How Do I Top Up My Boiler Pressure?

Please note, we can only give you basic advice on this page to do the job yourself. Make sure you use the right tools and protective equipment to avoid hurting yourself or damaging your home.

Reasons for low boiler pressure

There are two common reasons for your boiler to have low pressure:

1. If you’ve recently bled your radiators, releasing water from your central heating system can lead to pressure loss.

2. There may be a leak somewhere in your central heating system. Check for damp patches near your boiler, radiator and pipes. Don’t look inside the boiler unit itself – this is a job for a Gas Safe Registered engineer. If you locate a leak, an engineer will be able to fix the problem for you.

How to check if your boiler has low pressure

Look at the water pressure gauge on the front of your boiler. Different boilers will have different gauges; you can find more information about yours in the user instructions for your boiler. If you have a digital gauge, the pressure is typically represented by bars. If you have a hydraulic gauge, your reading is likely to be on a dial, where the red sections will indicate low and high pressure.

Ideally, you want to see a reading of between one and two bars. Anything below one bar is considered to be low pressure, while 2.75 bars and above is considered high pressure. (If you’re experiencing high pressure, see the steps to fix this below, however it’s best to call a Gas Safe Registered engineer to resolve the issue as soon as possible as this can be dangerous).

Topping up your boiler pressure

Before you re-pressurise your boiler, check your manufacturer’s instructions as every boiler is different.

If you have a filling loop, here are the basic steps:

1. Switch off your boiler and allow it to cool

2. Check the filling loop, making sure both ends are attached securely

3. Open the two valves; you should hear the cold water entering the system

4. Keep the valves open until the gauge reaching 1.5 bars

5. Close both valves, one at a time

6. Switch the boiler back on

7. Undo the filling loop and store in a safe place

If you have a re-pressure key:

1. Switch off your boiler and allow it to cool

2. Find the key and slot in the concealed tray underneath your boiler

3. Insert the key into the slot and turn to the ‘closed padlock’ symbol

4. Turn the square nut clockwise so that water flows into the boiler (you’ll be able to hear this)

5. Keep this going until the pressure reaches 1.5 bars

6. Turn the nut counter-clockwise until the water stops

7. Turn the key back to the ‘open padlock’ sign

8. Turn your boiler back on

Is your boiler pressure too high?

1. Switch off your boiler and allow it to cool

2. Check the boiler pressure gauge

3. If it’s above 2 bars, you’ll need to reduce the pressure

4. Ensure the filling loop or relief valve is tightly closed

5. Bleed your radiators to release any trapped air

6. Check to see if the boiler pressure returns to normal

If it’s still too high, the expansion vessel may need re-pressurising or there could be a fault with the filling loop. In either case, you’ll need to call a Gas Safe registered engineer who can come and fix the problem for you.