Low boiler pressure is one of the most common reasons a combi boiler stops working. In most cases you can fix it yourself in under five minutes by using the filling loop. This guide covers exactly how to repressurise your boiler safely — and when low pressure means something more serious.
What Pressure Should a Boiler Be?
A combi or system boiler operates as a sealed pressurised system — unlike older open-vented systems with a feed-and-expansion tank in the loft. The pressure in the system is shown on the boiler's pressure gauge, which is either a mechanical dial gauge or a digital display depending on the boiler model.
Correct operating pressures are:
- Cold system (boiler off and system at room temperature): 1 to 1.5 bar. Most manufacturers specify 1 bar as the minimum acceptable cold pressure.
- Hot system (boiler running, system at operating temperature): Typically 1.5 to 2 bar. As water heats up it expands, and the system pressure rises. A reading of 1.8–2 bar with the heating on is normal. A reading above 2.5 bar when hot indicates overcharging or a failing expansion vessel — this is a different problem requiring an engineer.
Most boilers will display a low pressure warning or stop firing entirely when pressure drops below 0.5 bar. The boiler may show an error code — common ones include F22 on Vaillant, EA on Worcester Bosch, and E119 on Baxi — all of which relate to insufficient water pressure.
Signs of Low Boiler Pressure
- Boiler firing and then cutting out, or refusing to fire at all
- Pressure gauge reading below 0.5 bar (or the needle well into the red zone)
- Radiators not heating despite the boiler appearing to attempt operation
- Error code displayed on the boiler's digital panel
- No hot water despite the boiler's indicator lights being on
Finding the Filling Loop
The filling loop is the connection between the cold mains water supply and the sealed heating system. It is used specifically to add water to the system when pressure is low. On most installations, it looks like:
- A flexible stainless steel braided hose, typically 15–30cm long
- With a valve at each end — often a slotted screw valve (turned with a flathead screwdriver) or a lever-handle valve
- Usually located underneath the boiler, behind the lower access panel, or running between the cold water supply pipe and the heating return pipe nearby
Some boilers have an internal filling loop that is accessed via a key or lever on the front panel rather than an external hose — check your boiler's instruction manual if you cannot locate an external filling loop. Common examples include certain Vaillant ecoTEC and Ideal Logic models.
If the filling loop is not connected — it has been removed between uses, which is best practice — you will need to reconnect it before proceeding. The hose connects to two identical-looking 15mm compression fittings.
Step-by-Step: How to Repressurise Your Boiler
- Turn the boiler off. Switch it to standby or off mode. Do not repressurise a hot, running boiler — wait for the system to cool if it has been running recently. Repressurising a hot system can cause pressure to overshoot dangerously when the system cools.
- Locate the filling loop as described above. If it is a separate hose, connect it to both ports now — there should be a click or a tight fit confirming the connection is secure.
- Watch the pressure gauge. Note the current reading before opening any valves.
- Open both filling loop valves slowly. Turn both valves (there should be two — one on each end of the hose) a quarter turn to open them. You will hear water flowing into the system. Open them slowly rather than fully — this gives you better control over how fast the pressure rises.
- Watch the gauge as the pressure rises. Stop when the gauge reads approximately 1.5 bar. Do not overshoot — if you go above 2 bar cold, you have overcharged the system.
- Close both valves. Turn both valves back to their closed position. Confirm both are fully closed — a valve left slightly open will continue to allow water in and the pressure will rise further.
- Disconnect or stow the filling loop if it is a separate hose. Leaving it connected while the boiler operates is not best practice — if either valve has a minor leak, pressure will continue to rise.
- Restart the boiler. Switch it back on. The low pressure error should clear and the boiler should fire normally. Check the pressure gauge after 5–10 minutes of operation to confirm it remains within the normal range (1.5–2 bar when hot).
What to Do if Pressure Drops Again Quickly
If you repressurise the boiler and the pressure drops back to low within a few days or weeks, there is a leak in the system. The leak may be:
- Visible and external: Check under all radiators and at all visible pipework joints for water marks, staining, or active drips. Check around the boiler itself, particularly at the connections where pipes enter the unit. Check the pressure relief valve discharge pipe (typically exits through an external wall) — if it has been weeping, there will be lime-scale deposits or staining below the outlet.
- Hidden: Micro-leaks in pipework concealed under floors or within walls will not be visible. These are identified by a plumber using a pressure test — pressurising the system and monitoring for decay over time.
Do not repeatedly repressurise a system that keeps losing pressure — this introduces fresh oxygenated water repeatedly, accelerating corrosion and scale build-up. Call a Gas Safe registered engineer to find and fix the leak.
When NOT to Repressurise
- If the boiler pressure gauge consistently reads above 2.5 bar when cold — this indicates overcharging or a failing expansion vessel, not low pressure. Do not add more water.
- If you see water coming from the pressure relief valve discharge pipe — this indicates the system is already over-pressurised. Do not repressurise; call an engineer.
- If you have already repressurised the system more than twice in the past six months without identifying and fixing a leak — stop and call an engineer.
Safety Note on Scalding
Always repressurise with the boiler off and ideally with the system cooled down. Hot system water under pressure can scald seriously if connections are made incorrectly or filling loop valves are faulty. If you are uncertain about any part of the process, call a Gas Safe registered engineer — a repressurisation check is a quick, low-cost call-out.
Frequently asked questions
What pressure should my combi boiler be at when cold?
A combi boiler should read between 1 and 1.5 bar on the pressure gauge when the system is cold (boiler off, system at room temperature). Below 0.5 bar, most boilers will stop firing. When the heating is running and the system is hot, pressure rises to 1.5–2 bar — this is normal. If pressure exceeds 2.5 bar when cold, the system is overcharged and needs an engineer, not more water.
Where is the filling loop on my boiler?
The filling loop is usually a flexible stainless steel braided hose with a valve at each end, located underneath the boiler or behind its lower access panel. Some boilers (particularly Vaillant ecoTEC and certain Ideal Logic models) have an internal filling mechanism accessed via a lever or key on the front panel rather than an external hose. Check the boiler manual if you cannot locate it — the manual diagram shows the filling loop position for your specific model.
Why does my boiler keep losing pressure?
If a boiler loses pressure repeatedly after repressurising, there is a leak in the sealed system. Check all visible radiator connections, pipework joints, and the area around the boiler for water marks or active drips. If nothing visible is found, the leak may be in concealed pipework. Do not keep repressurising a leaking system — repeated topping up with fresh oxygenated water accelerates internal corrosion. A Gas Safe engineer can pressure-test the system to locate the fault.
Is it safe to repressurise my boiler myself?
Yes — repressurising using the filling loop is a straightforward task designed to be done by the householder, not a Gas Safe certified operation. The key safety rules are: always turn the boiler off before you start, repressurise to 1.5 bar and stop, close both filling loop valves fully before restarting the boiler, and do not repressurise if the system is already showing high pressure or if the pressure relief valve is weeping. If you are unsure, a quick call to an engineer is a low-cost reassurance.