How to Top Up a Vaillant Boiler

What Model Do I Have?

Vaillant boilers typically use one of the following filling systems:

• Internal tap filling loop
• External filling loop

Before topping up your boiler, it’s important to identify which filling loop you have, as the steps differ slightly for each type.

Vaillant Boiler – Internal Tap Type Filling Loop 

A common setup on Vaillant boilers is an internal lever filling loop, usually operated by two small taps underneath the boiler.

Step 1

You can identify an internal filling loop by locating two valves.

🟦 Blue Flame Engineer Tip

Make sure you close both taps. If one is left open – No hot water will comefrom the hot tap, untill both are closed.

This is very common and nothing to worry about.

Also note that the taps can be difficult to see when the boiler is installed low to the ground or inside a cupboard.
Using a torch or phone light can make them much easier to locate.

Step 2 – Open the First Valve (Door)

Slowly open either one of the grey levers — it does not matter which one you open first.

Think of this like “opening the first door”
At this stage, water is allowed to travel up to the second valve, but nothing will happen yet.

You may hear a faint sound of water, but the boiler pressure will not increase at this point.

Step 3 – Open the Second Valve (Door) VERY Slowly

Now, very slowly open the second grey lever.

This is the second door.
Once this door is opened, water is allowed into the boiler and heating system.

Open this valve a tiny amount only — do not open it fully.
You should hear water running, and the pressure on the gauge will begin to rise.

Move the lever gently, as if you are just cracking a door open, not swinging it wide.

Blue Flame Tip

Opening this valve too quickly can cause the pressure to rise too fast.

Step 4 (Be very careful here)

Keep a close watch on the pressure gauge.

Once the pressure reaches just over 1 bar (do not exceed 1.5bar), close both taps.

Safety note (See gauge)

Do not overfill the pressure past 1.5 bar.
Overfilling can damage the boiler and heating system.

Vaillant Boiler – Internal Tap Type Filling Loop 

A common setup on Vaillant boilers is an internal lever filling loop, usually operated by two small taps underneath the boiler.

Step 1

You can identify an internal filling loop by locating two valves.

🟦 Blue Flame Engineer Tip

Make sure you close both taps. If one is left open – No hot water will comefrom the hot tap, untill both are closed.

This is very common and nothing to worry about.

Also note that the taps can be difficult to see when the boiler is installed low to the ground or inside a cupboard.
Using a torch or phone light can make them much easier to locate.

Step 2 – Open the First Valve (Door)

Slowly open either one of the grey levers — it does not matter which one you open first.

Think of this like “opening the first door”
At this stage, water is allowed to travel up to the second valve, but nothing will happen yet.

You may hear a faint sound of water, but the boiler pressure will not increase at this point.

Step 3 – Open the Second Valve (Door) VERY Slowly

Now, very slowly open the second grey lever / tap.

This is the second door.
Once this door is opened, water is allowed into the boiler and heating system.

Open this valve a tiny amount only — do not open it fully.
You should hear water running, and the pressure on the gauge will begin to rise.

Move the lever gently, as if you are just cracking a door open, not swinging it wide.

Blue Flame Tip

Opening this valve too quickly can cause the pressure to rise too fast.

Vaillant Boiler – External Filling Loop 

A common setup on Vaillant boilers is an external filling loop.

This is usually a silver braided hose connecting the mains cold water supply to the heating system pipework.

Step 1

You can identify an external filling loop by locating two valves:

Note: Some set-ups only have 1 lever – this is normal.

Step 2 – Open the First Valve (Door)

Slowly open either one of the black levers — it does not matter which one you open first.

Think of this like “opening the first door”
At this stage, water is allowed to travel up to the second valve, but nothing will happen yet.

You may hear a faint sound of water, but the boiler pressure will not increase at this point.

Step 3 – Open the Second Valve (Door) VERY Slowly

Now, very slowly open the second black lever.

This is the second door.
Once this door is opened, water is allowed into the boiler and heating system.

Open this valve a tiny amount only — do not open it fully.
You should hear water running, and the pressure on the gauge will begin to rise.

Move the lever gently, as if you are just cracking a door open, not swinging it wide.

Blue Flame Tip

Opening this valve too quickly can cause the pressure to rise too fast.

Step 4 (Be very careful here)

Keep a close watch on the pressure gauge.

Once the pressure reaches just over 1 bar (do not exceed 1.5bar), close both taps.

Safety note (See gauge)

Do not overfill the pressure past 1.5 bar.
Overfilling can damage the boiler and heating system.

Vaillant Boiler – External Filling Loop 

A common setup on Vaillant boilers is an external filling loop.

This is usually a silver braided hose connecting the mains cold water supply to the heating system pipework.

Step 1

You can identify an external filling loop by locating two valves.

Note:  Some set-ups only have 1 lever – this is normal

Step 2 – Open the First Valve (Door)

Slowly open either one of the black levers — it does not matter which one you open first.

Think of this like “opening the first door”
At this stage, water is allowed to travel up to the second valve, but nothing will happen yet.

You may hear a faint sound of water, but the boiler pressure will not increase at this point.

Step 3 – Open the Second Valve (Door) VERY Slowly

Now, very slowly open the second black lever.

This is the second door.
Once this door is opened, water is allowed into the boiler and heating system.

Open this valve a tiny amount only — do not open it fully.
You should hear water running, and the pressure on the gauge will begin to rise.

Move the lever gently, as if you are just cracking a door open, not swinging it wide.

Blue Flame Tip

Opening this valve too quickly can cause the pressure to rise too fast.