A Vaillant F22 fault code means your boiler has low water pressure (not enough water in the heating system), so it shuts down to protect itself.
- What Does the F22 Code Mean?
- What pressure should a Vaillant boiler be?
- What causes low water pressure (and F22)?
- How to fix F22: topping up/repressurising your Vaillant boiler
- Cost of hiring a heating engineer to fix F22
- Will my warranty cover repairs for an F22 fault?
- Looking for a great deal on a new boiler?
Like all boiler brands, Vaillant's are capable of breakdown.
And like all good boiler brands, Vaillant's at least let you in on why they aren't happy. Opposed to the silent treatment.
These fault codes inform homeowners what the problem is - or at least spit out something you're able to Google.
Most of the time, you can fix F22 by repressurising (topping up) the boiler back to a normal range. If the pressure drops again soon after, you likely have a leak or a faulty component that needs a Gas Safe engineer.
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Do this now (safe homeowner checks):
[1] Check your boiler pressure gauge
Most Vaillant boilers run happily around 1.0–1.5 bar when cold (varies by system).
If your pressure is very low (often around 0.3–0.6 bar, depending on model/system), F22 can appear and the boiler may lock out.
[2] Top up the boiler to the normal range
If you’re comfortable using the filling loop, repressurising often fixes it in minutes (steps below).
[3] Watch what happens over the next week
If pressure holds: congrats, you’ve avoided the engineer visit.
If pressure drops again: you probably have a leak (or a failing part) and need a professional.
[4] Don’t keep topping up forever
Repeatedly losing pressure is not “a quirky boiler personality”. It’s a problem.
🔑 Key takeaways:
F22 means low water pressure.
The usual fix is repressurising via the filling loop.
If pressure keeps dropping, suspect a leak or component fault.
Engineer needed for leaks and internal parts.
If your boiler is old and repair-prone, replacement may be the better move.
What Does the F22 Code Mean?
F22 means your boiler has detected insufficient system water pressure.
Combi boilers and sealed heating systems need the right water pressure to circulate hot water through the system.
If there’s not enough water, the boiler protects itself by shutting down - which is why you suddenly have no heating and no hot water.
What pressure should a Vaillant boiler be?
As a general rule, many homes see:
Around 1.0–1.5 bar when the system is cold
Higher when hot (pressure rises as water expands)
If pressure drops very low (commonly around 0.3–0.6 bar, depending on system/model), your boiler may trigger F22 and stop working.
(If your boiler manual gives a specific range, follow that - manuals outrank blog posts, even ours.)
What causes low water pressure (and F22)?
Low pressure usually happens for one of these reasons:
[1] A slow leak somewhere in the system
The most common culprit. Leaks aren’t always dramatic puddles - they can be:
a small radiator valve seep
a joint under the floor
a weeping PRV pipe outside
corrosion in an older system
If you top up and it drops again soon after, leak is top of the list.
[2] You’ve bled radiators recently
Bleeding radiators releases air - and usually reduces system pressure too.
(You did the right thing. The boiler is just being dramatic about it.)
[3] Ageing system / gradual pressure loss
Over time, systems can lose pressure gradually without one obvious “event” causing it.
[4] Faulty components
Less common than leaks, but still possible:
pressure sensor issues
pump/valve problems
PRV issues
expansion vessel problems (often shows as pressure rising too high when hot, then dumping water)
Can I fix an F22 fault code myself?
Sometimes, yes - repressurising is usually DIY-able.
But if the cause is a leak or a faulty internal component, that’s not a homeowner job.
Here’s the clean rule:
✅ DIY likely works if - pressure dropped and stays stable after topping up
❌ Engineer needed if - pressure keeps dropping, you see leaks, or you’re topping up repeatedly
How to fix F22: topping up/repressurising your Vaillant boiler
Before you start:
Let the boiler cool down
Make sure you know where your pressure gauge is
Find your filling loop (built-in or external)
Watch our step-by-step video below:
Step-by-step (typical process)
Turn the boiler off and allow it to cool.
Find the filling loop (usually a silver braided hose with valves, or a built-in key/lever setup depending on model).
Open the valve(s) slowly to allow water into the system.
Watch the pressure gauge rise. Aim for around 1.0–1.5 bar when cold.
Close the valve(s) firmly once you hit the target pressure.
If you have an external filling loop, remove it after topping up (if your system requires removal).
Turn the boiler back on and check everything runs normally.
If pressure jumps too high
If you accidentally overfill and pressure goes much higher than normal, don’t panic - but don’t leave it that way.
You may need to bleed a radiator slightly to bring pressure back down (or call an engineer if you’re unsure).
Important: If you’re not confident locating/using the filling loop, stop and book a Gas Safe engineer. No shame. Better safe than soggy.
Fixing a leak (DIY? Nope.)
You can’t reliably diagnose and repair boiler/heating leaks without the right tools and experience - and sometimes the leak isn’t even visible.
If you see signs like:
pressure drops repeatedly
water marks under radiator valves
dripping overflow/PRV pipe outside
corrosion around pipework
…it’s time for a professional.
Cost of hiring a heating engineer to fix F22
Costs vary by region and what the underlying issue is, but as rough ballparks:
Repressurising / simple check: £30–£60
Leak investigation + repair: £60–£120+
Replacing a part: £150–£400+
Replacing the boiler entirely: £1,800–£3,800+ (depending on boiler choice, complexity, controls, system condition)
If your boiler is old and you’re paying for repeated repairs, replacement can be the saner financial move.
Will my warranty cover repairs for an F22 fault?
Sometimes - but it depends on:
whether the boiler is still within warranty
whether annual servicing conditions have been met
whether the fault is classed as a manufacturing defect vs. system issue (like leaks/radiators/pipework)
Warranties typically require:
annual servicing
correct installation/commissioning by a qualified engineer
adherence to warranty terms and timelines
If F22 is caused by a system leak or external heating issue, warranty cover is less likely.
Related guides:
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