Boiler Losing Pressure: Causes, Fixes and When to Call a Plumber in London

Why Boilers Lose Pressure

A sealed central heating system is designed to hold a stable water pressure — typically between 1 and 1.5 bar when cold. That pressure is set when the system is first filled, and in a healthy, properly sealed circuit it should remain largely constant for months or years at a time. When pressure drops regularly, something is allowing water to escape the sealed circuit.

The heating system in most London homes — whether a Victorian terrace in Hackney, a converted mansion block in Kensington, or a new-build flat in Stratford — consists of the boiler itself, a network of copper or plastic pipework, one or more radiators, and ancillary components including the expansion vessel, the pressure relief valve (PRV), and the filling loop. A fault in any one of these components can cause pressure to fall.

It is important to understand the difference between a system that loses pressure slowly over months (which may be within normal tolerances) and one that loses pressure quickly — falling 0.5 bar or more within days or weeks. The latter always indicates a fault that needs investigation. If your boiler has stopped working entirely, low pressure is one of the most common causes and the first thing to check.

Most Common Causes of Boiler Pressure Loss in London Homes

1. Leaking radiator valves

Radiator valves — both the lockshield valve at the return end and the thermostatic radiator valve (TRV) at the flow end — are among the most frequent sources of pressure loss in London heating systems. The valve bodies contain rubber seals and O-rings that harden and crack over time, particularly in the capital's older housing stock where original 1970s and 1980s radiator valves may still be in service.

The leak is often so small that it evaporates before it reaches the floor, leaving only a faint white mineral deposit or rust staining around the valve body as evidence. In older Edwardian and Victorian terraces across boroughs like Islington, Lambeth, and Southwark, corroded valve bodies can weep imperceptibly for months while steadily bleeding system pressure.

2. Leaking pipework joints

Compression fittings and solder joints in copper pipework can develop slow weeps as they age. This is particularly common in London homes built before 1980, where original lead or early copper pipework may have been subject to decades of thermal cycling — the repeated expansion and contraction of pipes heating and cooling every day. Under concrete floors, behind plasterboard walls, or inside kitchen and bathroom boxing-in, these leaks may be entirely invisible until the damp finally makes itself known through staining or peeling plaster.

3. Faulty pressure relief valve (PRV)

The pressure relief valve is a safety device fitted to the boiler — its job is to open and discharge water if system pressure rises dangerously high (typically above 3 bar). PRVs have a limited lifespan. Over time they can become stuck partially open, allowing a continuous trickle of water to discharge even at normal operating pressure. This water exits through a small copper pipe that typically terminates outside the building at low level, and in many London flats and terraced houses this discharge point goes unnoticed for months.

A PRV that is continuously discharging is not just causing pressure loss — it is also a sign that the system may be repeatedly overpressurising, which is itself a fault that needs investigation. Never simply replace the PRV without understanding why pressure is building high enough to trigger it.

4. Boiler internal leak

The boiler itself contains numerous seals, O-rings, and heat exchanger connections that can leak. A slow internal weep may evaporate from the warm boiler casing before reaching the floor, but it will steadily deplete system pressure. If you notice water staining on the floor directly beneath the boiler, rust marks on the boiler casing, or a musty smell near the unit, suspect an internal leak.

In older boilers — particularly those more than 10–12 years old — the primary heat exchanger can develop hairline cracks as a result of thermal stress, London's hard water limescale build-up, or corrosion from an uninhibited system. A cracked heat exchanger is a serious fault: combustion products can mix with the heating water, and the component is expensive to replace. In many cases a cracked heat exchanger makes replacement of the entire boiler more economical than repair.

5. Air in the system after bleeding radiators

Bleeding a radiator releases trapped air — but it also releases a small volume of water. If you bleed multiple radiators without adding water back to the system, pressure can drop noticeably. This is normal and expected. Topping up the system pressure via the filling loop after bleeding is a straightforward task most homeowners can do themselves.

However, if your radiators regularly need bleeding, that air has to come from somewhere. Air ingress most commonly indicates a micro-leak drawing air in at a low-pressure point, a corroding system where hydrogen gas is being generated, or a faulty auto air vent. Repeated need to bleed radiators warrants investigation — it should not be an annual routine.

6. Filling loop left open

Some boilers have a permanently connected filling loop (a flexible hose or built-in valve assembly that connects the mains cold water supply to the heating circuit). If the filling loop valve is left slightly open — or its internal seal fails — mains water can trickle into the system continuously, which paradoxically causes pressure to fluctuate wildly rather than stabilise. If your boiler pressure is erratic rather than steadily falling, check whether the filling loop valves are fully closed.

How to Find a Boiler Leak

Finding the source of a pressure loss is more important than topping up the pressure. If you repressurise the boiler without fixing the leak, the pressure will simply fall again — and you will be diluting the corrosion inhibitor in the system with every fill, accelerating internal corrosion of the pipework and radiators.

Step-by-step leak investigation

1. Check visible pipework around the boiler. Look for damp patches, rust staining, or white mineral deposits on the floor, wall, or boiler casing itself. Run your hand along the underside of visible pipes — even a very slow drip will leave the pipework surface damp to the touch.
2. Inspect every radiator valve in the property. Check both the TRV and lockshield ends. Look for discolouration, white calcium deposits, or damp on the floor below each valve. Check the valve body where it connects to the radiator tail.
3. Find the PRV discharge pipe. On most London properties this is a 15 mm copper pipe that exits through the outside wall at low level, often near the boiler or in the airing cupboard. Check whether water is dripping or trickling from this pipe — if it is, the PRV is either discharging (overpressure) or faulty.
4. Check the filling loop. Ensure both valves on the filling loop are fully closed. If the loop is permanently connected, confirm there is no slow trickle through the loop by temporarily isolating it and monitoring pressure.
5. Repressurise to 1.2 bar and monitor closely. Note the date and time. Check the gauge again after 24 hours, 48 hours, and one week. A drop of more than 0.3 bar in a week confirms an active leak that a Gas Safe engineer should locate.

If you discover a burst or fractured pipe rather than a slow weep, turn off the water supply immediately using the stopcock. In London flats, locating the stopcock can sometimes be challenging — our guide on isolating water in a London flat covers the most common scenarios.

Expansion Vessel Faults: The Misdiagnosed Cause

The expansion vessel is a sealed steel tank — typically located inside or immediately adjacent to the boiler — that absorbs the increase in water volume as the heating system warms up. Inside the vessel is a rubber bladder or diaphragm under nitrogen gas pre-charge. When water in the circuit expands due to heat, it compresses into this vessel against the nitrogen charge, preventing pressure from rising dangerously high.

Over time, two things go wrong with expansion vessels. First, the nitrogen pre-charge gradually escapes, leaving the vessel unable to absorb expansion effectively. Second, the rubber bladder can fail entirely, allowing water to fill the entire vessel and removing its buffering capacity. When this happens, pressure in the system behaves erratically — rising sharply when the boiler fires (as hot water expands with nowhere to go), triggering the PRV to discharge, and then dropping to low pressure when the system cools. This pattern — pressure spiking high then falling low, repeatedly — is the diagnostic signature of expansion vessel failure.

How to check if the expansion vessel has failed

On most combi boilers, the expansion vessel has a Schrader valve (identical to a car tyre valve) accessible from outside the boiler casing. With the system cold and depressurised, pressing the valve pin should release a brief puff of air — the nitrogen charge. If water comes out, the bladder has failed and water has filled the vessel. If nothing comes out, the nitrogen charge has escaped.

Expansion vessel failure is frequently misdiagnosed as a leak, because the end result — dropping system pressure — looks identical. The key diagnostic difference is the pressure pattern: a leak causes steady, gradual decline; a failed expansion vessel causes pressure to rise then fall with every heating cycle. If your boiler pressure is unpredictable rather than steadily declining, suspect the expansion vessel before assuming a leak.

Expansion vessels in older London properties

In older open-vented systems — common in Victorian and Edwardian properties across Hackney, Camden, and Kensington that have not been converted to sealed pressurised systems — there is no expansion vessel. Instead, a cold water feed-and-expansion tank in the loft provides the buffer. These systems do not have a pressure gauge and work on a gravity-feed principle. If you have a tank in your loft feeding the heating circuit, the advice in this article does not directly apply to your setup — consult a Gas Safe engineer for specific guidance.

When to Call a Gas Safe Engineer

Many homeowners in London are understandably reluctant to call out an engineer for what seems like a minor pressure issue — particularly when topping up the pressure takes only a few minutes and the boiler fires immediately afterwards. However, there are clear situations where a professional inspection is not optional.

Call a Gas Safe engineer if:

• You are topping up boiler pressure more than two or three times per year
• Pressure drops to zero or below 0.5 bar within 24–48 hours of topping up
• You can see water actively dripping from the PRV discharge pipe outside the building
• You notice water staining, damp patches, or rust marks on the boiler casing or immediately below it
• Pressure rises sharply when the boiler fires and the PRV activates — this suggests a failed expansion vessel and overpressure situation
• You can smell damp or see mould forming on walls near pipework runs — indicating a concealed leak
• The boiler is more than 10 years old and showing repeated pressure faults — a heat exchanger inspection is warranted

If the situation involves water visibly flowing or spurting — rather than a slow weep — treat this as an emergency. Knowing when a plumbing situation requires emergency attendance can save significant damage to your property. For London properties where water is flowing uncontrolled, isolating the water supply is the priority — our guide on finding and operating your stopcock explains how to do this quickly.

What an engineer will do

A Gas Safe engineer attending a boiler pressure fault will typically: verify the drop rate by monitoring pressure over a defined period; systematically inspect all visible pipework, joints, and valves; test the PRV and expansion vessel; carry out a pressure test using nitrogen if the source is not visually apparent; and advise on repair options. Most straightforward pressure faults — leaking valves, failed PRV, expansion vessel recharge — are diagnosable and repairable in a single visit.

Repair Costs in London

Boiler pressure repair costs in London vary depending on the cause and complexity. The table below gives typical market rates for Gas Safe registered engineers in the capital in 2026. For a broader view of what emergency plumbing attendance costs, see our emergency plumber cost guide.

Trusted Plumber London charges a £60 call-out fee with no additional call-out charge on top of the labour rate for jobs that run over two hours. Labour is charged at £95 per hour with most standard pressure repairs completed within two hours. Trusted Plumber London's engineers carry all Gas Safe ID and are fully insured — always ask to see these before any gas work proceeds.

For ongoing boiler health, particularly in London's hard-water zones, a power flush every five to seven years removes the magnetite sludge and limescale that accelerates seal failure and corrosion — the same corrosion that causes many of the leaks described in this article. If you have not had a power flush in more than eight years and are experiencing repeated pressure issues, a flush and inhibitor top-up can address multiple underlying problems in a single service visit.

Frequently Asked Questions