What should the pressure be on a Glow-worm?
The pressure on a Glow-worm boiler should be between 1 and 1.5 bar with the heating off and the system cool. That is the figure in Glow-worm's own homeowner advice on repressurising, and it matches the recommended filling pressure of 1 to 1.5 bar in the installation instructions for the Energy7 and Betacom 4 combis. The Ultimate 3 operating instructions ask you to check monthly that the pressure sits in the same 1 to 1.5 bar band.
At the top end, Glow-worm's advice is to avoid going over 2.5 bar. The pressure rises a little when the heating is running, which is normal, but a cold reading well above 1.5 bar usually means the system has been overfilled.
For how these figures compare with other brands and what the readings mean hot and cold, see our guide to normal boiler pressure.
How Glow-worm boilers show the pressure
Current Glow-worm boilers show the pressure on the digital display rather than on a dial. The details differ slightly between ranges.
Energy
The Energy7 combi operating instructions describe a pressure symbol on the display. When it is lit steadily, the pressure is in the permitted range. When it flashes, the pressure is outside that range. The installation instructions explain that the pressure value itself flashes once it falls below 0.5 bar.
Ultimate
The Ultimate 3 combi shows the filling pressure of the heating system on its digital display. Its operating instructions ask you to check it monthly and make sure it sits between 1 and 1.5 bar, topping up if it has fallen below that band.
Betacom
The Betacom 4 has a digital pressure reading. According to its installation instructions, the value flashes below 0.5 bar, and below 0.3 bar the boiler switches off, the display shows 0.0 bar and fault F22 is stored.
Older Glow-worm boilers
Older ranges, including earlier Betacom and Ultimate models, may have a dial gauge on the front or underneath. The same 1 to 1.5 bar target applies unless your user manual gives a different figure.
Glow-worm pressure at a glance
| Model | How pressure is shown | Recommended (cold) | Warning | Boiler stops |
|---|---|---|---|---|
| Energy7 combi | Digital value and pressure symbol | 1 to 1.5 bar | Symbol or value flashes, below 0.5 bar | Below 0.3 bar, shows F.22 |
| Ultimate 3 combi | Digital value on the display | 1 to 1.5 bar | Reading outside 1 to 1.5 bar at the monthly check | F.22 low water pressure |
| Betacom 4 combi | Digital value on the display | 1 to 1.5 bar | Value flashes below 0.5 bar | Below 0.3 bar, shows 0.0 bar and stores F22 |
| Older models | Dial gauge on some models | 1 to 1.5 bar | Needle below 1 bar | Check your user manual |
Figures are from the Glow-worm operating and installation instructions for each model, which Glow-worm publishes on its website.
Glow-worm low pressure and F22
When a Glow-worm boiler runs short of water, it shows F22 (written F.22 on some displays), which the Energy7 instructions describe as water deficiency in the heating installation. The fix they give is to fill the heating installation. Our Glow-worm F22 page covers the code in more detail.
If the display shows a normal pressure but you get a pressure or circulation fault, the problem may be the pressure sensor or pump rather than the water level. Our Glow-worm F75 page explains that fault.
On a combi, a low pressure shutdown stops the hot water as well as the heating, because the same burner and heat exchanger serve both. If the pressure is back in range and the radiators work but the taps stay cold, the cause lies elsewhere, and our guide to a Glow-worm boiler with no hot water covers it.
Where is the filling loop on a Glow-worm?
On most Glow-worm combi boilers the filling loop is underneath the boiler. Glow-worm's homeowner advice describes it as a flexible silver hose that connects two small valves under the boiler. The Energy7 installation instructions say a filling device is supplied with the boiler and connects through the cold water isolation valve, with a double check valve to stop heating water flowing back into the mains.
Glow-worm also notes that some older combi boilers use a filling key instead of a hose. On a system boiler with a hot water cylinder, the installer fits the filling loop on the pipework, often under the boiler or near the cylinder. Our filling loop guide shows what each type looks like.
If the hose has been taken off and stored, look in the cupboard under the boiler and with the boiler paperwork before buying a new one. A hose that screws on with rubber washers is the part you need.
How do I repressurise a Glow-worm boiler?
To repressurise a Glow-worm boiler, let it cool, open the filling loop valves slowly until the display reads 1 to 1.5 bar, then close them. These steps follow Glow-worm's homeowner advice.
- Turn off the heating and hot water and wait until the pipes and radiators have cooled.
- Find the filling loop under the boiler. If the hose is detached, connect both ends hand tight.
- Open the valves slowly. Turn both valves 90 degrees anticlockwise. You will hear water entering the system.
- Watch the pressure. Keep your eye on the display or gauge as it rises.
- Stop at 1 to 1.5 bar. Turn both valves clockwise to close them fully.
- Remove the loop if it is detachable, and store it safely for next time.
The Energy7 operating instructions make the split of jobs clear: the installer is responsible for the first fill and for the water quality, and topping up afterwards is a job for you as the householder.
What happens after you top up
After you repressurise, some Glow-worm boilers run an automatic purge for about five minutes to clear air from the system. Glow-worm says the pressure may move up and down during this purge, and that this is normal. Wait until it has finished before judging the reading or topping up again.
If a radiator is cold at the top or you hear gurgling, bleed it once the purge has finished, then check the pressure again. Bleeding lets out air and the pressure falls a little, so you may need a second, smaller top-up.
Glow-worm boiler pressure too high
If the cold reading is above 1.5 bar after topping up, you have added too much water. The Ultimate 3 operating instructions say to bleed a radiator to reduce the pressure, or ask your installer. Check that both filling valves are fully closed, because a valve left slightly open will keep pushing the pressure up.
If the pressure is right when cold but climbs towards 3 bar with the heating on, the expansion vessel is the likely cause. The Energy7 has a relief valve preset to 3 bar that discharges through a pipe to the outside, so a dripping pipe outside is another sign. Our guides to boiler pressure that is too high and the boiler pressure relief valve cover what happens next.
When to call a Gas Safe registered engineer
Glow-worm says that a pressure that keeps dropping suggests a problem with the system that needs a Gas Safe registered engineer. Book one if:
- You need to top up again within days or weeks.
- F22 returns even though the display showed a normal pressure after filling.
- The pressure climbs above 2.5 bar when the heating runs.
- Water drips from the relief valve pipe outside or from under the boiler.
Only use the filling valves outside the boiler case. Never remove the casing to look for a leak or reach the pressure sensor; those parts sit next to gas and electrical connections and are for a Gas Safe registered engineer only.
Before the engineer arrives, our guide to a boiler losing pressure lists the checks you can make yourself, such as radiator valves and the pipe outside, so you can tell them what you found.