A boiler that keeps losing pressure usually has a small leak somewhere in the heating system, a faulty pressure relief valve, or an expansion vessel that has lost its charge. Bleeding radiators also lowers the pressure. You can safely top it up yourself using the filling loop, but if it keeps dropping, the cause needs finding by an engineer.
What should my boiler pressure be?
Most combi and system boilers like to sit at around 1 to 1.5 bar when the heating is cold. It rises a little when the heating is on, often towards 2 bar, which is normal. Check your boiler’s manual for its exact range. Below about 0.5 to 1 bar, many boilers stop working and show a low-pressure fault code. Close to 3 bar, the pressure relief valve will open to protect the system.
How to repressurise a combi boiler
- Turn the heating off and let the system cool down, so you get a true reading.
- Find the filling loop, usually under the boiler: either a flexible silver hose with a valve at each end, or a built-in key or lever on the boiler itself.
- Open the valves slowly (or follow your boiler’s built-in filling instructions) and watch the gauge rise.
- Close the valves when it reaches about 1 to 1.5 bar. Don’t overfill.
- If the boiler shows a fault code, follow the manual’s reset instructions.
If your filling loop is a detachable hose, it’s normal to disconnect it afterwards.
Common causes of pressure loss
- A small leak on a radiator valve, a joint, or pipework under the floor. Look for damp patches, rust marks, staining on ceilings, or drips under the boiler.
- Recently bled radiators. Letting air out lets pressure out too, so you’ll need a top-up afterwards.
- The pressure relief valve. If it’s letting by, you may see drips from a small copper pipe on an outside wall near the boiler.
- The expansion vessel. If it has lost its air charge, the pressure jumps up when the heating is on, the relief valve opens, and the pressure falls again when it cools. This is a job for an engineer.
- A leak inside the boiler. Only a Gas Safe registered engineer should open the boiler casing.
What if the pressure is too high?
A reading well above 2 bar when the system is cold usually means the filling loop has been left open, or is letting water through slowly. Check it’s fully closed. You can bring the pressure down a little by bleeding a radiator into a cloth. If it keeps rising, or creeps up towards 3 bar when the heating is on, call an engineer: it can point to a faulty filling valve or expansion vessel.
When to call a professional
Call a Gas Safe registered engineer if you need to top up more than once every few weeks, if you can see water around the boiler or the relief valve pipe dripping outside, if the pressure shoots up when the heating comes on, or if the boiler shows a fault you can’t reset. Never take the boiler casing off yourself. If you ever smell gas, call the National Gas Emergency number on 0800 111 999.
Quick questions
Is it dangerous if my boiler loses pressure?
Low pressure isn’t usually dangerous. The boiler simply stops working to protect itself. But the water is going somewhere, so a repeating drop should be checked before it causes damage.
Why does my boiler lose pressure but there’s no visible leak?
It may be a leak hidden under a floor or behind a radiator, a pressure relief valve letting water out through the pipe outside, or a faulty expansion vessel. An engineer can test each in turn.
How often should I need to top up my boiler?
Ideally rarely. Topping up after bleeding radiators is normal. Needing to top up every week or two isn’t.
Does a boiler service check the pressure?
Yes. Our boiler service includes checking the system pressure and the expansion vessel, which is one of the most common causes of pressure loss.
This guide is general advice for simple checks. Never attempt work on gas appliances yourself: by law, gas work must be done by a Gas Safe registered engineer. If you smell gas, call the National Gas Emergency number on 0800 111 999.