Vaillant ecoTEC F.22 fault code: low water pressure explained
What Vaillant F.22 means on ecoTEC boilers, how to bring the pressure back to 1 to 1.5 bar with the filling loop, and when a repeat drop means a leak.
What this means
Vaillant assigns F.22 to low water pressure in the sealed heating circuit, and the boiler stops firing to protect itself from running dry. Vaillant names the normal range as 1 to 1.5 bar, and lists a faulty pressure sensor, loose sensor or pump wiring, and a failed internal expansion vessel alongside a genuine water shortage.
Equipment and model context
Vaillant ecoTEC plus and ecoTEC sustain boilers that list F.22
- ecoTEC plus combi
- ecoTEC plus system
- ecoTEC sustain combi
This meaning follows the Vaillant United Kingdom fault-code list for ecoTEC boilers that display F.22. Other Vaillant ranges and the same code on another appliance can be mapped differently, so confirm the model from the data badge before you top up or reset.
What you may notice
- The ecoTEC display shows F.22 and the boiler will not start central heating or hot water.
- The pressure gauge or the digital pressure readout sits below 1 bar when the system is cold.
Likely causes
- Water has left the sealed circuit through bleeding a radiator, a slow weep at a valve or joint, or a discharge from the pressure relief pipe.
- The pressure sensor, its wiring to the board, or the internal expansion vessel can misreport or fail to hold pressure even when the system is full.
Diagnostic paths
F.22 cleared after one top-up and has stayed away
The gauge was low, one fill to 1.5 bar with the filling loop brought it back, the code reset, and the pressure has held over several heating cycles since.
Treat it as a single loss from bleeding or a minor event. Note the date and the starting pressure, and watch the gauge weekly. No further action is needed while it holds.
F.22 returns within days of every top-up
The pressure falls back below 1 bar within hours or days each time it is refilled, or the gauge swings a long way between cold and hot before dropping.
Stop repeatedly refilling and arrange a Vaillant or Gas Safe registered engineer. A steady loss points at a leak to find, and a wide cold-to-hot swing points at the expansion vessel.
F.22 is the boiler acting on a number from the pressure sensor. Every element below sits between the water in the system and that number, which is why the code cannot name the fault on its own.
- Sealed circuit waterFills pipes and radiators to a set pressure
- Expansion vesselHolds pressure steady as water heats and cools
- Pressure sensorTurns the water pressure into an electrical reading
- Sensor wiringCarries that reading to the boiler control board
- Control boardBlocks ignition and shows F.22 below the limit
| What the gauge does | What it points to | What it does not settle |
|---|---|---|
| One drop, then holds after a fill | A single loss from bleeding or a small event | Whether a slow weep will show up over months |
| Falls again within days of each top-up | Water leaving the system somewhere | Where the leak is, which needs tracing on site |
| Low cold, then a large fast rise when hot | An expansion vessel that has lost its air charge | Whether the pressure sensor is also reading true |
The top-up is the owner task, the pattern is the diagnosis
Vaillant treats a low reading as something a homeowner can correct with the filling loop, so the first response to F.22 is to check the gauge with the system cold and bring it to about 1.5 bar if it is below 1 bar. Open both filling-loop valves slowly, watch the needle rise, then close both valves and reset the boiler as the manual describes. That clears the lockout when the only problem was a genuine shortage.
What the top-up cannot do is tell you why the pressure fell. A system that holds afterwards was short for a reason that has passed, such as air released from a radiator. A system that falls again is losing water, and each refill adds fresh oxygenated water that speeds corrosion inside the radiators and the heat exchanger. The number of top-ups per month is the measurement that decides whether this is maintenance or a repair.
When F.22 is not really about water
Vaillant lists the pressure sensor, its wiring, and the internal expansion vessel among the causes, which matters because all three can produce F.22 on a system that is actually full. A sensor reading low, a connector that has worked loose, or a vessel with no air charge behind its diaphragm each present as low pressure to the control board even when a gauge shows a normal figure.
This is why the fault is not a parts order. An engineer confirms the true system pressure independently, checks the expansion vessel air side with the system depressurised, and inspects the sensor connection before replacing anything. Opening the boiler casing to reach those parts is where owner work stops, because the case encloses gas and mains electrical connections.
What to give the technician
Give the engineer the ecoTEC model from the badge, the cold and hot gauge readings, how often you have refilled and by how much, whether any radiator was bled recently, and whether water has been seen at the outside pressure relief pipe.
Safe homeowner checks
- Read the pressure gauge with the system cold, and top up to about 1.5 bar with the filling loop only if it is below 1 bar, then close both valves fully.
- Record how many times the pressure has needed topping up in the last month and whether it drops fastest hot or cold.
Is a reset appropriate?
Clearing the fault once, after the gauge has been raised to 1.5 bar and stayed there, is fine. Do not keep clearing F.22 while the pressure drops away, because every refill adds oxygen to the water.
When to call a technician
- A repeated pressure loss, a suspected leak, an expansion vessel check, or any sensor or wiring inspection inside the boiler casing needs a Vaillant or Gas Safe registered engineer.
Questions people ask about this
What pressure should a Vaillant ecoTEC be at?
Vaillant names 1 to 1.5 bar as the normal range with the system cold. Fill to the middle of that band rather than to the top, and check it cold rather than while the heating is running.
Why does F.22 keep coming back after I repressurise?
The circuit is losing water through a leak, or the internal expansion vessel is no longer holding pressure. An engineer needs to establish which, and each extra top-up in the meantime introduces more dissolved oxygen.
Can F.22 show when the boiler is not actually low on water?
Yes. Vaillant lists a faulty pressure sensor, loose sensor or pump wiring, and a failed expansion vessel as causes, and each can make a full system read as low pressure to the control board.
Evidence recordHow this page was checked
official manufacturer support article, service manual · checked 2026-09-09
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Vaillant UK and Vaillant technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- official manufacturer support article, service manual
- Scope of the definition
- Vaillant ecoTEC plus and ecoTEC sustain boilers that list F.22
- Last checked
- 2026-09-09