Glow-worm F9 Fault Code: Water Pressure Problem

What does F9 mean on a Glow-worm boiler?

Glow-worm’s trade fault tables describe F9 as a pressure sensing fault, and the wording varies by range:

• Flexicom cx and the Ultracom cx, cxi, hxi and sxi: water pressure sensor fault, blocked flow filter or pump fault
• Flexicom hx and sx: low water pressure sensor
• CI, CI Plus, cxi, SI, sxi and Xtramax HE: water pressure sensor fault
• Xtrafast 96/120: F09, system pressure sensor fault

In practice the sensor is usually reporting a real drop, and topping up clears it. When the gauge already reads 1 to 1.5 bar, the boiler is reading the pressure wrongly or water is not moving, and that is the engineer’s list. On the hxi range F9 appears on earlier models only. Newer Glow-worm boilers with dot codes show low pressure as F.22 instead.

How do I clear F9 myself?

Read the gauge first, with the heating off and the boiler cool. Glow-worm’s repressurising guidance asks for 1 to 1.5 bar. Below that, find the filling loop, usually a braided hose or a pair of taps under the boiler, and open the valves slowly while watching the gauge. Stop at 1.5 bar, close both valves fully, then press reset once.

Do not overfill. The relief valve on a sealed system opens at 3 bar and, once it has lifted, it often drips afterwards. If you overshoot, bleeding a radiator lowers the pressure again. Give the boiler a minute after the reset: if F9 clears and the gauge holds steady through one heating cycle, the top-up was the fix. Our Glow-worm pressure guide has more on topping up.

Why does my Glow-worm keep going to F9?

A sealed heating system should hold its pressure for months. If you are topping up every few days, water is escaping. Check radiator valves, the joints under the boiler and the copper pipe on the outside wall that the pressure relief valve drains into. A pipe that is dripping, or leaves a damp patch below it, means the relief valve is letting by. A flat expansion vessel is the other common reason: the pressure rises sharply when the heating runs, the relief valve dumps water, then it falls when the system cools. The article on pressure that keeps dropping goes through that pattern.

If the gauge reads correctly and F9 stays, the boiler’s own pressure sensor may be faulty or its wiring loose, which is why Glow-worm lists it first for most ranges. On the Flexicom cx and Ultracom models the flow filter inside the boiler can block with sludge, and a seized pump gives the same result. None of these are inside-the-casing jobs for a householder.

What will the engineer do about F9?

The engineer compares the boiler’s gauge with the reading the control board sees in its diagnostic menu. If they disagree, the sensor or its plug is the fault. If the pressure is genuinely right and the sensor agrees, they check the pump runs and clean the flow filter, which on a sludged system may mean a power flush later. If the system keeps losing water, they pressure test it to find the leak, and check the expansion vessel charge and the relief valve.

Gas work is carried out by Gas Safe registered engineers. The visit starts with the £60 call-out and the repair price is agreed with you on site before any work starts. Book a Glow-worm boiler repair, or see boiler pressure repair if you cannot get the pressure to hold.