Vaillant F20 Fault Code: Low Water Pressure Explained & How to Fix It

What does Vaillant F20 mean?

The Vaillant F20 fault code indicates that your boiler's water pressure has dropped below 0.5 bar. This is one of the most common Vaillant fault codes and is usually straightforward to resolve.

When your boiler detects low pressure, it automatically locks out and stops operating to prevent damage to internal components. This is a safety feature, but it also means you'll lose heating and hot water.

What causes F20 on a Vaillant boiler?

1. Simple pressure loss (most common)

Heating systems naturally lose a small amount of pressure over time due to temperature fluctuations. If your boiler pressure drops below 0.5 bar, you'll see the F20 code. This is normal and easily fixed by re-pressurizing.

2. A leak in your system

The most common cause of sudden pressure loss is a leak somewhere in your central heating pipework, radiators, or boiler connections. Even a tiny leak can cause significant pressure loss over time.

3. Failed expansion vessel

Your heating system includes an expansion vessel that accommodates pressure changes as water heats and cools. If this vessel fails, your system will lose pressure rapidly.

4. Faulty pressure relief valve

The pressure relief valve is designed to release excess pressure to protect your system. If it becomes stuck open, pressure will drop continuously.

How to fix the Vaillant F20 fault code

Step 1: Locate the filling loop

Your Vaillant boiler should have a flexible hose called a 'filling loop' beneath or behind the unit. This is used to manually re-pressurize the system.

Step 2: Re-pressurize your system

Follow these steps:

1. Turn off your boiler and allow it to cool for 20 minutes
2. Locate both ends of the filling loop (usually two small levers or taps)
3. Attach the loop between the cold water inlet and the system return pipe
4. Slowly open both isolation valves to allow water into the system
5. Watch the pressure gauge — it should rise to between 1.0 and 1.5 bar
6. Close both isolation valves when pressure is correct
7. Remove the filling loop
8. Switch your boiler back on and reset it if needed

Step 3: Reset the boiler

Once pressure is restored, press the reset button on your boiler. It should fire up normally.

When to call a Gas Safe engineer

If the F20 code returns after a few weeks, you likely have a leak. Call RARE Plumbing immediately — we'll use leak detection equipment to pinpoint the source without damaging your property.

If the filling loop is faulty or won't hold pressure, a qualified engineer will need to repair or replace it.

Do NOT repeatedly re-pressurize without investigating why pressure is being lost — this wastes water and indicates an underlying problem.

How to prevent F20 in future

Annual boiler servicing is the best prevention. During a service, we check your expansion vessel, pressure relief valve, and all connections for leaks.

Adding a magnetic filter to your system also helps — it catches corrosion particles that could damage seals and cause pressure loss.

Book a Vaillant engineer

If you're unable to re-pressurize your boiler, or if pressure keeps dropping, RARE Plumbing's Gas Safe registered engineers are available 24/7 across London. We'll diagnose the cause and fix it the same day in most cases.