Glow-worm Boiler Pressure: Low, High and the F22 Code

Updated July 2026 · Emergency Plumber Chelsea

Pressure matters because a sealed heating system is a closed loop of water that the boiler pumps round the radiators. The boiler needs a minimum pressure so the pump has water to push and the heat exchanger never runs dry. Too little and the boiler shuts down to protect itself. Too much and the pressure relief valve opens to let water out. Glow-worm boilers, like most modern combis, watch the pressure with a sensor and will lock out rather than run outside the safe range. Before you do anything, find the gauge. On older Glow-worm models it is a round dial under the boiler or behind a flap on the front. On models with a digital display, the pressure is shown on the screen, usually in bar.

Read the gauge with the heating off and the boiler cold. Pressure rises as water heats and expands, so a reading taken while the radiators are hot will always look higher. Around 1 to 1.5 bar cold is normal. Anything below 1 bar is on the low side, and below about 0.5 bar most Glow-worm boilers will refuse to fire. Up to about 2 bar while the heating runs is fine. If the needle sits near 3 bar, or goes into the red zone on the dial, the pressure is too high. Our general guide on what pressure your boiler should be explains the reading in more detail.

F22 on a Glow-worm display means low water pressure. It is the code people see most often, and it usually appears on the first cold morning after the heating has been off for the summer, or straight after someone has bled the radiators. Bleeding lets water out along with the air, so the pressure drops. A single F22 after bleeding is normal and a top-up fixes it. F22 that keeps coming back every few days or weeks is different: the water is escaping somewhere, and topping up only treats the symptom.

Topping up is done through the filling loop, which connects the heating system to the mains cold water. Glow-worm has used two arrangements over the years. Many combis have a filling device built into the underside of the boiler, operated by a lever, a tap or a small key. Older models, and many system boilers, use an external loop: a braided silver hose between two small valves on the pipes under the boiler. Your manual shows which you have. With the heating cold, open the valves slowly and watch the gauge rise. Close them as soon as it reaches about 1 to 1.5 bar, then press reset if the display still shows F22. If you have an external hose, close both valves fully afterwards so the loop does not leak through and slowly push the pressure too high.

If the pressure keeps dropping, look for where the water is going. Check under every radiator, around radiator valves and along any pipework you can see for drips, damp patches or green crusting on copper joints. Look under the boiler itself. Then go outside and find the copper pipe that comes through the wall near the boiler. That is the discharge pipe from the pressure relief valve, and if it is dripping or the ground beneath it is wet, the valve is letting water out. Our guide to a boiler that keeps losing pressure covers hidden leaks under floors and in walls, which are common in Chelsea's period houses and mansion flats.

A leaking pressure relief valve is often the result of a separate fault, not the cause. When the expansion vessel inside the boiler loses its air charge, it can no longer absorb the extra volume as the water heats. The pressure then shoots up each time the heating runs, the relief valve opens, water goes down the discharge pipe, and when the system cools the pressure drops below 1 bar. That cycle of too high when hot and too low when cold is the classic sign of a flat expansion vessel. An engineer can recharge the vessel, and if it has failed, replace it, and will usually replace the relief valve at the same time because a valve that has opened repeatedly rarely seals properly again. Our guide on a pressure relief valve that is leaking explains the discharge pipe in more detail.

If the pressure is too high, the usual cause is the filling loop. A valve left slightly open, or one that has started to let water through when closed, keeps feeding mains water into the system until the pressure reaches 3 bar and the relief valve opens. Check the loop is fully shut. If the pressure is only a little high, you can bring it down by bleeding a radiator with the heating off, letting out a small amount of water at a time and checking the gauge between each. Our guide to boiler pressure that is too high walks through this safely.

Other Glow-worm codes can look like a pressure problem. On many current models, F75 means the boiler did not detect the pressure rise it expected when the pump started. That can be low water, but it can also be a failing pump or pressure sensor, or air trapped in the system. F28 and F29 relate to ignition and flame loss, not pressure. If the gauge reads correctly but the boiler still shows a code, write it down with the model name from the front panel and check the manual, because code meanings vary between Glow-worm ranges. Our guide on how to reset a boiler explains when a reset is worth trying and when it is not.

Some pressure faults need a Gas Safe engineer. Call if you need to top up more than once a month, if the relief valve pipe outside keeps dripping, if the pressure swings between very high and very low, if you can see water inside the boiler casing, or if F22 or F75 returns straight after a top-up. Do not take the front casing off to look for the leak. The expansion vessel, pressure sensor and relief valve are all inside the sealed part of the appliance. Our boiler repair service covers Glow-worm and the other common makes.

Emergency Plumber Chelsea sends Gas Safe registered engineers to Glow-worm boilers in Chelsea, Knightsbridge, Sloane Square, South Kensington and Fulham. Tell us the model, the code on the display and what the gauge reads cold and hot, and we can often narrow the fault down on the phone. We agree the work before we start, and there is no call-out fee on jobs over 3 hours. Call 020 3002 6667.

Quick tips

• Read the gauge with the heating cold: about 1 to 1.5 bar is normal.
• F22 means low pressure; top up through the filling loop, then press reset.
• Close both filling loop valves fully afterwards, or the pressure will creep too high.
• A dripping copper pipe outside by the boiler means the relief valve is opening.
• Pressure that is high when hot and low when cold usually points to the expansion vessel.

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