A boiler should sit at about 1 to 1.5 bar when the heating is off. Pressure loss is often harmless: recently bled radiators, a faulty expansion vessel, or the pressure relief valve letting water out. But if you are topping it up every week or month and there is no obvious cause, a hidden leak in the pipework is the usual reason, and that needs finding, not just refilling.
Watching the pressure gauge creep down again is one of the more annoying parts of owning a combi boiler. Sometimes it is nothing. Sometimes it is a sign that water is escaping somewhere you cannot see. Here is how to tell the difference, and what to do about it.
What pressure should a boiler be?
For most combi boilers, the gauge should read around 1 to 1.5 bar when the heating is off, usually the green zone on the dial. It rises a little when the heating is running and settles back down afterwards. That movement is normal. A reading that keeps falling below the green, so the boiler cuts out or shows a low-pressure fault, is what you are trying to explain.
The common causes
- Recently bled radiators. Letting air out of the radiators also lets pressure out of the system. This is the most harmless cause: just top the pressure back up and you are done.
- A faulty expansion vessel. This part absorbs the pressure changes as water heats and cools. When it fails, the pressure can swing, high when hot, low when cold. A heating engineer can recharge or replace it.
- The pressure relief valve (PRV). If system pressure climbs too high, the valve releases water through a pipe that usually runs to outside. A sticking or faulty PRV can let water out steadily, so the pressure keeps dropping.
- A leak in the system. Water escaping from a pipe, joint, radiator or valve anywhere in the heating circuit will pull the pressure down. Visible leaks are easy. The tricky ones are hidden under floors or in walls, where the only clue is the gauge.
How to top it up
If the cause is minor, repressurising is a simple job. With the boiler switched off and cool, find the filling loop (a flexible silver pipe with a valve at each end), open both valves to let cold mains water in, watch the gauge climb to between 1 and 1.5 bar, then close the valves. Your boiler manual shows where the loop is for your model. If you are not comfortable doing it, a heating engineer will sort it quickly.
When it points to a hidden leak
Here is the rule of thumb that matters most: topping up once or twice a year is normal. Doing it every month or week is not. A system that needs constant refilling is losing water somewhere, and if there is no puddle, no dripping valve and no obvious PRV discharge, that water is usually going into the floor or wall.
Watch for these alongside the falling pressure:
- A patch of floor that is warm, damp or never quite dries.
- A cold spot on a floor that should be warm, or a radiator slow to heat.
- The faint sound of running water, or a musty smell in one area.
That combination points to a leak in the heating pipework. Our guide to finding an underfloor heating leak goes into the signs in more detail, and the water leak guide covers hidden leaks generally.
When to call a specialist
If you are repressurising constantly, or you have spotted any of the signs above, the fix is to find the leak rather than keep refilling. We locate hidden heating leaks with non-invasive equipment, so only the spot over the leak is opened up, not the whole floor. See how our central heating leak detection and underfloor heating leak detection services work across Cornwall. If you suspect a fault with the boiler itself, a Gas Safe registered heating engineer should check it.
Frequently asked questions
What pressure should my boiler be?
For most combi boilers, around 1 to 1.5 bar when the heating is off, often shown as a green zone on the gauge. It rises a little when the heating is running, which is normal. Check your boiler manual for the exact figure for your model.
Is a boiler losing pressure always a leak?
No. It can be down to recently bled radiators, a faulty expansion vessel, or a pressure relief valve releasing water. But if the pressure keeps dropping and there is no obvious cause, a leak somewhere in the system, often hidden under floors, is a common culprit.
How often should I need to top up my boiler?
Once or twice a year is generally fine. If you are repressurising every month or every week, the system is losing water somewhere and it is worth investigating rather than just topping up again.
How is a hidden heating leak found?
With non-invasive equipment, thermal imaging, tracer gas and acoustic listening, that pinpoints the leak through the floor or wall. Only the small area over the leak is opened up, so you avoid lifting a whole floor on guesswork.
Boiler losing pressure with no obvious leak in Cornwall?
If you are topping up again and again, we find the hidden heating leak with non-invasive kit and give you a clear, insurance-ready report. Fast response across Cornwall.