heating loops
Plastic underfloor pipe in a screed rarely fails on its own, but it does at manifold connections and where a floor fixing has gone through it. The pattern is the same as a leak on a radiator circuit, but it can be isolated by closing individual loops at the manifold, which is one of the first things a survey does.
The hot water cylinder coil
On a system with a hot water cylinder, the heating water passes through a coil inside it. If the coil splits, heating water leaks into the hot water and heating pressure drops, while the cylinder itself stays dry. Rusty hot water at the taps is the clue.
Reading the pattern before anyone arrives
| What you notice | Most likely | What to do |
|---|---|---|
| Pressure drops slowly and steadily, hot or cold | A genuine leak, probably on pipework | Note the rate; a survey with pressure testing and imaging |
| Pressure high when hot, relief pipe wet outside, low when cold | Expansion vessel | Heating engineer to recharge or replace |
| Relief pipe damp or stained all the time | Relief valve not reseating | Heating engineer to replace the valve |
| Radiators need bleeding repeatedly | Air entering at a leak, or corrosion gas | Points towards a leak; check inhibitor |
| A warm patch on a floor with no pipe you know of | Heating pipe leaking in the screed | Do not lift the floor on a guess; have it located |
| Rusty or discoloured hot water | Cylinder coil | Plumber to inspect the cylinder |
Where radiators are cold at the bottom as well, sludge may be part of the story and the guide to cold spots on radiators, sludge or leak helps tell the two apart. And a warm patch on the floor with no obvious reason is close to a diagnosis on its own.
How the survey finds it
Once the relief valve and expansion vessel have been checked (either by you or by a heating engineer), the leak is on the system and the survey works through it in stages.
- Pressure testing. The system is isolated in sections where the pipework allows, and each section is pressurised and watched. The section that will not hold pressure contains the leak. This narrows a whole house to one floor or one circuit before anything else is done.
- Thermal imaging. With the heating running, every pipe under the floor shows as a warm line on the camera. A leak shows as a bloom of heat spreading away from the line where hot water is wetting the screed or the joist. On a suspended floor the image is taken from below where there is access, or from the room above through the boards. This is the fastest way to find a pipe in a screed nobody has a drawing of.
- Tracer gas. Where imaging is ambiguous, the drained section is charged with a safe hydrogen and nitrogen mixture that escapes at the leak and is detected at the surface with a sensitive probe. It works through tiles, screed and floorboards.
- Acoustic listening. Useful on a pressurised circuit where the leak is large enough to hiss, and for confirming the exact point once imaging has found the area.
The output is a mark on the floor and a written report naming the circuit, the location and the method used. The plumber then lifts one board or cuts one small section of screed. That is the difference a survey makes on a solid floor: a single opening rather than the floor coming up room by room. It is also what trace and access cover in most buildings policies is designed to pay for, alongside making good the access afterwards. The pipe repair itself is normally excluded, but the finding, the opening and the reinstatement are commonly covered.
Two things to do this week
- Log the pressure. Read the gauge cold every morning for a week without topping up, and note whether the heating ran the previous day. A steady fall regardless of use points to a pipe; a fall only after hot running points to the vessel and relief valve.
- Look at the relief pipe outside. Find where the small copper pipe from the boiler comes through the wall and check whether it is wet, stained or dripping. Photograph it. That single observation often saves a survey visit, or focuses one.
Topping up every fortnight is not harmless in the meantime. Each refill brings fresh oxygenated water into the system, which speeds up corrosion in radiators and dilutes the inhibitor. If the leak is in a screed the escaping water is also wetting the floor structure and possibly a wall, and that damage is the part that becomes expensive. We provide leak detection across Cornwall, and a heating pipe in the floor of a 1970s bungalow in Newquay or under the slate floor of a farmhouse on the edge of Bodmin Moor is found the same way.
Frequently asked questions
Does losing boiler pressure always mean a leak?
No. A failed expansion vessel or a pressure relief valve that will not reseat both cause falling pressure without any pipe leaking. The pattern gives it away: pressure high or the relief pipe running when hot, low when cold. Have those two components checked before assuming the pipework has failed.
Why does the water meter not show a heating leak?
A sealed heating system is filled through the filling loop and then closed off, so it does not draw mains water while running. Water escaping from a radiator pipe never passes through the meter. The only meter for a heating leak is the pressure gauge on the boiler, which is why logging it matters.
Can a heating leak be found under a solid concrete floor?
Yes. With the heating running the pipes show as warm lines on a thermal camera, and a leak shows as heat spreading from the line into the screed. Tracer gas and acoustic listening confirm the exact point. The floor is then opened in one small place rather than dug up in search of the pipe.
Is it safe to keep topping the system up?
For a short while, but not as a habit. Every refill introduces fresh oxygen, which corrodes radiators and weakens the inhibitor, and the escaping water is damaging whatever it lands on. If you are topping up more than once a month, treat it as a leak to be found rather than a routine.
Will trace and access cover a central heating leak?
Usually, where the policy includes it. Trace and access pays for locating the leak and making good the floor or wall opened to reach it, up to the policy limit. Repairing the pipe itself is normally excluded, as is a boiler component such as the vessel or relief valve. Check your own wording.
Could the leak be in the boiler itself?
It can, and a heating engineer checks that during a service: the heat exchanger, the pump seals and internal joints. If the boiler casing is dry inside and the relief valve and vessel are sound, the leak is on the system pipework or radiators, which is where leak detection takes over.
Topping up again?
We pressure test the system, image the pipes under the floor and mark the point where the water is escaping, so the repair is one small opening. The report supports a trace and access claim on your buildings policy.
