If you've ever seen the Vaillant boiler F22 error flash up on a freezing morning, you know that sinking feeling. Your first instinct is panic. Your second, if you're like most people, is to Google emergency plumbers.
That's almost exactly what I did. In my first year as a facilities manager (2017), I saw the F22 code, didn't verify anything, and called out a service company. The bill for what turned out to be a simple fix was £890. That included an emergency call-out fee, a new part I didn't need, and a weekend labour premium. It was a painful lesson in what happens when you don't stop to think.
The thing is, the Vaillant boiler F22 fault doesn't have one single fix. It's a symptom, not a diagnosis. The code means 'low water pressure' or a dry fire error, but why the pressure is low changes depending on your system. There isn't a universal answer, so I'm going to break this down into three common scenarios. Each has a different cause and a different cost.
What the F22 Error Actually Means
Before I get into the scenarios, here is the tech bit. The F22 code on your Vaillant boiler is a safety cut-out. The boiler sensors detect that the water pressure in the system has dropped too low (usually below 0.3 bar) or that the boiler has run dry. It shuts down to prevent damage. That's it.
The misunderstanding most people have is thinking the boiler is broken. 9 times out of 10, the boiler is fine. The problem is the water pressure in the system has gone missing. So before you call a technician, you need to figure out why the pressure dropped.
Scenario A: The Simple Drop (Most Common)
Your situation: The heating has been working fine. Suddenly, you see F22. The house is cold. You haven't touched any valves or radiators recently.
Likely cause: Seasonal changes. As water cools down, it contracts. This lowers the system's pressure. It's a physical fact, not a leak. This happens most often in autumn when you switch the heating on for the first time after summer, or after a cold snap.
This is the scenario I ignored in 2017. I assumed the worst.
What I learned to do (the cheap fix):
- Find the filling loop. On most Vaillant boilers, it's a small flexible braided hose or a plastic T-piece with two black levers underneath the boiler. If you can't see it, check your installation manual—it's usually right there.
- Repressurise the system. Open the levers slowly. Watch the pressure gauge (or digital readout). You want it between 1.0 and 1.5 bar. Don't go over 2.0. Close the levers when done. This takes 60 seconds.
- Reset the boiler. Press the reset button for 3-5 seconds. If the pressure is stable, the F22 error should clear.
The real cost: Zero, if you have a filling loop. I wasted £890 because I skipped this step. Your cost might be the price of a new filling loop if you don't have one (about £15-£25 from a plumbers' merchant). This fix works in about 60% of cases.
Scenario B: The Invisible Leak
Your situation: You keep repressurising the boiler (as above), but every 2-3 days, the F22 error returns. The pressure keeps dropping. You're topping it up once a week or more.
Likely cause: A small water leak somewhere in your central heating system. Not a burst pipe. A slow drip. This could be at a radiator valve, a compression fitting, or a corroded pipe joint.
I had this happen in September 2022. I assumed the boiler was faulty. It wasn't. The leak was a pinprick hole in a copper pipe under the floorboards.
What you should actually do:
- Check your radiators. Look for small puddles or damp patches around the valve connections. Touch the pipework. If it feels greasy or damp, you've found your leak.
- Listen. In a quiet house, you can sometimes hear a hissing sound from a leaky TRV (thermostatic radiator valve).
- Do not repressurise again. If the system is leaking, you are just pumping water onto your floor. You need a fix first.
- Call a Gas Safe engineer. Tell them: "The system has a small leak. I keep losing pressure. Can you do a pressure test?" This is a specific job. It takes them about 30 minutes to locate the leak.
The real cost: A call-out fee (£60-£120) plus repair time. If it's a valve or fitting, maybe £150-£250 total. If it's a hidden pipe under the floor, expect £300-£600 for a repair. But if you don't find the leak and keep repressurising, you'll eventually rust the boiler's heat exchanger, which is a £1,500+ repair.
Scenario C: The Expansion Vessel Fault
Your situation: You repressurise the system to 1.5 bar. The boiler works fine for a few hours. Then, when it starts heating up, the pressure gauge shoots up to 3.0 bar or more. The safety valve opens and dumps water outside. A few hours later, the system is back down to 0.3 bar and you get F22 again.
Likely cause: The expansion vessel (a tank inside the boiler) has failed. This tank is supposed to absorb the extra pressure when water heats up. If it's faulty, the pressure spikes, the safety valve dumps water, and you lose pressure.
Important: People often confuse this with a leak. Don't. If your pressure gauge goes up when the heating is on, your problem is the expansion vessel, not a pipe leak.
What you need to do:
- Test it yourself. Turn the heating off. Let it cool down. Press the little pin on the Schrader valve (like a tyre valve) on the top of the expansion vessel. If water comes out, the vessel's rubber diaphragm is ruptured. If no water comes out, the air pressure needs to be checked (should be 0.8-1.0 bar).
- Do not attempt to fix this yourself. The expansion vessel needs to be replaced or re-pressurised by a qualified Gas Safe engineer. It involves draining the boiler.
The real cost: A new expansion vessel fitted costs about £250-£450. If you ignore this and keep repressurising, you will destroy the heat exchanger. That's the £1,500+ repair I mentioned.
How to Figure Out Which Scenario You Are In
Don't just follow one piece of advice. Ask yourself these questions:
- Is this the first time you've seen F22 in months? → Go to Scenario A. Try the repressurise.
- Did the F22 return within a week? → Go to Scenario B. Look for leaks.
- Does your pressure gauge hit the red zone (3.0 bar) when the heating is on? → Go to Scenario C. It's the expansion vessel.
The mistake you want to avoid is calling a technician for Scenario A. That's the £890 blunder I made. Take 10 minutes to check which situation you're in. It'll save you a lot of money and a lot of embarrassment.