Why is my boiler losing pressure?

30 August 2023

Why is my boiler losing pressure?

A boiler works by circulating hot water through pipes and radiators. In domestic systems, the water that’s circulating around the radiators or cylinder is never anywhere near boiling point, however. It’s more likely to be at around 60–70 °C, and water coming out of the hot taps will be at around 50 °C.

One common issue that homeowners often encounter is a loss of pressure in their boiler system. This can be a frustrating problem, but understanding the reasons behind it can help to resolve the issue effectively. It’s also quite reassuring to know it’s not always a costly fix – indeed, it can sometimes be solved in minutes by doing it yourself.

That said, we’d always recommend calling out an engineer if your pressure keeps dropping, and we’re not just saying that because we’re engineers. Getting under the cover and poking around can be very dangerous, with the triple hazards of gas, scalding water and electric currents in there.

Most likely causes of pressure drops

Firstly, it is crucial to understand that a drop in boiler pressure may be a symptom of an underlying problem rather than the primary issue itself. In an ideal scenario, a boiler should maintain its pressure at a consistent level, typically between 1 and 1.5 bar. Check your boiler’s manual to find your broiler’s optimum pressure, but 99% of boilers operate around these levels.

If the pressure drops below this range, it probably indicates an issue within the system. That’s because once filled with water, the system is closed. There should be no way for water to get in or out. The same water is used again and again to heat your radiators.

Leaks in the system

One of the most common reasons for a boiler losing pressure is a water leak somewhere in the heating system. Boiler systems are complicated networks of pipes, valves and radiators, and any damage, wear and tear or poor fitting in these components can lead to leaks. Even a tiny drip of a leak may cause water to escape from the system, resulting in a decrease in pressure over time. These leaks can be difficult to detect, especially if they are hidden within walls or underneath floorboards. A qualified heating engineer can perform a thorough inspection and locate the source of any leaks.

Pressure relief valve

Another potential cause of pressure loss in a boiler is a faulty pressure relief valve. This valve is designed to release excess pressure from the system, preventing any damage due to overpressure. However, if the pressure relief valve is faulty or becomes stuck, it may release pressure more frequently than necessary, leading to a decrease in overall system pressure. In such cases, it is essential to have the valve inspected and replaced if needed by a professional.

Have you bled your radiators?

A drop in boiler pressure will almost inevitably occur due to bleeding radiators. Over time, air can accumulate within the system, causing pockets of trapped gases in the radiators. This reduces the efficiency of the heating system and can itself lead to a reduction in boiler pressure. However, by bleeding the radiators and releasing the trapped air, the system’s pressure can drop, as there’s less pressurised fluid (both liquid and gas) in the system.

Bleeding a simple DIY task that can be done without the need for professional assistance. It just requires you to open the small valve near the top of a radiator with a special key. Once you’ve completed the operations (i.e. when water starts coming out of the valve rather than gas), you will probably need to top up the water pressure using the filling loop, which puts water from the mains into your system.

Expansion vessel

A faulty expansion vessel can also be the culprit behind a loss of boiler pressure. The expansion vessel is a critical component that absorbs excess pressure within the heating system as it heats up, and maintains pressure when it cools down. It’s basically a strong rubber diaphragm in a vessel with compressed air on one side and the system’s water on the other.

If the vessel fails, it may no longer be capable of performing its function, resulting in pressure fluctuations. A heating engineer can examine the expansion vessel and replace it if necessary to restore proper functioning and pressure stability.

No single cause

A boiler losing pressure can have various reasons. Water leaks, faulty pressure relief valves, air trapped in radiators or faulty expansion vessels can all contribute to a drop in pressure. It might even simply be because the boiler is old and its pump and other components are worn out or partially blocked. Identifying the underlying cause is crucial in effectively resolving the issue. We recommend seeking professional help from a qualified heating engineer if bleeding and topping up doesn’t work permanently.

We can inspect and diagnose the problem accurately and provide the necessary repairs or maintenance. Annual servicing, regular maintenance and monitoring of the boiler system helps prevent pressure loss and ensures efficient and reliable operation for years to come.