Glow-worm Boiler Pressure: Low, High or Keeps Dropping
What pressure your Glow-worm boiler should be at, how to top it up safely, what F.22 and the other pressure codes mean, and why pressure that keeps dropping or rising needs an engineer.
Glow-worm gives the normal pressure range for its boilers as 1 to 1.5 bar. If your gauge or display reads below 1 bar, the boiler may stop and show F.22, and you can usually top it up yourself with the filling loop, slowly, until it reads between 1 and 1.5 bar, then close the loop fully. If the pressure is above 1 bar and the boiler still shows a pressure fault, Glow-worm advises calling an engineer rather than opening the boiler. Pressure that keeps dropping points to a leak, a dripping pressure relief valve or a failed expansion vessel. Pressure that keeps rising usually means the filling loop is letting water in.
Key Takeaways
- Normal pressure on a Glow-worm boiler is 1 to 1.5 bar, according to Glow-worm.
- Below 1 bar, the boiler may lock out with F.22. You can usually top it up yourself with the filling loop.
- If the gauge reads above 1 bar and a pressure code still shows, it is an engineer job.
- Topping up every few days is not a fix. Something is losing water.
- Pressure that climbs on its own usually means the filling loop is not fully closed or is letting by.
- Never open the boiler casing. Anything inside needs a Gas Safe engineer.
Glow-worm gives the normal pressure range for its boilers as 1 to 1.5 bar. If your gauge reads below 1 bar, the boiler may stop and show F.22, and you can usually fix it yourself by topping up through the filling loop until it reads between 1 and 1.5 bar. If the gauge reads above 1 bar and the boiler still shows a pressure fault, or you are topping up every few days, water is escaping somewhere or a part has failed. That needs an engineer, not another top-up.
What pressure should a Glow-worm boiler be?
Glow-worm's fault code guidance gives the normal range as 1 to 1.5 bar. Older boilers show this on a dial gauge, usually on the front panel or underneath. Newer models show it on the display. Check it with the heating off and the system cold, because the reading rises a little when the water is hot.
| Reading (cold) | What it means | What to do |
|---|---|---|
| Below 1 bar | Too low. The boiler may lock out with F.22 | Top up with the filling loop |
| 1 to 1.5 bar | Normal | Nothing, unless a code is showing |
| Well above 1.5 bar | Higher than it should be | Check the filling loop is shut. See the high pressure section below |
| Near 3 bar or water from the outside pipe | The pressure relief valve is likely to open | Call an engineer |
Low pressure and F.22
F.22 is Glow-worm's low water pressure code. Glow-worm is part of the same group as Vaillant and uses a similar set of codes, so you may also see pressure listed as a possible cause for F.75, a pump pressure fault, and F.83, where the boiler lights but the water does not heat up. Our guides to F75 and F83 cover both.
Glow-worm splits F.22 into two situations, and the difference matters.
- The gauge reads below 1 bar. The system is short of water. Glow-worm says you can top it up yourself if you are confident doing so, which is covered below.
- The gauge reads above 1 bar but F.22 is still showing. The water is there, but the boiler cannot see it. The cause might be the pressure sensor, its wiring, the pump or the internal expansion vessel. Glow-worm advises calling an engineer and not opening the casing.
Common reasons for a sudden low reading are bleeding radiators, a radiator being taken off the wall for decorating, or a small leak that has been going on for a while.
How to top up the pressure
Most Glow-worm combis are topped up with a filling loop: a short braided hose with a valve at one or both ends, fitted under the boiler between the cold mains and the heating return. Some models have a different filling arrangement, so if you cannot see a loop, check the user manual for your model before doing anything.
Turn off and cool down
Switch the boiler off and let the system cool, so you get a true cold reading.
Check the loop is connected
Both ends of the hose should be attached firmly. If it has been removed and stored, reconnect it by hand.
Open the valves slowly
Open the valve, or both valves one after the other, a little at a time. You will hear water flowing. Watch the gauge the whole time.
Stop between 1 and 1.5 bar
Close the valves fully as soon as the needle reaches the range. Do not overshoot, as you will then need to bleed a radiator to bring it back down.
Switch on and reset
Turn the boiler back on and, if a code was showing, reset it once as your manual describes.
For a fuller walk-through that covers other makes too, see our guide on how to repressurise a boiler.
Pressure keeps dropping
A sealed heating system should hold its pressure for months. If you are topping up every week or two, water is leaving the system or the expansion vessel has stopped doing its job. The usual causes are:
- A small leak at a radiator valve, a pipe joint under the floor or inside the boiler. Look for drips, rust marks or damp patches.
- The pressure relief valve letting water out through the copper pipe that runs outside, usually near the boiler. Check whether the end of that pipe is wet or dripping. Our guide to a pressure relief valve dripping explains why it happens.
- A failed expansion vessel. The pressure shoots up when the heating is on, the relief valve lets water out, and the pressure is then low when cold. That cycle repeats until the vessel is recharged or replaced.
Our guide to a boiler losing pressure goes through how an engineer tells these apart. Topping up over and over also brings fresh, oxygenated water into the system, which speeds up corrosion and sludge.
Pressure too high
If the reading is well above 1.5 bar when cold, the most common reason is that the filling loop was not closed fully after the last top-up, or one of its valves is letting water by. Make sure both valves are shut. You can bring the pressure down by bleeding a radiator with the heating off, catching the water in a cloth. If it climbs back up with the loop closed, the loop valve or the expansion vessel needs attention. See our guide to boiler pressure too high for the full picture.
When to call an engineer
Call an engineer when F.22 shows with a reading above 1 bar, when you are topping up more than once every few weeks, when the outside pipe is dripping, or when the pressure keeps rising with the filling loop shut. An engineer will check and recharge the expansion vessel, test the pressure relief valve and sensor, and trace any leak. Leaks under floors or behind walls can be found with our leak detection service.
Work inside the boiler must be done by a Gas Safe registered engineer. If your boiler is still under Glow-worm's guarantee, check the terms before booking a repair. If you rent, report it to your landlord or agent in writing.
Glow-worm losing pressure again?
Tell us the model, the reading and how often you have topped it up. Our labour is £95 an hour and the £60 call-out is waived on jobs over two hours. We confirm the price before any work starts.
📞 Call 020 7870 3200More on our boiler pressure repairs and expansion vessel replacement.
Frequently Asked Questions
What should the pressure be on a Glow-worm boiler?
Glow-worm gives the normal range as 1 to 1.5 bar. Read it with the heating off and the system cold. It is normal for the reading to rise a little when the heating is running.
How do I increase the pressure on my Glow-worm boiler?
Turn the boiler off and let it cool, find the filling loop under the boiler, and open its valves slowly while watching the gauge. Stop at 1 to 1.5 bar, then close both valves fully and switch the boiler back on. If your model has a different filling arrangement, follow the user manual for your model.
What does F.22 mean on a Glow-worm boiler?
F.22 is Glow-worm's low water pressure fault. If the gauge reads below 1 bar, topping up to 1 to 1.5 bar and resetting usually clears it. If the gauge reads above 1 bar and F.22 still shows, the pressure sensor, wiring, pump or expansion vessel may be at fault, and you need an engineer.
Why does my Glow-worm boiler keep losing pressure?
Pressure only drops when water leaves the system or the expansion vessel is not doing its job. Common causes are a small leak at a radiator valve or pipe joint, a pressure relief valve that lets water out through the outside pipe, a failed expansion vessel, or water lost after bleeding radiators.
Is high boiler pressure dangerous?
The boiler has a pressure relief valve that opens to release water if the pressure gets too high, so it is designed to protect itself. High pressure still needs sorting, because the valve can keep dripping afterwards and the cause, often a filling loop left open or a failed expansion vessel, will not go away on its own.
Sources and further reading
Source links checked 27 September 2026. Scheme rules and manufacturer instructions can change; use the current official guidance for your account or exact appliance.