A boiler showing low pressure is one of the most common reasons people call us out — and in many cases it's something you can resolve yourself in a few minutes without needing an engineer. This guide walks you through exactly what to do, what the pressure should read, and when low pressure is a sign of something more serious that does need professional attention.
What Pressure Should My Boiler Be At?
Most combi and system boilers should operate at between 1 and 1.5 bar when the heating is cold (not running). Some manufacturers specify a slightly different range — always check the label on your boiler or the manual if you have it — but 1 to 1.5 bar is the standard for the vast majority of domestic boilers in the UK.
When the heating is running and the system is hot, the pressure will naturally rise slightly — typically to around 2 to 2.5 bar. This is normal and nothing to worry about. If the pressure rises above 3 bar when hot, or above 2 bar when cold, that's too high and also needs looking at.
Low pressure is generally anything below 1 bar. Most modern boilers will display a fault code or a warning light when the pressure drops too low, and many will lock out entirely and stop firing until it's been corrected.
What Causes Low Boiler Pressure?
The most common cause is simply that the system has lost a small amount of water over time — this can happen gradually through micro-leaks at radiator valves, pipe joints or the boiler itself, or occasionally through a faulty pressure relief valve. It can also happen after bleeding radiators, which releases a small amount of water from the system along with the air.
Losing pressure occasionally and needing to repressurise once or twice a year is not unusual. If you find yourself topping the pressure up frequently — say, every few weeks — that points to a leak somewhere in the system that should be investigated properly.
What You'll Need
Most boilers are repressurised using the filling loop — a short braided metal hose that connects the boiler to the mains cold water supply. On some boilers the filling loop is permanently fitted inside the boiler casing; on others it's a separate flexible hose that you connect temporarily to two valves underneath the boiler.
You'll also need to be able to see the pressure gauge — either the analogue dial on the front of the boiler or the digital display, depending on your model.
How to Repressurise Your Boiler — Step by Step
Step 1 — Turn the boiler off and let it cool
The boiler should be switched off and cool before you repressurise it. Adding cold mains water to a hot system can cause thermal shock. Give it at least 30 minutes after the heating has been off.
Step 2 — Find the filling loop
Look underneath your boiler. The filling loop is usually a silver or grey braided hose with a valve at each end. On some Worcester Bosch models there's a built-in filling key on the front of the boiler. If you're not sure what you're looking for, your boiler manual will show the location — or give us a call and we can talk you through it.
Step 3 — Connect the filling loop (if it's a separate hose)
If your filling loop isn't permanently connected, attach it to both valves now by screwing the ends on hand-tight. Both valves should currently be closed — the slots on the valve heads will be at right angles to the pipe.
Step 4 — Open the valves slowly
Open both valves by turning them a quarter turn so the slots align with the pipe. You should hear water flowing into the system. Watch the pressure gauge carefully.
Step 5 — Stop at 1.5 bar
As soon as the gauge reaches 1.5 bar, close both valves. It's important not to overfill — too much pressure can cause the pressure relief valve to discharge, which means water will drip from a pipe outside the property (usually through an external wall). If this happens, don't panic, but do call an engineer.
Step 6 — Disconnect the filling loop
If it's a separate hose, remove it now and put it somewhere safe. It's worth keeping it near the boiler for next time.
Step 7 — Switch the boiler back on
Turn the boiler on and check that it fires up correctly. If it was locked out due to low pressure, it should now reset automatically — though on some models you may need to press a reset button. The pressure should hold steady as the boiler runs.
When to Call a Professional Instead
While repressurising your boiler is a straightforward task, there are situations where you should call an engineer rather than attempting it yourself:
- The pressure keeps dropping. If you're topping the system up regularly, there's a leak somewhere that needs finding and fixing. Repeatedly repressurising without addressing the cause isn't a solution.
- You can't find or identify the filling loop. Don't experiment with valves or connections you're not sure about — give us a call.
- The pressure won't hold after repressurising. If the pressure drops back below 1 bar within a day or two of topping it up, that's a sign of a more significant issue.
- The pressure is too high. If the gauge reads above 3 bar, do not attempt to repressurise further. The pressure relief valve may need replacing or there may be an expansion vessel fault.
- The boiler still won't fire after repressurising. Low pressure may not have been the only problem. If the boiler shows a different fault code after you've topped the pressure up, it needs a proper diagnosis.
If you're in Stourbridge or the surrounding area and you'd like us to take a look, give us a call on 07961 169 681 or drop us a message on WhatsApp. We'll diagnose the problem and give you a clear, upfront price before doing anything.