How to repressurise a boiler
6 min read ·
The five-minute job that resolves more no-heating calls than any other. Here is how to do it properly, and how to tell when topping up is the wrong answer.
Key takeaways
- Correct pressure is 1.0 to 1.5 bar with the system cold — aim for 1.2.
- Always fill cold. Water expands when it heats, so filling hot leaves you low later.
- Close both filling loop valves fully afterwards, and check them again the next day.
- Topping up more than twice a year means you have a leak, not a maintenance task.
- Never exceed 2.5 bar cold — the safety valve will dump the excess outside.
Why the pressure matters
A modern boiler runs on a sealed heating system, which means the water in your radiators is held at a slight positive pressure rather than being fed from a tank in the loft. That pressure keeps water in contact with every surface of the heat exchanger. Below about 0.5 bar the boiler can no longer guarantee that, so it locks out rather than risk running the exchanger dry.
Almost every manufacturer has a code for this. Vaillant calls it F.22, Worcester Bosch calls it CE 207, Ideal calls it F1, Baxi calls it E119. They all mean the same thing and they all clear the same way.
Find the filling loop
The filling loop connects your cold mains to the heating system so you can add water. It takes one of two forms.
The common one is an external loop: a braided silver hose, usually about 30cm long, with a small valve at each end, sitting on the pipework below the boiler. The valves are often small levers or slotted heads that need a flat screwdriver.
The other is an internal filling link, which is increasingly common on newer boilers. Instead of a hose there is a plastic key, often blue or white, clipped somewhere under the boiler. It slots into a fitting at the base and turns to open.
If you genuinely cannot find either, stop and ask an engineer to show you once. Do not start opening valves you cannot identify.
Fill it
- Turn the boiler off and let it cool. Filling a hot system leaves you underpressurised once it cools down.
- Find the pressure gauge — either a dial on the boiler front or a small gauge on the pipework beneath it.
- Open both filling loop valves slowly. You will hear water flowing.
- Watch the gauge. When it reaches 1.2 bar, close both valves.
- Reset the boiler if it is locked out.
That is the whole job. It takes five minutes.
Do not overfill
If you overshoot past about 2.5 bar cold, the pressure relief valve will discharge the excess through a copper pipe outside the wall. That is not dangerous, but it means the system will end up low again once it settles, and a valve that has discharged once sometimes fails to reseat properly afterwards and drips from then on.
If you do overshoot, bleed a small amount out of a radiator bleed valve with a cloth underneath until the gauge comes back to 1.2.
Close the loop properly
This is the step people miss. If a filling loop valve is left even slightly open, mains pressure slowly pushes into the heating system, and you will find the gauge creeping up over days until the safety valve starts discharging. Close both valves firmly and check the gauge again the following day.
On systems with an external braided hose, regulations require it to be disconnected after use in some circumstances. If yours is permanently connected, mention it at your next service.
When topping up is the wrong answer
Repressurising is a fix for gradual loss, which is normal. It is not a fix for a leak.
If you are doing this more than twice a year, water is escaping and you are steadily damaging the system by replacing it. Fresh mains water carries dissolved oxygen, and oxygen in a sealed heating system produces magnetite — the black sludge that blocks radiators, wears out pumps and eventually restricts the heat exchanger.
Signs it is a leak rather than normal loss:
- The gauge falls noticeably within days of filling.
- There is a damp patch, staining, or a musty smell anywhere near pipework.
- The copper discharge pipe outside the wall drips or shows limescale staining.
- Pressure climbs a long way when the heating runs — from 1.2 to 2.5 bar or more — which points at a failed expansion vessel rather than a leak.
Any of those is worth a call. A leak trace is £90 to £160 and it costs a great deal less than a system full of sludge.
What you should not do
Do not remove the boiler casing. That is gas appliance work and it is legally restricted to Gas Safe registered engineers. Everything described here is done from outside the case.
Do not keep resetting a boiler that will not hold pressure. The lockout is protecting the most expensive component in the appliance, and overriding it repeatedly is how a £150 repair becomes a new boiler.