F22 on a Vaillant boiler means low water pressure, which Vaillant calls a "dry fire" fault: there isn't enough water in the heating system for the boiler to run safely, so it has locked out. To fix it, top the pressure up to about 1.5 bar using the filling loop, then press reset. If F22 keeps coming back, water is escaping somewhere (a leak, the pressure relief valve or a faulty expansion vessel) or the pump or pressure sensor has failed, and you'll need a Gas Safe engineer.
F22 is one of the most common Vaillant fault codes in UK homes, and one of the few you can usually clear yourself in five minutes. Glow-worm boilers (made by the same group) use the same code for the same problem. This guide covers what F22 means, the step-by-step fix, why it comes back, and what repairs cost. For other codes, see our complete list of Vaillant fault codes.
What does F22 mean on a Vaillant boiler?
F22 (sometimes shown as F.22) means the boiler has detected too little water pressure in the heating system. Without enough water circulating, the heat exchanger could overheat ("dry fire"), so the boiler shuts down and stops heating and hot water until the pressure is restored.
You'll usually see F22 on Vaillant ecoTEC boilers (ecoTEC plus, ecoTEC pro, ecoTEC exclusive) and on Glow-worm models. The code is the same whether it appears as F22, F.22 or F 22.
| What you see | What it means |
|---|---|
| F22 / F.22 on the display | Low water pressure: boiler locked out |
| Pressure reading well below 1 bar | Confirms the system is short of water |
| No heating, no hot water | Normal: the boiler won't fire until pressure is restored |
Is F22 dangerous?
No. F22 is a safety lockout, not a dangerous fault. The boiler has stopped because it detected a problem, which protects it from damage. You won't have heating or hot water until you fix it, but there's no immediate danger.
If you can smell gas at any point, don't touch the boiler. Call the National Gas Emergency number on 0800 111 999, turn the gas off at the meter if it's safe to do so, and open the windows.
How to fix F22: step by step
You'll need the filling loop (a braided hose or built-in lever under the boiler) and about five minutes.
- Turn the heating off and let the boiler cool for 10–15 minutes so you get a true cold reading.
- Check the pressure. Vaillant ecoTEC boilers show pressure on the digital display (your user manual shows which button brings it up); older models have a round gauge underneath. If it reads below about 1 bar, you've found the cause.
- Find the filling loop. On most Vaillant combis it's underneath the boiler: either a flexible braided hose between two valves, or a built-in blue lever or key. Some Vaillant filling loops are a separate silver hose you clip on.
- Connect the hose if yours is detachable, hand-tight at both ends.
- Open the valves slowly. You'll hear water flowing. Watch the pressure rise.
- Close the valves at about 1.5 bar. Vaillant says the gauge should generally read 1 to 1.5 bar, so around 1.5 bar is a good target.
- Remove the hose if it's detachable (a drip of water is normal).
- Press the reset button on the boiler. On many ecoTEC models it's the button with the reset (flame or arrow) symbol; hold it for a couple of seconds.
- Turn the heating back on and check the boiler fires up.
Topped up too much? If the pressure is over 2 bar when cold, bleed a radiator with a radiator key until it drops back to about 1.5 bar. Pressure that's too high can make the pressure relief valve discharge, which causes F22 again later.
For more detail on pressure readings, see what boiler pressure should be.
Why does F22 keep coming back?
If you've topped up and reset, and F22 returns within days or weeks, water is escaping from the system or a part has failed. Here are the common causes, roughly from most to least likely.
1. A leak in the heating system
Check every radiator valve, exposed pipe joint and the area under the boiler with dry kitchen roll while the heating is on. Look for rust spots, white crusty deposits or brown stains on ceilings. Even a slow weep that evaporates will drain the pressure over a week or two.
2. You recently bled the radiators
Bleeding radiators lets out water as well as air, so pressure drops. This is normal: top up again once you've finished bleeding all of them.
3. The pressure relief valve is discharging
Look outside for a copper pipe coming through the wall near the boiler. If it's dripping or there's staining under it, the pressure relief valve is letting water out, either because it's worn or because pressure is spiking too high when the heating runs.
4. A faulty expansion vessel
If the pressure rises sharply (towards 3 bar) when the heating is on, then drops low when it cools, the expansion vessel has probably lost its air charge. The relief valve dumps water each time, and you end up back at F22. An engineer can re-charge or replace the vessel.
5. A faulty pump or pressure sensor
Sometimes the system pressure is fine but the boiler still shows F22. A seized pump, an air lock, or a faulty water pressure sensor can all make the boiler think it's short of water. These are engineer jobs.
We cover each of these in more depth in combi boiler losing pressure.
F22 vs other Vaillant pressure codes
| Code | Meaning | Common cause |
|---|---|---|
| F22 | Low water pressure (dry fire) | Low pressure, leak, relief valve, expansion vessel |
| F23 | Temperature difference between flow and return too large | Pump not circulating, air, low pressure |
| F24 | Temperature rising too quickly | Pump, air in system, low pressure |
| F73 / F74 | Water pressure sensor signal fault | Faulty or disconnected pressure sensor |
| F75 | No pressure change when the pump starts | Pump or pressure sensor fault, air, low pressure |
If you see F23, F24 or F75 alongside low pressure, topping up often clears those too. If not, call an engineer.
How much does it cost to fix F22?
| Fix | Typical UK cost |
|---|---|
| Top up pressure and reset yourself | Free |
| Engineer call-out and diagnosis | £60–£120 |
| Fix a leaking radiator valve | £60–£150 |
| Re-charge the expansion vessel | £60–£120 |
| Replace the expansion vessel | £150–£350 |
| Replace the pressure relief valve | £100–£200 |
| Replace the pump | £200–£400 |
| Replace the water pressure sensor | £100–£200 |
Prices vary by region and model. Check whether your boiler is still under Vaillant's warranty (it requires an annual service) before paying for repairs.
Topping up the pressure and pressing reset are householder jobs covered in your Vaillant user manual. Don't remove the boiler casing or attempt to adjust the expansion vessel, relief valve or any internal part: work inside a gas boiler must be done by a Gas Safe registered engineer.
FAQs
What does F22 mean on a Vaillant boiler?
It means low water pressure, also called a "dry fire" fault. The boiler has detected there isn't enough water in the heating system and has locked out to protect itself.
How do I fix F22 on my Vaillant boiler?
Top up the pressure to about 1.5 bar using the filling loop under the boiler, then press the reset button. If F22 comes back, there's a leak or a faulty part that an engineer needs to find.
Why does my Vaillant boiler keep showing F22?
Because water is escaping from the system (a leak, bled radiators, or a pressure relief valve that's discharging) or because the expansion vessel, pump or pressure sensor is faulty.
How do I reset a Vaillant boiler after F22?
Restore the pressure first, then press and briefly hold the reset button on the front of the boiler. Resetting without topping up won't work, because the boiler will lock out again straight away.
What pressure should a Vaillant boiler be?
Vaillant says the gauge should generally read between 1 and 1.5 bar, and advises avoiding pressure above 2.5 bar. It's normal for pressure to rise a little when the heating is running.
Is F22 the same on a Glow-worm boiler?
Yes. Glow-worm boilers use F22 for low water pressure too, and the fix is the same.
Can I use my boiler with F22 showing?
No. The boiler won't provide heating or hot water until the pressure is restored and it's been reset.
- F22 on a Vaillant (or Glow-worm) boiler = low water pressure.
- Top up to about 1.5 bar with the filling loop, then press reset.
- If F22 returns, look for leaks and check the relief valve pipe outside.
- Expansion vessel, pump and sensor faults need a Gas Safe engineer.
