Clear it first, then read what it tells you
Vaillant records F22 as "Safety switch-off: Low water pressure", with the cause given as no or insufficient water in the product, or water pressure that is too low. Their installation instructions give the thresholds precisely: if the filling pressure falls below 0.8 bar the boiler flashes the pressure value as a warning, and if it falls below 0.5 bar the boiler switches off and displays F22. The value keeps flashing until the pressure is back above 1.1 bar.
So the immediate fix is a refill:
- Switch the boiler off and let it cool. A hot system reads high and you will undershoot.
- Find the filling loop, usually a braided silver hose under the boiler with a valve at each end, or on some Vaillant models an internal filling key behind the front flap.
- Open both valves slowly and watch the gauge, not the clock.
- Stop between 1.0 and 1.5 bar cold. Vaillant's homeowner guidance suggests around 1.5 bar cold; 1.2 is a safe middle for a typical flat or house.
- Close both valves fully and detach the loop if yours is removable. A loop left cracked open lets mains pressure creep into a sealed system.
- Reset the boiler. Most clear the lockout on their own once the pressure is above 1.1 bar.
If you overshoot, bleed a radiator briefly to come back into the band.
The question the code is really asking
A sealed heating system is closed. In normal service it should not lose water at all. A small drop across a year is ordinary, and a drop straight after bleeding radiators is expected. Anything more than that is water leaving the circuit, and the boiler shutting down on F22 is the last symptom rather than the first.
| How often F22 comes back | What it usually means |
|---|
| Once, after a radiator was bled or a boiler serviced | Normal. Refill and forget it. |
| Once or twice a year | Within tolerance for an older system. Keep an eye on it. |
| Every month or two | A weeping joint, valve or towel rail, or an expansion vessel that has lost its charge. Worth finding. |
| Weekly | An active leak. Usually under a floor, in a wall, or inside the boiler case. |
| Daily, or it will not hold at all | A significant leak, or the pressure relief valve dumping to the discharge pipe outside. |
The reason weekly topping up is worse than it sounds: every refill puts fresh oxygenated mains water into pipework that was designed to hold the same deaerated water for years. That drives internal corrosion, which creates more pinholes, which means more refills. In the meantime, all the water you have added is somewhere in the building.
Rule out the things that are not a leak
The expansion vessel
If the pressure climbs unusually high when the heating runs and falls below normal when it cools, the vessel has probably lost its air charge and the system has nowhere to put the expansion. Vaillant has the engineer measure the pre-charge at the vessel valve during maintenance and top it up where it reads below 0.75 bar. That is a vessel recharge or replacement. No detection required.
The pressure relief valve
Go outside and look at the small copper or plastic discharge pipe that exits near the boiler. If it drips or runs after the boiler fires, the system is dumping water to the drain, which will empty it steadily. The valve itself or the vessel behind it is the fault, and again this is a boiler repair, not a search.
The pressure sensor
Less common, but a drifting sensor can report a pressure that is not real and lock the boiler out on F22 with a full system. The clue is a reading that jumps around or refuses to move at all while you are filling.
If it is a leak, where it will be
Check what you can see first, in this order: the pressure relief discharge pipe outside, under and behind the boiler, every radiator valve spindle and the nut where the tail meets the pipe, towel rails in bathrooms and kitchens, and the underside of any radiator where rust has bloomed. A weeping valve leaves a white or green crust rather than a puddle, because it evaporates as fast as it seeps.
If all of that is dry, the loss is hidden. A pinhole in a 15mm copper pipe under a screed floor can drop the gauge weekly for months while producing no visible damp at all, because the screed absorbs the water and the heating dries the surface. In flats, the first sign is often a stain in the ceiling below rather than anything in your own home. Underfloor heating circuits behave the same way, with the added clue of a warm stripe or a patch that stays warm when everything else has cooled.
What we do about it
The circuit is isolated and pressure-tested first. That confirms the loss is real and gives its rate, which routinely ends the job before anything is opened, because a circuit that holds on a gauge for an hour is not leaking and the fault is at the boiler. Where the loss is genuine, the pipe run is traced with thermal imaging, acoustic listening and moisture mapping, and with tracer gas on a drained circuit where the screed is too thick for thermal to read through.
Then the pipework is repaired or re-run, and the floor, screed, plaster or ceiling is put back. Our rate is £150 per hour, fixed before we attend, and if we do not find the leak you do not pay for the time we spent looking. The rate does not move; the total depends on the hours. Where a leak has damaged the building, the trace and the access are often recoverable from your insurer, which is covered under insurance claims and trace and access.
When not to call us
- This is the first F22 you have ever seen. Refill it and carry on.
- You have just had radiators bled, a valve changed or the boiler serviced. Refill, and only worry if it happens again.
- The discharge pipe outside is dripping. That is a valve or vessel fault for a heating engineer.
- You cannot find the filling loop or the boiler is under warranty. Call Vaillant or your installer rather than guessing.
Call us when the gauge will not hold, nothing is visibly wet, and you are refilling more often than you should have to. The wider pattern is set out on the boiler and heating pressure hub, and the repair side under boiler and heating leak repair.
