How to Top Up a Worcester Boiler With a Filling Key

Not Sure If You Have a Keyed System?

Worcester boilers typically use one of three filling systems: an external filling loop, an internal keyless (blue lever) filling loop, or an internal keyed filling loop with a removable white key.

This page applies to internal keyed filling loop systems only. These systems require a small white plastic key to be inserted and locked into position before the boiler can be topped up.

If your boiler has a blue lever underneath and does not require a removable key, follow our main Worcester boiler top-up guide instead.

Still unsure which system you have? Look underneath the boiler:
• If you see a blue lever, you have a keyless system.
• If you need to insert a white plastic key before turning a tap, you have a keyed system.

Worcester Boiler – Internal Keyed Filling Loop
(This uses a removable filling key)

Many Worcester boilers use an internal keyed filling loop. This means you’ll need to insert a small white filling key (often stored under the boiler) before you can top up the pressure.

Step 1 – Find the filling key (and the key housing)

The filling key is sometimes:

• Loose near the boiler (on a shelf / in a drawer), or

• Resting on top of the boiler casing

However, many Worcester boilers have a key housing on the decorative panel underneath the boiler.

Remove the bottom decorative panel to access it.
✅ You can’t damage the boiler by removing this panel — it’s purely a cover for appearance.

Step 2 – Remove the key and locate the key slot

Once the panel is off, you’ll normally see:

• The white filling key stored in the panel/key holder

• The key slot / orifice where it inserts (use your photo with the blue arrow here)

Take the key out and line it up with the slot.

Step 3 – Insert the key (push + twist like a bayonet bulb)

Push the key into the slot firmly.

It fits like a bayonet light bulb, so you need to:

• Push in, then

• Twist to lock

You’ll feel it seat properly — usually an extra half inch as it locks into place.

If it doesn’t feel secure, remove it and try again — don’t force it at an angle.

Step 4 – Open the white filling tap slowly (anti-clockwise)

Now turn the white square filling tap (the one you’ve marked with the blue arrow).

Turn it anti-clockwise slowly until you hear water running.

Tips:

• If you feel unsure, open it slightly, then close it again to build confidence

• Don’t rush — the pressure rise can lag slightly behind what you hear

Step 5 – Watch the pressure gauge and stop just over 1 bar

Keep your eyes on the pressure gauge as you fill.

• Aim for just over 1.0 bar (cold system)

• Do not exceed 1.5 bar

Once you hit the target:

1. Turn the white filling tap fully clockwise to close

2. Wait 2–3 seconds to ensure the needle stops rising

Step 6 – Remove the key and refit the panel

To finish:

• Twist and remove the filling key

• Put it back into its holder (so it’s there next time)

• Refit the bottom decorative panel

✅ Job done.

Pressure Note

• Do not allow the pressure to exceed 1.5 bar

• Overfilling can damage the boiler and heating system

Worcester Boiler – Internal Keyed Filling Loop
(This uses a removable filling key)

Many Worcester boilers use an internal keyed filling loop. This means you’ll need to insert a small white filling key (often stored under the boiler) before you can top up the pressure.

Step 1 – Find the filling key (and the key housing)

The filling key is sometimes:

• Loose near the boiler (on a shelf / in a drawer), or

• Resting on top of the boiler casing

However, many Worcester boilers have a key housing on the decorative panel underneath the boiler.

Remove the bottom decorative panel to access it.
✅ You can’t damage the boiler by removing this panel — it’s purely a cover for appearance.

Step 2 – Remove the key and locate the key slot

Once the panel is off, you’ll normally see:

• The white filling key stored in the panel/key holder

• The key slot / orifice where it inserts (use your photo with the blue arrow here)

Take the key out and line it up with the slot.

Step 3 – Insert the key (push + twist like a bayonet bulb)

Push the key into the slot firmly.

It fits like a bayonet light bulb, so you need to:

• Push in, then

• Twist to lock

You’ll feel it seat properly — usually an extra half inch as it locks into place.

If it doesn’t feel secure, remove it and try again — don’t force it at an angle.

Step 4 – Open the white filling tap slowly (anti-clockwise)

Now turn the white square filling tap (the one you’ve marked with the blue arrow).

Turn it anti-clockwise slowly until you hear water running.

Tips:

• If you feel unsure, open it slightly, then close it again to build confidence

• Don’t rush — the pressure rise can lag slightly behind what you hear