Why Does My Boiler Keep Losing Pressure?
A boiler that loses pressure once in a while is not unusual, but a boiler that needs repressurising every few days, or that drops to zero within hours of being topped up, is telling you something is wrong. Ignoring it rarely makes the problem go away, and in some cases a persistent pressure loss can lead to the boiler locking out completely, leaving a household without heating or hot water.
Pressure loss can stem from something as simple as a slightly weeping valve, or something more involved such as a hidden leak in the central heating pipework. Because sealed heating systems rely on maintaining a stable pressure to circulate water efficiently, understanding what is normal and what needs investigating can save you from repeated call-outs and unnecessary inconvenience.
This article explains what boiler pressure actually means, the most common reasons it drops, and when it is time to book in a proper diagnosis rather than continuing to top it up yourself.
What is normal boiler pressure?
Most domestic boilers should sit between 1 and 1.5 bar when the heating is cold, as shown on the pressure gauge on the front of the unit. This rises slightly when the heating is on due to the water expanding. If the needle regularly falls below 1 bar, or you find yourself topping up the system every week, that is a sign of an underlying issue rather than normal wear.
Hidden leaks in the system
The most common cause of gradual pressure loss is a small leak somewhere in the sealed central heating circuit. This could be at a radiator valve, a pipe joint under a floor, or within the boiler itself. These leaks are not always visible, sometimes appearing only as a faint damp patch, a small stain on a ceiling below a pipe run, or limescale build-up around a fitting. A plumber can use pressure testing and, where necessary, leak detection equipment to narrow down the source without unnecessary disruption.
Faulty pressure relief valve
The pressure relief valve is designed to release water if the system pressure gets too high, protecting the boiler from damage. Over time these valves can wear out and begin releasing water even at normal pressure, often dripping from a small pipe outside the property. If you notice water dripping from an external overflow pipe, this is usually a sign the relief valve needs replacing.
A failing expansion vessel
The expansion vessel inside the boiler absorbs the extra volume of water as it heats up and expands. It contains a rubber diaphragm and a cushion of air, and if that diaphragm fails or the air charge is lost, the vessel can no longer do its job properly, causing the system pressure to fluctuate or the relief valve to open unnecessarily. Replacing or recharging the expansion vessel usually resolves this.
Problems with the filling loop
The filling loop is the flexible hose used to top up pressure manually. If its valves do not seal properly, water can seep through very slowly, gradually lowering the system pressure without any obvious leak elsewhere. This is a common and often overlooked cause, and a straightforward one for an engineer to check during a boiler service or repair visit.
When to call an engineer
If you are topping up your boiler more than once a fortnight, or the pressure drops suddenly rather than gradually, it is time to book a repair rather than keep managing it yourself. A qualified engineer will check the whole system, from radiator valves to the boiler's internal components, and diagnose the actual cause rather than treating the symptom. For households across Wimbledon and neighbouring areas, our engineers carry out boiler repairs at £99 + VAT per hour on weekdays, £120 + VAT per hour in the evening, and £185 + VAT per hour overnight and on Sundays and bank holidays, all excluding VAT, with a one-hour minimum.
Gas boilers must only be worked on by a Gas Safe registered engineer, so please do not attempt to dismantle or adjust any gas components yourself.
Annual servicing helps catch problems early
Many pressure issues are picked up during a routine boiler service, which is why an annual check is worthwhile even if the boiler seems to be running fine. Our boiler service costs £99 + VAT on weekdays and covers up to two appliances, with £40 + VAT for each additional appliance, and a combined service and Gas Safety Certificate is available for £160 + VAT.
Need a plumber or heating engineer?
We arrange appointments across Wimbledon and south west London at clear hourly rates. Call 020 3727 2578 or request an appointment.