F22 Vaillant
Vaillant F22 Fault: Low Water Pressure Explained
F22 on a Vaillant — ecoTEC models especially — is the brand's low water pressure lockout: on most models it triggers when system pressure falls to around 0.5 bar or below, at which point the boiler refuses to run rather than operate dry. It's Vaillant's most common code and usually the easiest to resolve at home.
What the F22 code means
Vaillant's control board watches the sealed system's pressure via its internal sensor. When the reading drops through the lockout threshold, the boiler stops the pump and burner and holds F22 on the display until pressure is restored and the fault reset. The logic errs deliberately safe: running a heating circuit on insufficient water risks the pump and, ultimately, the heat exchanger — the components no one wants on an invoice.
What you can safely do yourself
Check the gauge first: a cold system wants roughly 1–1.5 bar, and readings under 1 bar explain the lockout. Re-pressurising is a homeowner-safe job on almost every boiler: open the filling loop valve(s) under the boiler gently, watch the needle rise to around 1.2–1.5 bar, shut the valves tight, then reset the boiler. If the display clears and the boiler fires, you've fixed today's symptom.
Note the word symptom: a sealed system shouldn't lose pressure. Once is unremarkable — pressure drifts over months, and bleeding radiators lowers it too. But if you're topping up weekly, water is escaping somewhere (a radiator valve, the relief pipe, or a part inside the boiler), and that pattern needs an engineer rather than a routine.
What causes it
Water has left the sealed circuit. The candidate exits, roughly in order of frequency: radiator valve glands weeping invisibly slowly, the pressure-relief valve passing to its outdoor pipe, pressure legitimately dropped by radiator bleeding, an expansion vessel that has lost its air charge (classically causing pressure to climb when hot and collapse when cold), or an internal boiler leak. On ecoTEC models, a failing pressure sensor can also report low when pressure is fine — which is where F22's story occasionally meets F75's.
When to call a Gas Safe engineer
One F22 a season is life with a sealed system; F22 as a recurring character is a leak or component fault that needs tracing. Book a Gas Safe engineer if you're topping up more than a couple of times a year, if pressure visibly falls over days, or if F22 alternates with other codes. Expansion vessel recharges, valve gland repairs and sensor swaps are all routine single visits on this brand.
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Frequently asked questions
How do I fix F22 on my Vaillant boiler?
Re-pressurise: open the filling loop beneath the boiler until the display or gauge reads about 1.2–1.5 bar, close it fully, and reset. If low pressure was the whole story, the ecoTEC returns to service immediately.
What causes Vaillant F22?
Anything that removes water from the sealed system: weeping radiator valves, a passing relief valve, recent bleeding, a discharged expansion vessel, an internal leak — or occasionally a pressure sensor misreporting. Recurrence is the signal to have it traced.
My Vaillant shows F22 every few weeks — is that bad?
It's a leak wearing a polite disguise. Beyond the inconvenience, every top-up dilutes the system's corrosion inhibitor, so chronic F22 quietly feeds the sludge problem too. An engineer visit to find the escape route is the economical move.
Is F22 dangerous?
No — it's protective. The boiler is refusing to run under-filled, which is exactly what you want it to do. The danger is only ever in ignoring what recurrence implies about the system's condition.
Does bleeding radiators lower boiler pressure?
Yes — releasing air reduces system pressure, so re-check the gauge after bleeding and top up to 1–1.5 bar if needed. It's the one 'pressure loss' cause that's entirely normal and self-inflicted.
Can low boiler pressure be dangerous?
Not acutely — the lockout exists to protect the boiler from running dry, and it errs safe. The risks are consequential: interrupted heating, and long-term component damage if the underlying leak dilutes inhibitor or lets air keep entering the system.
If you'd rather hand this over
Plenty of readers reach this point, weigh the afternoon involved, and sensibly hand it to someone who does it weekly. No judgement — that's what the network exists for. Every engineer is Gas Safe registered (plus OFTEC for oil and MCS for heat pumps), insured and locally based; prices are agreed up front; and genuine no-heat emergencies are prioritised. Describe the symptoms you've identified here and you'll get an accurate price rather than a guess — informed customers reliably get better quotes.