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Repressurising a boiler

Checked against its sources: Updated

To repressurise a boiler you let mains water in through its filling device, watch the gauge, stop at the figure in your own user manual, then close and disconnect the device.1 The device and the figure vary by model.2

Covers
Sealed systems, UK. Not for open-vented systems, which refill from a loft tank.
You
Topping up with the filling device, where your own user manual describes it. Nothing inside the casing.
A professional
Repeated top-ups, leaks, a device that will not close, any boiler where the manual sends you to a competent person.

1 Before you start

Find your boiler on the boiler pressure table, or open its manual, so you know the figure to stop at. Your installer should have shown you where the filling device is3. Vaillant's 2023 ecoTEC plus manual asks that the competent person explains it first6. Where a guide only points to a video, as Baxi's current quick start guides do, use that video and do not borrow steps from another model7.

Advice on the boiler's state differs. Vaillant's website says to turn the boiler off and let the system cool1. Worcester's 4000 manual says to ensure the central heating is on, to aid filling3. Follow your own manual.

2 Which device do you have

Filling devices, from named manuals
DeviceHow to spot itNamed in
Keyless linkLever under the boiler, pulled while fillingWorcester 4000, i ErP38
Keyed linkRemovable key with padlock symbolsWorcester 4000, i ErP38
External hose loopTwo valves with a hose between themWorcester i ErP; Main; Ideal892
Spring-loaded linkGreen lever under the boilerBaxi Platinum Easy-Fill4
Double non-return set-upFill line at a cold-water valveVaillant ecoTEC plus 20236
Automatic fillOptional accessory set by the installerWorcester 40003

3 Steps by named manual

These are in plain wording, for those models only. Other boilers differ.

Worcester Greenstar i ErP, keyless link

  1. Pull the blue lever down to start filling, if the gauge reads under 1 bar8
  2. Watch the gauge on the facia8
  3. Release the lever between 1 and 1.5 bar8

Worcester Greenstar i ErP, keyed link

  1. Push the key in with its arrow on the open padlock, then turn it right to the stop8
  2. Turn the white knob left to let water in8
  3. Between 1 and 1.5 bar, turn the knob right to close, without over-tightening8
  4. Turn the key back left and pull it out8

Main Eco Compact Combi, temporary loop

  1. Connect the filling pipe between the two taps and tighten its nuts9
  2. Fully open one tap, then carefully open the second while watching the gauge9
  3. Turn both off between 1 and 1.5 bar9
  4. Disconnect the pipe, keep it safe and refit any caps9

Ideal Logic+, 2014 issue

  1. Check the loop valves are closed, take off the left cap and attach the loop2
  2. Open the valves and wait for 1 to 1.5 bar2
  3. Close the valves, disconnect the loop at the left, and refit the cap2

4 Afterwards

Worcester says the key must come out or the external loop be disconnected, to meet water regulations; more under filling loop3. Main says its boiler then resets automatically once the correct pressure is restored9. Bleeding radiators lowers pressure again, so Baxi's glossary says to recheck the gauge after doing so10.

A little water from a disconnected hose is normal in Worcester's and Main's descriptions911.

5 When to stop

Baxi says a regular need to top up indicates a fault or leak, and to seek advice from the installer4. Vaillant's website says that if pressure drops as soon as the valve is closed, a significant leak is likely, and not to keep trying1. Also close the loop and call an engineer if the gauge does not rise5. See pressure keeps dropping. If you overfill, see pressure too high.

6 Where maker pages and manuals disagree

Worcester's web page says to stop at about one bar, while its manuals give 1-1.5 bar or the green zone; use the manual118. Glow-worm's site allows a competent user to top up to 1-1.5 bar "at your own risk"12.

From the VaillantUK channel on YouTube. Vaillant shows topping up the pressure with the built-in filling loop under the boiler by hand to 1.2 to 1.5 bar, and it applies only to the ecoTEC plus; other boilers have different loops and targets. Nothing loads from YouTube until you press play.

Sources

  1. Vaillant, how to repressurise a boiler read 3 November 2026. Web page.
  2. Ideal Logic+ Combi user guide UIN 204979 A07, p11, May 2014, merchant-hosted, read 3 November 2026.
  3. Worcester Greenstar 4000 manual 6720891170, pp8-9, 2020/09, read 3 November 2026.
  4. Baxi Platinum Combi user guide 7697821-01, p6, 1/18, read 3 November 2026.
  5. Vaillant, what if my boiler has no pressure read 3 November 2026. Web page.
  6. Vaillant ecoTEC plus instructions 0020308119_03, pp14-15, 28.11.2023, read 3 November 2026.
  7. Baxi 600 Combi 2 quick start guide 7876395-01, p10, read 3 November 2026.
  8. Greenstar i ErP user instructions 6720806946, pp19-21, 2015/03, read 3 November 2026.
  9. Main Eco Compact guide 7716571-04, p18, read 3 November 2026.
  10. Baxi's glossary read 3 November 2026.
  11. Worcester boiler pressure page read 3 November 2026. Web page.
  12. Glow-worm boiler fault codes page read 3 November 2026. Web page.