Why Does My Boiler Keep Losing Pressure? Causes and Fixes in London

Normal Boiler Operating Pressure

A sealed central heating system should maintain a stable pressure between 1 and 1.5 bar when cold and rise to approximately 2 to 2.5 bar when the heating is running at full temperature. This range is normal and is caused by the thermal expansion of water as it heats. A pressure gauge in the green zone at both cold and hot indicates a healthy system. If the pressure drops below 1 bar, most modern boilers will display a low pressure fault code and refuse to fire. Topping up the pressure by opening the filling loop until the gauge reads 1.2 to 1.5 bar will restore operation — but if the pressure drops again within a few days or weeks, the system is losing water somewhere, and simply topping up repeatedly without finding the cause is masking a fault that will eventually become more serious or more expensive to fix.

How to Repressurise a Combi Boiler

The filling loop is a flexible braided hose connecting the cold mains supply to the sealed heating system. On most London combi boilers it is located underneath the boiler, accessible through the front panel. Open both valves on the filling loop slowly — you will hear water entering the system — and watch the pressure gauge. When the needle reaches 1.2 to 1.5 bar, close both valves in sequence. Never exceed 2 bar when cold; overpressure when the system heats will cause the pressure relief valve to discharge. Some boilers have a built-in filling loop or a keyed filling system; consult the boiler manual for the specific procedure on your model.

Causes of Repeated Pressure Loss

If the system requires repressurising more than once every few months, something is allowing water to leave the sealed system. The most common causes in London properties are as follows. Leaking radiator valves — the most frequent culprit — can drip so slowly that the puddle evaporates before it is noticed, but the gradual pressure loss accumulates over weeks. Check underneath all radiator valves and around TRV (thermostatic radiator valve) bodies, particularly on older radiators in London Victorian and Edwardian properties where the valves may not have been replaced since installation. Small drips from valve glands are easily mistaken for condensation.

Pipe joint leaks in concealed or underfloor pipework are harder to find but cause the same gradual pressure loss. Adding a tracer dye compatible with your system inhibitor can help identify the source; the dye will be visible at the point of leakage under UV light when the system is pressurised and run. A pressure test with the heating isolated at 1.5 bar can confirm whether the loss is occurring when the system is running (suggesting thermal expansion stress at a joint) or consistently (suggesting a static leak).

Waterlogged Expansion Vessel

The expansion vessel is a pressurised vessel containing a rubber diaphragm that separates a cushion of nitrogen or air from the system water. As water expands when heated, the diaphragm compresses, preventing overpressure. Over time, the diaphragm can fail or the pre-charge pressure can be lost, causing the vessel to become waterlogged — entirely filled with water. A waterlogged expansion vessel cannot accommodate thermal expansion, causing the system pressure to spike during each heating cycle and the pressure relief valve to open and discharge a small amount of water. Each heating cycle the PRV discharges, the system loses a small amount of water, and the pressure when cold gradually falls. This pattern — pressure fine when cold but high when hot, with the PRV releasing — is the classic sign of a waterlogged expansion vessel.

Diagnosing a waterlogged vessel requires checking the pre-charge pressure via the Schrader valve on the vessel when the system is cold and at zero bar pressure. The pre-charge should typically match the system fill pressure — usually 0.5 to 1 bar. If the vessel releases water rather than air from the Schrader valve, the diaphragm has failed and the vessel needs replacement. Re-pressurising the vessel nitrogen charge is an option if the diaphragm is intact; a failed diaphragm requires full vessel replacement at a cost of £120 to £200 including labour.

Faulty Pressure Relief Valve

The pressure relief valve (PRV) is a safety device set to open and discharge if system pressure exceeds a safe level — typically 3 bar. Over time, PRVs can begin to weep at normal operating pressures, either because the valve seat has deteriorated or because scale from London hard water has prevented the valve from sealing fully after a previous discharge event. A weeping PRV causes slow, steady pressure loss that is easy to overlook. You can identify this by checking whether the PRV discharge pipe (which typically runs outside the property or to a drain) shows signs of water or staining. A faulty PRV should be replaced rather than adjusted; the cost is typically £80 to £150.

Microbore Pipework and Corrosion

Many London properties built or refurbished in the 1960s and 1970s have microbore pipework — 8 or 10mm copper tube running to each radiator from a larger bore manifold. In London hard water areas, microbore pipework is particularly susceptible to internal corrosion and pinhole leaks because the smaller bore creates higher flow velocities, the thin copper wall offers less corrosion resistance, and decades of hard water without inhibitor treatment accelerates scale and corrosion at fittings. Pinhole leaks in microbore pipework often occur under floorboards or within wall chases where they are not immediately visible. Specialist leak detection — thermal imaging or tracer dye in the system — is the most reliable way to locate concealed pinhole leaks.

When to Call a Heating Engineer

Repressurising the system yourself is safe and appropriate as a temporary measure. Finding the cause of repeated pressure loss requires a heating engineer in most cases. If you cannot identify a visible leak at radiator valves or pipe joints, if the pressure loss is rapid (dropping from 1.5 bar to below 1 bar within 24 hours), or if repressurising is required more than once a month, call a Gas Safe registered heating engineer. Continuing to top up without investigation risks further damage — water that escapes the sealed system is replaced by fresh water, accelerating scale and corrosion in the heat exchanger and pipework. In London hard water areas, this effect is particularly damaging.