Glow-worm Boiler Pressure: What It Should Be and Why It Changes

A Glow-worm boiler should normally read around 1 to 1.5 bar with the heating cold, rising a little when the system is hot. Your manual gives the exact figure for your model. If the pressure falls too low, the boiler shows an F22 code and stops. If it climbs well above the normal range when the heating runs, the expansion vessel is the usual suspect. Either way, one top-up or one bleed is fine. Needing to do it again and again means something needs repair.

What Pressure Should a Glow-worm Boiler Be?

With the heating off and the system cold, a Glow-worm combi or system boiler usually sits between 1 and 1.5 bar. When the heating is running, it is normal for the reading to rise by around half a bar as the water expands.

On older models there is a dial gauge on the front or underneath. On many current Glow-worm boilers the pressure is shown on the digital display instead, either on the main screen or through a button that switches the display to pressure. The manual for your model shows which. If you have a regular (heat-only) Glow-worm fed from a tank in the loft, it may not have a pressure gauge at all, because that type of system is open vented rather than sealed.

What Does Low Pressure Look Like on a Glow-worm?

When the pressure drops below the minimum the boiler needs, it shuts down and shows F22, the Glow-worm low pressure code. You get no heating and, on a combi, no hot water.

A low reading without a code is an early warning. The boiler may still run, but it is getting close to the cut-off point. A reading of zero usually means either a significant loss of water or a faulty pressure sensor, and an engineer can tell the difference by comparing the display with a separate gauge.

Can I Top Up the Pressure Myself?

Yes. Topping up a sealed system through the filling loop is a job Glow-worm documents for homeowners, and it does not involve any gas work. The method depends on whether your boiler has an external braided filling loop or a built-in filling key or lever underneath.

With the boiler off and the system cool, open the filling valve slowly, watch the reading rise to about 1 to 1.5 bar, then close the valve fully. Turn the boiler back on and reset it if F22 is still showing. Our guide to repressurising a boiler safely covers each type of filling loop in more detail. Do not remove the boiler casing to reach anything, and leave the filling loop closed afterwards.

Why Does My Glow-worm Keep Losing Pressure?

If you need to top up more than once or twice a year, water is leaving the system. The three usual causes are a leak, a pressure relief valve letting water through, and an expansion vessel that has lost its air charge.

• A leak: look for damp around radiator valves, pipe joints and the boiler itself. Leaks under floors or behind boxing can be invisible for months.
• The pressure relief valve: it vents to a copper pipe that usually ends outside. If that pipe drips, or is wet at the end, the valve is letting water out.
• The expansion vessel: a vessel that has lost its air charge cannot absorb the water expanding as it heats, so the pressure rises when hot, the relief valve opens, and it drops again when cold.

A pattern where the reading swings high when hot and low when cold points strongly at the expansion vessel. A steady fall over days, even with the heating off, points more to a leak. Our wider article on why a boiler keeps losing pressure goes through how each one is narrowed down.

What If the Pressure Is Too High?

A reading that climbs toward 3 bar when the heating runs is too high. Sealed systems have a relief valve that opens at around 3 bar, so a system that reaches it will start discharging water through the outside pipe.

The most common reasons are a filling loop left slightly open, so mains water keeps creeping in, or an expansion vessel that needs recharging or replacing. First check that both filling loop valves are fully closed. If the pressure is high only because you overfilled it, bleeding a radiator will bring it down. If it keeps rising on its own with the filling loop closed, the expansion vessel needs checking, and water dripping from the relief valve pipe outside is a sign it has already been opening.

When Should You Call an Engineer?

Call an engineer when a top-up does not last, when the pressure swings widely between hot and cold, or when water is dripping from the relief valve pipe. Those are signs of a fault, and topping up again will not repair it.

An engineer tests the expansion vessel charge with a gauge, checks whether the relief valve is seating properly, and inspects the boiler and the visible system for leaks before looking under floors. Some of these parts sit inside the boiler casing next to gas and electrical components, which is why the testing and repair are not a homeowner job. Tell the engineer how often you have been topping up, what the reading is hot and cold, and whether you have seen F22. That history usually narrows the cause before any tools come out. Glow-worm is part of the same manufacturer group as Vaillant, so the parts and the approach are familiar to any engineer who works on either brand. For London, our boiler losing pressure repair service focuses on finding where the water is going.