Why Is My Boiler Losing Pressure? 7 Common Causes (and Fixes) – If you’ve noticed your heating isn’t working as it should, or your boiler keeps switching itself off, one of the first things to check is the pressure gauge. A boiler that’s losing pressure is one of the most common issues we come across at OJ Gas & Heating, and it can leave homeowners in London and the South East without reliable heating or hot water.
But what does low pressure actually mean, and why does it happen? In this guide, we’ll explore the most common causes of a boiler losing pressure, the signs to look out for, and what you can do to fix it.
Understanding Boiler Pressure
Most modern boilers are sealed systems that rely on the correct water pressure to circulate hot water efficiently. You can usually see the pressure on the front of the boiler, a small gauge with a needle. Ideally, it should sit around 1 to 1.5 bar when the heating is off, and rise slightly when it’s on. If the pressure drops below 1 bar, your boiler may struggle to operate or switch off entirely.
A sudden drop in pressure is usually a sign that something isn’t quite right. While sometimes it’s a simple fix, other times it can point to a bigger issue that needs professional attention.
Common Reasons Your Boiler Is Losing Pressure
One of the most frequent causes of pressure loss is a leak somewhere in the heating system. This doesn’t always mean water is pouring out onto your floor – often the leak is small and hidden within pipework under floors or behind walls. Over time, even a tiny leak can cause the pressure to drop. If you suspect a leak, it’s best to call a Gas Safe engineer, as finding and repairing it properly usually requires professional tools and expertise.
Another reason boilers lose pressure is a faulty pressure relief valve. This valve is designed as a safety measure, releasing excess pressure from the system. If it becomes faulty or has been triggered due to previous over-pressurisation, it can allow water to escape, lowering the pressure. In this case, the valve may need to be repaired or replaced.
Problems with the expansion vessel are also a common culprit. The expansion vessel is an internal component that helps manage changes in pressure as the water heats up and cools down. If the diaphragm inside fails, it can no longer regulate pressure effectively, leading to frequent drops. This is something that can usually be repaired, but in some cases replacement is the best option.
Bleeding radiators too often can also cause low pressure. While bleeding is useful to remove trapped air, it releases water from the system each time. If you find yourself bleeding radiators regularly and not topping up the pressure afterwards, you may gradually reduce it to a level where the boiler can no longer function properly.
Air trapped in the system itself can also lead to pressure issues. If air pockets build up in the pipework, the circulation of water is interrupted, and the gauge may show a drop. This can usually be resolved by a professional power flush or a more thorough check of the system.
Another overlooked cause is simply human error. If the filling loop, which is used to add water to the boiler, isn’t closed properly after repressurising, it can allow water to seep out and pressure to fall. Making sure valves are correctly secured is a simple but important step in keeping your boiler working as it should.
Finally, in some cases, the issue lies within the boiler itself. Faulty seals, worn parts or internal leaks can all affect pressure levels. With older boilers, these problems tend to become more frequent, and while repairs are possible, sometimes replacing the unit is the most cost-effective long-term solution.
How to Fix a Boiler Losing Pressure
If your boiler has lost pressure, the first step is to check the gauge and consult the manufacturer’s instructions for repressurising. Most modern combi boilers allow you to top up the pressure using a filling loop, a simple process of opening two valves until the gauge reaches around 1.5 bar. This is a straightforward fix and can restore heating quickly.
However, if your boiler repeatedly loses pressure even after topping it up, it’s a clear sign that something more serious is going on. Continuing to repressurise without addressing the cause can mask the issue and put strain on the system. That’s why it’s always recommended to call a Gas Safe registered engineer if the problem persists.
Book a Boiler Repair with OJ Gas & Heating
At OJ Gas & Heating, we help homeowners and landlords across London and the South East with fast, reliable boiler repairs. If your boiler is losing pressure, our qualified team can diagnose the issue, carry out the necessary repairs, and make sure your system is running safely and efficiently again.
