Boiler Losing Pressure in Bexleyheath? Causes and Fixes

What normal boiler pressure looks like

Most combi and system boilers should read between 1 and 1.5 bar when the heating is cold. When the heating is running, the water expands and it is normal for the gauge to climb to around 2 bar. Most boilers lock out and show a fault code once pressure falls below roughly 0.5 to 0.8 bar, which is usually when people notice the problem.

A single top-up after bleeding radiators is nothing to worry about. If you are topping up every week or two, something is letting water out of the system or failing to absorb its expansion.

The usual reasons pressure drops

In our experience across Bexleyheath, Welling, Barnehurst and Crayford, the cause is nearly always one of the following:

• A small leak on a radiator valve, pipe joint or towel rail. It is often just a slow weep that dries before you see it, so look for green or white crusting on valves and brown staining on carpets or skirting boards.
• A leak under the floor. Many 1930s semis around Bexleyheath have their heating pipes under suspended timber floors, and older homes may have pipes buried in solid floors, so a leak there can go unnoticed for months.
• The pressure relief valve discharging. Look for a short copper pipe coming through an outside wall near the boiler. If it drips or leaves a wet patch on the wall, the valve is releasing water.
• A flat expansion vessel. This vessel inside or next to the boiler absorbs the expansion as the water heats up. When it loses its air charge, pressure climbs too high when the heating is on and then drops once it cools, often pushing water out through the relief valve.
• Recently bled radiators. Letting air out lowers the pressure, so one top-up afterwards is expected.

Safe checks you can do yourself

Before calling anyone, it is worth spending ten minutes checking. None of these involve opening the boiler casing, which should only ever be done by a Gas Safe registered engineer.

• Write down the pressure with the heating off and cold, then check it again after 24 hours with the heating still off. A drop with no heat points towards a leak.
• Check it again with the heating running. A jump from 1.2 bar to 2.5 bar or more usually points to the expansion vessel.
• Run dry kitchen roll around every radiator valve, joint and the pipework under the boiler. Even a damp spot is useful evidence.
• Check the relief valve pipe outside for drips or staining.
• Make sure the filling loop taps are fully closed after topping up. A tap left slightly open can let pressure creep up and set off the relief valve.

Hard water and older systems in Bexley

Bexley sits on chalk and our tap water is very hard. Over the years limescale can build up in the filling loop valves and the relief valve seat, and these parts can then leak by a small amount without anyone noticing. Older systems that have never been flushed also collect sludge. This speeds up corrosion in radiators, and eventually they develop pinhole leaks.

If your system is over 15 years old and the radiators feel cold at the bottom, a powerflush or a magnetic filter may be worth considering alongside fixing the pressure problem.

What a repair is likely to involve

Costs depend on the cause, how easy it is to get to, and your boiler make. As a rough guide in the Bexleyheath area, recharging an expansion vessel is often one of the cheaper fixes, typically around £80 to £150. Replacing a relief valve or a failed vessel usually costs more, often £150 to £350 including parts. Tracing a hidden leak under floors is the hardest to price, because it depends on how much flooring has to be lifted.

A good engineer will test the vessel charge and do a pressure drop test before replacing anything, so you are not paying for parts on guesswork.