F22 Glow-worm
Glow-worm F22 Fault: Low Water Pressure Fix
F22 on a Glow-worm means the same as it does on a Vaillant — no coincidence, since Glow-worm is the Vaillant Group's value badge and shares its code table. On most models it's the low water pressure lockout: the sealed system has dropped below threshold, and the boiler has paused rather than run under-filled.
What the F22 code means
The control board monitors sealed-system pressure and locks the boiler out below roughly 0.5 bar, holding F22 until pressure is restored and the fault reset. Cold-system target is the standard 1–1.5 bar. The group inheritance is practically useful: any engineer fluent in Vaillant pressure faults reads a Glow-worm F22 at sight, and the diagnostic sequence — and most parts — carry straight across.
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
Standard sealed-system arithmetic: water out somewhere means pressure down everywhere. The exits to check: radiator valve glands (slow, invisible weeps), the external pressure-relief pipe (drips mean the valve is passing), legitimate loss from bleeding radiators, an expansion vessel with a lost air charge (swinging pressure is its tell), or an internal boiler seep. On value-tier components, chronic under-pressure running plus the inhibitor dilution of repeated top-ups accelerates wear — the recurrence, not the code, is the real cost.
When to call a Gas Safe engineer
Book a Gas Safe engineer for the recurring version: multiple top-ups a season, pressure falling over days, relief-pipe drips, or F22 mixing with ignition or sensor codes. Group parts commonality keeps Glow-worm pressure fixes cheap and fast — vessel recharges, gland repairs and sensor swaps are routine. Time how long a top-up holds and report it; that single number does more for diagnosis than a paragraph of description.
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Frequently asked questions
How do I clear F22 on a Glow-worm boiler?
Top up via the filling loop beneath the boiler to about 1.2–1.5 bar, close the valve(s) firmly, then press reset. If pressure was the whole problem, normal service resumes immediately.
Is Glow-worm F22 the same as Vaillant F22?
Yes — shared group platforms mean a shared code table. Same meaning (low water pressure), same safe response, same diagnostic sequence when it recurs.
Why does my Glow-worm keep dropping pressure?
A leak is finding its way out — radiator valves, the relief pipe, the expansion vessel or an internal component. Value-tier parts feel the side-effects (inhibitor dilution, under-pressure wear) fastest, so recurring F22 is worth tracing promptly.
What pressure should my Glow-worm show?
Around 1–1.5 bar cold, in the gauge's green band, rising modestly when hot. Below 1 bar risks the F22 lockout; a system that needs frequent help staying in the band has a story an engineer should hear.
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.
How often is topping up the pressure 'too often'?
More than two or three times a year deserves investigation; weekly means an active leak. Every top-up also dilutes the corrosion inhibitor, so chronic re-pressurising quietly causes sludge damage on top of whatever's leaking.
When to bring in a Gas Safe engineer
Every guide on this site draws the same line in the same place: homeowner-safe checks stop at the boiler's casing. Re-pressurising via the filling loop, thawing a frozen condensate pipe, bleeding radiators, checking the thermostat and pressing reset once — all yours. Anything requiring the case to come off involves gas, and gas work is legally restricted to Gas Safe registered engineers for reasons that predate any website's advice. Our network handles exactly that side of the line, with engineers matched to the job and prices agreed before anyone starts.