Glow-Worm Boiler F9 Fault Code: Causes & Fixes

The F9 fault code means your Glow-Worm boiler has detected low water pressure or a problem with the pressure sensor. The system cannot circulate water safely, so the boiler locks out and you lose heating and hot water. 

The most common causes are simple low system pressure, a small leak somewhere in the heating circuit, a faulty or dirty pressure sensor, and less often a pump or circulation issue.

UK homeowners and landlords see this code regularly, especially after bleeding radiators or during colder months. In many cases you can fix it yourself by topping up the pressure. 

Having boiler cover in place means you are never left facing unexpected repair bills when pressure problems keep returning, and it can also protect you from more expensive issues that develop if an underlying leak is left unresolved.

What Does the Glow-Worm Boiler F9 Fault Code Mean?

The Glow-Worm boiler F9 fault code means a water pressure or pressure sensor fault. The boiler monitors the pressure in the central heating system through a sensor linked to the control board. When the pressure drops too low, or the sensor cannot send a reliable reading, the boiler shuts down and displays F9. On newer models the same issue often appears as F22.

This is a safety measure. Without enough water pressure the pump cannot circulate heat properly and components can overheat. You will usually notice the pressure gauge sitting below 1 bar and the boiler refusing to fire until the pressure is restored or the sensor fault is cleared.

What Causes the Glow-Worm Boiler F9 Fault Code?

The F9 fault code is almost always linked to system pressure or the sensor that measures it. The main causes are low water pressure, a leak in the heating system, a faulty pressure sensor, and occasionally a circulation or pump problem.

Low System Water Pressure

Heating systems naturally lose a little pressure over time. Bleeding radiators, small amounts of air escaping, or simply the normal expansion and contraction of water can drop the pressure below the safe minimum. When the gauge falls under about 1 bar the boiler displays the F9 code and locks out.

How to Fix

1. Turn the boiler off and allow it to cool.
2. Locate the filling loop, usually a flexible braided hose or a pair of valves underneath the boiler.
3. Open the valves slowly and watch the pressure gauge rise.
4. Close the valves when the gauge reaches between 1.0 and 1.5 bar.
5. Reset the boiler by pressing the reset button or turning the control knob to the reset position for a few seconds.
6. Check that the F9 code has cleared and the boiler restarts normally.

Small Leak in the Heating System

Even a tiny leak at a radiator valve, pipe joint or the pressure relief valve can cause pressure to drop steadily. You may not see a visible puddle, yet the system slowly loses water and the F9 code keeps returning after each top-up.

How to Fix

1. After repressurising, monitor the pressure gauge over the next few days.
2. If the pressure falls again, look carefully under radiators, around valves and under the boiler for signs of moisture.
3. Check the pressure relief valve discharge pipe for drips.
4. Call a Gas Safe engineer who will pressure-test the system, locate the leak and repair it.
5. Once the leak is fixed the pressure should stay stable and the F9 code should not return.

Faulty or Dirty Pressure Sensor

The pressure sensor (sometimes called a pressure transducer) can become blocked with sludge or simply fail with age. When it sends an incorrect reading, or no reading at all, the control board assumes the pressure is wrong and displays the F9 error code even if the actual pressure looks fine on the gauge.

How to Fix

1. Confirm the pressure gauge is reading correctly between 1.0 and 1.5 bar when the system is cold.
2. If the gauge is in the normal range yet the F9 code remains, the sensor is likely at fault.
3. Do not attempt to remove or clean the sensor yourself.
4. A Gas Safe engineer will test the sensor, clean or replace it as needed, and reset the boiler.
5. After the repair the F9 code should clear and the boiler should operate normally.

Circulation or Pump Problem

On some Glow-Worm models the F9 code can also appear when the boiler cannot detect the small pressure rise that occurs when the pump starts. A seized pump, airlock or heavy sludge build-up can prevent this rise and trigger the same fault.

How to Fix

1. Check that all radiator valves and isolation valves under the boiler are fully open.
2. Listen for the pump running when the boiler calls for heat.
3. If the pump is silent or the F9 code persists after repressurising, call a Gas Safe engineer.
4. The engineer will test the pump, clear any airlocks or sludge, and replace the pump if necessary.
5. Once circulation is restored the boiler is reset and the F9 code should disappear.

How to Prevent the Glow-Worm Boiler F9 Fault Code

Preventing the Glow-Worm boiler F9 fault code is largely about keeping system pressure stable and the water clean. A few simple habits make a real difference.

1. Check the pressure gauge once a month when the system is cold and top up if it has dropped below 1 bar.
2. After bleeding any radiators, always repressurise the system straight away.
3. Book an annual boiler service with a Gas Safe engineer. During the service the pressure sensor, pump and system are checked for early signs of trouble.
4. Consider fitting a magnetic filter if your system is prone to sludge. This catches debris before it reaches the sensor or pump.
5. Address any visible leaks promptly rather than repeatedly topping up the pressure.
6. Consider a boiler cover plan that includes annual servicing and prioritised call-outs. This keeps your Glow-Worm appliance in good condition and means any future pressure or sensor fault is dealt with quickly without unexpected costs.

Protect Your Glow-Worm Boiler with Cover from PlusHeat

Pressure faults such as the Glow-Worm boiler F9 fault code can leave a home without heating at short notice. A boiler cover plan from PlusHeat gives you 24/7 access to Gas Safe engineers, parts and labour included, and an annual service that helps catch leaks and sensor problems early. 

Whether you own your home or manage rental properties, cover means one less worry when the boiler suddenly stops working.

FAQs

How often should I need to top up the pressure on a Glow-Worm boiler?

Most healthy systems only need topping up once or twice a year. If you find yourself repressurising every few weeks there is likely a small leak that needs investigating.

What pressure should a Glow-Worm boiler show when cold?

The ideal cold pressure is between 1.0 and 1.5 bar. Many engineers aim for around 1.2 to 1.3 bar as a comfortable mid-point.

Can a faulty expansion vessel cause the F9 code?

Yes. A failed expansion vessel can cause pressure to rise when the system is hot and then drop sharply when it cools, eventually triggering the F9 lockout. An engineer can test and recharge or replace the vessel.

Will a boiler cover plan cover repeated F9 repairs?

Yes. PlusHeat boiler cover plans include parts and labour for covered faults such as pressure sensor failures and leak repairs linked to the F9 code, so you avoid paying the full call-out cost each time.

Does the F9 code mean I need a new boiler?

In the vast majority of cases, no. Low pressure or a sensor fault is almost always repairable. Only if the boiler is very old and other major components are failing would replacement become the more sensible option.