What the F22 code means
On most ecoTEC models, F22 is the dry fire or low water pressure fault. The boiler has a pressure sensor that constantly checks there is enough water in the sealed heating system. If pressure falls too low, the boiler cannot be sure the heat exchanger is full of water, so it refuses to fire. Running a boiler without enough water could damage the heat exchanger, so this lockout is a safety feature, not a sign that the boiler itself is broken.
The good news is that F22 is one of the few Vaillant fault codes a homeowner can usually clear without help. The key question is why the pressure dropped in the first place. A one-off drop after bleeding radiators is nothing to worry about. Pressure that keeps falling every few days points to a leak or a component that needs attention.
Common causes of F22
Normal pressure loss over time
Sealed heating systems lose a small amount of pressure over months. If your boiler has not been topped up for a long time, a slow drift down is often all that has happened.
Bleeding radiators
Every time you bleed a radiator, air and a little water escape and the pressure drops. Bleeding several radiators in one go can easily take a system from 1.5 bar to below the level the boiler needs.
A leak somewhere in the system
Small leaks at radiator valves, pipe joints or towel rails can go unnoticed for weeks. Look for damp patches, staining on ceilings, rust marks or crusty deposits around valves. Our guide on a boiler leaking water covers the signs in more detail.
Pressure relief valve discharging
If the pressure climbs too high when the system heats up, the pressure relief valve opens and dumps water through a copper pipe that usually runs outside. That water then has to be replaced, so the pressure ends up too low when the system cools. This cycle often points to an expansion vessel that has lost its charge.
Faulty pressure sensor
Less often, the gauge shows normal pressure but the boiler still reports F22. That suggests the pressure sensor or its wiring is at fault, which is an engineer job.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Normal pressure loss | Gauge low, no other symptoms | You, via the filling loop |
| Recently bled radiators | Pressure dropped straight after bleeding | You, via the filling loop |
| Leak in the system | Pressure keeps dropping, damp or staining | Gas Safe engineer or plumber |
| Expansion vessel or relief valve | Pressure swings high when hot, drips from outside pipe | Gas Safe engineer |
| Faulty pressure sensor | Gauge reads normal but F22 persists | Gas Safe engineer |
Safe checks you can do yourself
- Check the pressure gauge with the heating off and the system cool. If it reads below 1 bar, the system needs topping up.
- Find the filling loop, usually a braided hose or a built-in key and lever under the boiler. Your manual shows where it is on your model.
- Open the filling loop valves slowly and watch the gauge. Close them once the pressure reaches 1 to 1.5 bar. Do not overfill.
- Make sure both valves are fully closed afterwards. A filling loop left slightly open can push the pressure too high.
- Press reset (or the mode button on some newer models) as your manual describes, and let the boiler try to fire.
- Walk round the house and check radiator valves, exposed pipework and the area under the boiler for signs of water.
- Note the pressure reading and check it again each day for a week.
Our step-by-step guide on how to repressurise a boiler with the filling loop has more detail if you have not done this before.
If you plan to bleed radiators, do it with the heating off and cool, then top up the pressure afterwards. Doing both jobs together avoids a surprise F22 later that day.
When to call a Gas Safe engineer
Topping up once is routine. Needing to top up every few days or every week is not, and you should book a Gas Safe registered engineer if:
- The pressure keeps dropping after you have topped it up.
- You can see water dripping from the pipe outside, or from under the boiler.
- The gauge shows normal pressure but F22 will not clear.
- The pressure shoots up close to 3 bar when the heating is on.
- The code returns after two or three resets.
Do not remove the boiler casing to look for a leak inside. Internal leaks, the expansion vessel, the pressure sensor and the relief valve must be dealt with by a Gas Safe registered engineer.
Typical repair costs in the UK
If topping up solves the problem, F22 costs nothing. If there is an underlying fault, these are the typical ranges we see. Prices vary by region and by the engineer, so treat them as a guide only.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Fixing a small leak on a radiator valve or joint | Typically £80 to £200 |
| Recharging the expansion vessel | Typically £80 to £150 |
| Replacing the pressure relief valve | Typically £120 to £220 |
| Replacing the pressure sensor | Typically £120 to £220 |
How to stop F22 coming back
Keep an eye on the gauge once a month, especially before winter. A system that holds steady between 1 and 1.5 bar when cold is in good health. Top up after bleeding radiators, and check that the filling loop valves are fully closed each time. A yearly service gives an engineer the chance to check the expansion vessel charge and spot small leaks early. See our boiler service cost guide for what that usually involves. If the pressure has swung the other way, our guide on boiler pressure that is too high explains what to do.