⚠️ Difficulty: Medium — This guide involves checking the system pressure gauge, bleeding radiators, and operating the filling loop. You do not need to remove the boiler casing.
Prerequisites: A radiator key and the ability to read your boiler's digital display. Expect this process to take 1 to 2 hours, including waiting for the system to cool and checking for leaks.
What to expect: You will learn to identify the root cause of pressure loss and perform safe repressurisation. If the issue involves the expansion vessel or pressure relief valve, you must call a Gas Safe registered engineer.
⚠️ Important: Any intervention on your boiler may void the manufacturer's warranty if it is still active. Proceed only if you are comfortable with the steps described.
Your Vaillant combi boiler was designed to run smoothly, silently, and efficiently — maintaining a steady pressure between 1.2 and 1.5 bar when cold. But when that needle keeps dropping below 0.8 bar, the boiler enters safety lockout mode, refusing to ignite. You are left with no heating and no hot water. It is frustrating, but the good news is that many causes of repeated pressure loss can be identified and resolved without a technician. By the end of this guide, you will have a clear understanding of what is happening and how to restore your system to its original factory condition — or know exactly when to call for help.
Do not worry about the brand. While we use a Vaillant combi boiler as our example, the principles and procedures described here apply to virtually any modern sealed heating system. Whether you have a Worcester Bosch, Ideal, Baxi, or Glow-worm, the steps for checking pressure, bleeding radiators, and using the filling loop are universal.
📝 Expert Note: This guide follows the manufacturer's recommended procedures for equipment maintenance. The F.22 error code on Vaillant boilers specifically indicates low water pressure — a condition that must be addressed before the boiler will operate.
How It Works: The Boiler's Pressure System
Your combi boiler's heating circuit is a sealed system. When everything is working correctly, the water pressure remains stable. The ideal operating pressure is between 1.2 and 1.5 bar when the system is cold. The expansion vessel — a sealed component with a rubber diaphragm and a nitrogen gas charge — absorbs pressure fluctuations as the water heats and expands. The pressure relief valve (PRV) is a safety device calibrated to open at 3.0 bar, discharging water if the pressure exceeds this limit.
When pressure drops repeatedly, something is wrong. The most common causes are a small leak in the system, a failed expansion vessel, or a faulty PRV. If the pressure increases significantly when the heating is on and drops when it is off, the expansion vessel has likely lost its gas charge or its diaphragm is compromised. If the external discharge pipe is dripping, the PRV is releasing water — either from over-pressurisation or a defective valve.
What Happens If You Delay the Repair?
| Timeframe | Symptom | Potential Consequence |
|---|---|---|
| First week | Pressure drops below 0.8 bar once or twice | Boiler enters safety lockout. You lose heating and hot water temporarily. |
| One month | You repressurise weekly, but pressure still drops | A small leak is wasting water and increasing your utility bills. Corrosion may begin inside the system. |
| Three months | PRV starts dripping; expansion vessel fails completely | Water damage to floors or ceilings. Boiler components may fail, leading to a costly full system replacement. |
Estimated Costs for Common Repairs
| Material / Part | Estimated Price Range | Where to Find |
|---|---|---|
| Radiator key | £2 – £5 | Hardware stores, online retailers |
| Filling loop (if missing or damaged) | £10 – £25 | Plumbing merchants, boiler spares websites |
| Expansion vessel replacement (parts only) | £40 – £80 | Boiler spares specialists, online |
| Gas Safe registered engineer call-out (diagnosis) | £60 – £120 | Local heating engineers, Gas Safe Register website |
Tools and Materials You Will Need
- A radiator key (for bleeding radiators)
- A cloth or small towel (to catch drips)
- Access to the boiler's filling loop (usually a flexible hose with two valves)
- Your boiler's user manual (for locating the filling loop and pressure gauge)
Step-by-Step: Diagnosing and Fixing Pressure Loss
We know that staring at a boiler with a flashing error code can feel overwhelming. Take a breath. The steps below are straightforward, and you can stop at any point if you feel unsure. Let us start with the simplest check first.
1. Check the Boiler's Digital Display for Error Codes
Look at your boiler's front panel. If you see an error code such as F.22, this indicates low water pressure. Write the code down before proceeding. If the display shows no code but the pressure gauge reads below 0.8 bar, the boiler is in safety lockout. This is your starting point.
💡 Expert Tip: An F.22 code on a Vaillant boiler is almost always caused by low pressure. Do not panic — it is one of the easiest issues to resolve yourself, provided there is no underlying leak.
2. Bleed All Radiators in the House
Air trapped in the system can cause pressure to drop. Starting with the radiator furthest from the boiler, use the radiator key to open the bleed valve slightly. Hold a cloth underneath to catch any water. You will hear a hissing sound as air escapes. When water starts to drip steadily, close the valve. Repeat for every radiator. This step alone can restore pressure, but it will also reduce it — so you will need to repressurise afterwards.
⚠️ Common Mistake: Do not open the bleed valve fully. A quarter turn is usually enough. Over-opening can cause the valve core to come out, leading to a sudden gush of water.
3. Repressurise the System Using the Filling Loop
Locate the filling loop — it is a flexible braided hose with one or two isolation valves, usually under the boiler. Ensure both valves are closed. Connect the loop to the mains water supply and the heating circuit if it is not already attached. Slowly open one valve, then the other. Watch the pressure gauge rise. Stop filling when the needle reaches between 1.2 and 1.5 bar. Close both valves securely. Critical: Disconnect the filling loop after use. Leaving it connected risks contaminating your potable water supply and can cause over-pressurisation.
💡 Expert Tip: If the pressure rises above 1.5 bar, do not panic. Open a radiator bleed valve slightly to release water until it drops back to the correct level. Then close the valve.
4. Check for External Leaks and PRV Discharge
After repressurising, walk around your home. Look for damp patches on ceilings, walls, or floors near radiators and pipework. Check the external discharge pipe from the boiler — it is usually a small copper pipe that exits through an outside wall. If it is dripping, the pressure relief valve (PRV) is releasing water. This indicates either over-pressurisation (which you may have just corrected) or a faulty PRV. If the dripping continues after you have set the pressure correctly, a technician is required.
5. Monitor the Pressure Over 24 Hours
Turn the heating on and let it run for an hour. Observe the pressure gauge. If it climbs significantly (above 2.5 bar) and then drops sharply when the heating turns off, the expansion vessel is likely the culprit. This is not a DIY repair. The vessel must be inspected by a Gas Safe registered engineer. If the pressure stays stable overnight, your issue was simply trapped air or a minor pressure fluctuation — and you have fixed it.
Time to Call a Professional?
Ask yourself these three questions:
- Do you have a radiator key and know how to use it?
- Have you ever used a boiler's filling loop before?
- Are you comfortable with the idea that removing the boiler casing is not allowed under UK regulations?
If you answered 'no' to any of these, calling a Gas Safe registered engineer is the right choice — no shame. Your safety and the integrity of your heating system come first.
Curiosity and Data: The Scale of the Problem
Here is a sobering thought: a significant percentage of boiler breakdowns each year are caused by simple issues like low pressure or trapped air — problems that a homeowner can often resolve in under an hour with a radiator key. Yet many people call out an engineer immediately, incurring a call-out fee for what is essentially a maintenance task. Understanding how your system works empowers you to make that decision yourself.
⚠️ This information is provided for reference only. You are responsible for any repair you undertake. If you are unsure about any step, or if the problem persists after following this guide, please contact a Gas Safe registered engineer. Working on a boiler without proper training can be dangerous.
Conclusion: Know When to Stop and Call a Professional
If you have followed the steps above and your boiler is holding pressure between 1.2 and 1.5 bar, congratulations — you have solved the problem. If the pressure continues to drop, or if you notice the external discharge pipe dripping, or if the pressure fluctuates violently with the heating cycle, stop. These are signs of a failed expansion vessel or a faulty PRV, both of which require a Gas Safe registered engineer. There is no shame in calling for help. The important thing is that your home is safe and your heating works reliably.
FAQ
What is the ideal pressure for my Vaillant combi boiler?
The ideal operating pressure is between 1.2 and 1.5 bar when the system is cold. Below 0.8 bar, the boiler will enter safety lockout.
Can I repressurise my boiler without a filling loop?
No. The filling loop is the only safe way to add water to a sealed heating system. If your boiler does not have one, or if it is damaged, you must have a new one fitted by a professional.
How much does it cost to replace an expansion vessel?
The part itself typically costs between £40 and £80. Labour from a Gas Safe registered engineer will add to this, so expect a total cost of £150 to £300 depending on your location and the accessibility of the vessel.
When should I stop trying to fix the pressure myself?
If the external discharge pipe is dripping, if the pressure fluctuates violently when the heating cycles, or if you see error codes other than F.22, stop and call a Gas Safe registered engineer. These indicate issues with the PRV or expansion vessel that require specialist tools and training.
