Why Does My Boiler Keep Losing Pressure?
If you find yourself topping up your boiler pressure every few days or weeks, you almost certainly have a leak somewhere in your central heating system. A sealed heating system, which is the type used with combi boilers and most modern system boilers, is a closed circuit. Water does not evaporate or disappear from it under normal operation. If pressure is dropping, water is escaping somewhere.
Many London homeowners live with this problem for months or even years, topping up the pressure via the filling loop as part of their routine. This is not harmless. Constantly introducing fresh oxygenated water into the system accelerates corrosion of radiators and pipes, leads to sludge build-up, and can eventually cause boiler components to fail. More importantly, the water has to go somewhere, and it may be silently damaging your property.
Common Causes of Central Heating Leaks
Central heating leaks in London properties tend to occur at predictable locations:
Pipe Joints and Connections
Compression fittings, solder joints, and push-fit connections can all fail over time. Thermal expansion and contraction as the heating cycles on and off puts repeated stress on joints. In London's older properties, original copper pipework from the 1960s and 1970s may have joints that have become loose over decades of thermal cycling.
Pipes in Concrete Floor Slabs
Many London properties, particularly those built between 1950 and 1990, have central heating pipes buried in the concrete floor slab. These pipes are subject to corrosion from the outside (where the concrete contacts the copper) and are impossible to inspect visually. Leaks in slab-buried pipes are among the most common and most difficult to find without specialist equipment.
Radiator Connections and Valves
Thermostatic radiator valves (TRVs) and lockshield valves are common leak points. The small sealing washers inside these valves deteriorate over time, and the compression connections at the base can weep. These leaks are often so slow that the water evaporates before it drips, making them difficult to spot visually.
Radiator Pinhole Leaks
Internal corrosion caused by sludge and dissolved oxygen creates tiny pinhole leaks in radiator panels. These may only weep when the system is hot and the metal expands. You might notice a small rust stain on a radiator but no visible drip.
Pipes Passing Through Walls
Where heating pipes pass through walls, slight building movement over time can stress the pipe, particularly at the point where it enters the wall. These leaks often go unnoticed because the water tracks into the wall cavity and is absorbed before it becomes visible.
Why DIY Methods Rarely Work
The first thing most people do when their boiler loses pressure is call a plumber. With respect to plumbers, most are not equipped to find hidden heating leaks. A typical plumber will check the visible pipe connections and radiator valves, and if they cannot see a drip, they may suggest a costly and disruptive approach like lifting all the floors or re-running the entire heating system.
DIY leak sealant products (sometimes called system sealers) are available and are added to the heating water. These products contain particles that are supposed to find and seal small leaks from the inside. While they can work as a temporary fix for very small leaks, they are not a reliable long-term solution and can cause problems by blocking valves, heat exchangers, and narrow pipe sections.
How Specialists Find Heating Leaks
Professional heating leak detection uses a combination of methods to find the source without unnecessary disruption:
Thermal Imaging
A FLIR thermal camera is used to scan floors, walls, and ceilings. Heating pipes running through a concrete slab will show as warm lines on the thermal image. A leak will appear as an anomalous warm spot that does not follow the expected pipe route, or as a larger heated area where hot water has spread into the surrounding screed.
Pressure Testing
The engineer will isolate different sections of the heating system and test each one independently. By closing valves and monitoring pressure on isolated sections, they can determine which part of the circuit is leaking. This narrows down the search area significantly.
Acoustic Detection
If the system can be pressurised to a sufficient level, acoustic listening equipment can detect the sound of water escaping from the pipe. This works particularly well on copper pipes in concrete where the pipe material conducts sound effectively.
Tracer Gas
For very difficult leaks, the heating system can be drained and filled with a tracer gas mixture, typically a blend of hydrogen and nitrogen. This gas escapes through the leak point and rises to the surface, where it is detected by a sensitive gas detector moved across the floor. This method is extremely accurate and can find leaks that other methods miss.
The Detection Process Step by Step
- Initial assessment — the engineer checks the boiler, expansion vessel, pressure relief valve, and all visible pipework to rule out obvious causes
- System isolation — different zones of the heating circuit are isolated and pressure tested to identify the leaking section
- Non-invasive survey — thermal imaging, acoustic listening, and moisture metering are used to locate the leak point without opening up
- Confirmation — the suspected leak point is confirmed using a combination of methods before any opening up work is recommended
- Report — a detailed report with thermal images and findings is provided, suitable for insurance claims
Repair Options
Once the leak is found, the repair approach depends on its location:
- Accessible pipe joints — can often be repaired by a plumber by tightening, resoldering, or replacing the fitting
- Pipes in concrete slabs — the leaking section can be abandoned and a new pipe run on the surface or through a less disruptive route. This avoids the need to break out large areas of concrete
- Radiator leaks — the radiator is typically replaced. Individual radiators are relatively inexpensive compared to the investigation cost
- Valve leaks — valves can usually be repacked or replaced with minimal disruption
Cost of Heating Leak Detection in London
A specialist central heating leak detection survey in London typically costs between 250 and 500 pounds, depending on the size of the property and the complexity of the system. This is almost always cheaper than the alternative of opening up floors speculatively or re-plumbing the entire system. Many London home insurance policies cover this cost under their trace and access provisions, so check your policy before paying out of pocket.
