Vaillant F22 Fault Code: Low Water Pressure Fix

What the Vaillant F22 fault code means

F22, printed as F.22 on most Vaillant displays, is the low water pressure lockout. A sealed heating system holds a set amount of water under pressure, and when that drops too far the boiler stops rather than fire into a circuit that is part empty. On most Vaillant boilers nothing is broken: it is a top-up you can do yourself in about ten minutes.

Vaillant publishes the code on its own fault code pages as follows.

Source: Vaillant UK boiler fault codes. That list splits in two. One cause is a genuine loss of water, which you can put right. The rest are the boiler being handed a wrong reading by a sensor, its wiring or the expansion vessel, and topping up will not touch them.

Check the gauge first: below 1 bar or above 1 bar

Everything about F22 turns on one reading, so take it cold before you touch anything: a hot system always reads higher. Depending on the model, that is a round dial on the front or underside of the boiler, or a bar figure on the digital display.

Vaillant then divides F22 into two completely different jobs.

• Under 1 bar. This is a straightforward loss of water. Vaillant says: “This can be solved without an engineer if you are competent to do so.” The steps below are the ones Vaillant sets out.
• 1 bar or above. The pressure is fine and the boiler is still locking out, which points at the sensor, the wiring or the expansion vessel. Vaillant says: “This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.”

That 1 to 1.5 bar range is the figure Vaillant publishes on its fault code pages, and it is a cold reading.

How to repressurise a Vaillant and clear F22

These steps stay inside what Vaillant asks a homeowner to do. If any of it does not match your boiler, check your user guide rather than improvise.

1. Turn the boiler off and let the system cool. A cold system gives an honest reading, and stops the pressure climbing past your target as the water heats.
2. Find the filling loop. Vaillant’s instruction is simply to “locate your filling loop”, and it points anyone unsure at the user manual for their model. On many installations it is a short braided hose linking two pipes below the boiler, with a valve at each end; on others the filling device is built into the underside of the boiler.
3. Open it slowly and watch the gauge. Vaillant’s wording is to “slowly increase the water pressure to be between 1 to 1.5 bar”. You should hear water moving. Aim for the middle of that range rather than the top of it, so there is room for the pressure to rise when the heating comes on.
4. Close both valves fully and watch the gauge for a minute. If it creeps up, a valve has not shut properly.
5. Reset the boiler once. Vaillant says that “often resetting your boiler can resolve a fault or temporarily get your boiler working again”, and points you at your user manual for the procedure on your model. Do it once. If F22 comes straight back, the pressure was not the whole story, and repeated resets only clear the display.

Two limits worth stating plainly. Vaillant’s own instruction on its fault code pages is “do not gain access inside your boiler”, so the casing stays on whatever the code says. And if you overshoot past 1.5 bar cold, the pressure comes back down by letting a little water out of a radiator bleed valve. It works the other way too: bleeding radiators lowers system pressure, and can take it below the level F22 trips at.

If you smell gas at any point, leave the property and ring the National Gas Emergency Service on 0800 111 999, the number Vaillant also gives.

Model-specific detail on gauges and filling loops across the Vaillant range sits in our Vaillant boiler pressure guide.

When F22 needs a Gas Safe registered engineer

Book somebody if any of the following is true.

• The gauge already reads 1 to 1.5 bar and F22 still shows. Vaillant sends this case straight to an engineer. The candidates are the water pressure sensor, its wiring, or a failed internal expansion vessel, and all three sit behind the casing.
• The pressure falls again within days or weeks. A sealed system does not consume water. If it keeps disappearing, it is going somewhere, and that needs finding rather than refilling.
• You can see damp patches, staining or wet pipework. Check under radiators, around valves and along visible pipe runs, then tell the engineer where you found it.
• The pressure will not rise at all with the filling loop open. Either the loop is isolated, or it has failed.
• You cannot find the filling loop, or you are not confident. Vaillant frames the top-up as a job for someone competent to do it. There is no prize for guessing.

Work inside the casing and on the gas side of a boiler may only be done by a Gas Safe registered engineer. Vaillant runs a homeowner technical support line on 0344 736 0049 and a line for booking a service or repair on 0330 100 3143 if you want the manufacturer’s own engineer. Whoever attends, note the code, the gauge reading and how fast the pressure dropped before they arrive.

Why your Vaillant keeps losing pressure

A one-off F22 after bleeding radiators is housekeeping. An F22 that keeps returning is a symptom, because water in a sealed system has nowhere legitimate to go.

• You bled the radiators recently. Bleeding lets water and air out of a sealed system, so the pressure falls with it. Working through a whole house of radiators can take the gauge under 1 bar on its own, so check it when you have finished.
• There is a leak somewhere on the system. Radiator valves, towel rail connections, pipework under floors and joints hidden in walls are the usual suspects, and a small one can lose pressure for months without leaving a puddle.
• The internal expansion vessel has failed. Vaillant lists this among the causes of F22. The vessel absorbs the expansion of heating water, and once it loses its charge the pressure swings about and can be pushed out through the discharge pipe outside. That is engineer work.
• The pressure sensor is reporting a fault that is not there. Vaillant lists both a faulty sensor and loose or damaged wiring to the pump and sensor. The gauge and the code then disagree, and the code is wrong.

Resist the urge to keep refilling. Each top-up hides how fast the pressure is falling, which is the most useful thing an engineer can be told. If you are reaching for the filling loop more than occasionally, read our guide on a boiler losing pressure.

One piece of prevention is worth the diary entry. Vaillant recommends the boiler is serviced once a year by a Gas Safe registered engineer. An annual service is when a slow leak or a tired expansion vessel gets caught, rather than in January.

If you would rather not pay for the repair

Most F22s cost nothing: you top the pressure up and carry on. When it is a leak, a pressure sensor or an expansion vessel, it becomes a breakdown repair with parts and labour attached. Smart Plan’s boiler and central heating cover covers parts and labour up to the cover limit for that module, which is £500 where the boiler is under 7 years old and £200 where it is over 7, per 12-month period. A £95 call-out fee, paid in advance, applies in your first 30 days, and to faults the plan does not cover, no-access visits and early annual-service requests; cover itself still applies right through those 30 days. Using a service starts a 12-month agreement period, and leaving early costs the remaining months or 75% of the outstanding balance. Smart Plan is a service plan, not insurance.

F22 and the other Vaillant pressure codes

Vaillant uses several codes around water pressure and circulation, and they get confused with one another. Here is our summary of what Vaillant publishes for each.

F.75 is the one most often mistaken for F22, because it starts the same way: if the gauge is under 1 bar, top it up. Our page on the Vaillant F75 fault code goes through it, and the full Vaillant fault code list covers the rest.

Vaillant F22: quick answers

Is the Vaillant F22 fault code dangerous?

No. F22 is the boiler protecting itself by refusing to fire on a system short of water, so the risk is loss of heating and hot water rather than danger. It is not a gas fault. Take one instruction seriously, the one Vaillant repeats on every code: do not gain access inside your boiler. If you ever smell gas, leave and ring 0800 111 999.

What pressure should a Vaillant boiler be at?

Between 1 and 1.5 bar with the heating cold. Vaillant gives that range on its fault code pages. The reading sits higher when the system is hot, which is normal, so always judge it cold and fill to the middle of the range.

The gauge reads 1.5 bar and F22 is still showing. What now?

That is the version of F22 Vaillant sends straight to an engineer: a reading above 1 bar with the code still showing is “a case for a Vaillant engineer”, and you should not try to solve it yourself, because opening the casing can be dangerous. The usual causes are the water pressure sensor, the wiring to the pump or sensor, or the internal expansion vessel.

Does an F22 need reporting if I am renting?

Yes. Tell your landlord or letting agent as soon as it happens and let them arrange the visit, because the boiler is their responsibility. Note the date and the gauge reading: a boiler that keeps losing pressure is evidence worth having.