How To Repressurise a Boiler (Step-by-Step)
Repressurising a combi boiler is a 2-minute homeowner job. Here's exactly how, what pressure to aim for, and the one moment to stop and call an engineer.
When you need to repressurise
You need to repressurise when the gauge reads below 1.0 bar (cold), or the boiler has locked out with a low-pressure fault code such as Vaillant F22 or Baxi E119. The job takes about two minutes and needs no tools.
What you're looking for: the filling loop
The filling loop is a braided flexible hose (or sometimes a rigid loop) connecting a cold-water pipe to a central-heating pipe, usually underneath the boiler. It will have one or two small valves. Some boilers (Vaillant ecoTEC, some Ideal and Worcester models) have an internal filling loop built into the boiler — a single knob or key underneath. Find which you have before starting.
Step-by-step
Follow these in order. The aim is to raise the pressure slowly to between 1.0 and 1.5 bar when the system is cold.
- 1.Switch the boiler off at the controls and let the system cool if it's been running.
- 2.Locate the filling loop and its valve(s).
- 3.Open the valve(s) slowly — you'll hear water flowing and see the gauge creep up.
- 4.Stop when the gauge reads between 1.0 and 1.5 bar. Do not overfill — above 3 bar the PRV will discharge water outside.
- 5.Close the valve(s) fully — a leaky filling loop is a common cause of pressure that won't stay down.
- 6.Switch the boiler back on and press reset if a fault code is showing. The low-pressure code should clear.
When to stop and call an engineer
Repressurising fixes the symptom, not the cause. Call a Gas Safe engineer if:
- The pressure drops back down within a few days — you have a leak or a failed expansion vessel.
- The pressure shoots above 3 bar while heating — the expansion vessel has likely failed.
- You can't find the filling loop, or the valve won't open — some boilers need a filling key.
- Water is leaking from the boiler or filling loop when you open it.
Why it keeps happening
If you're repressurising every week, stop — there's a fault to find. The most common culprits are a micro-leak on a radiator valve, a passing pressure-relief valve, or a discharged expansion vessel. Any of these is a straightforward Gas Safe repair, and fixing it protects the rest of your system from corrosion and air ingress.
Frequently asked questions
What pressure should my combi boiler be at?
Between 1.0 and 1.5 bar when the system is cold. Below about 0.5–0.6 bar the boiler will lock out; above 3 bar the pressure-relief valve discharges water outside.
Where is the filling loop on my boiler?
Usually a braided hose connecting a cold-water pipe to a heating pipe underneath the boiler. Some modern boilers (Vaillant ecoTEC, some Ideal and Worcester models) have an internal filling loop — a single knob or filling key on the boiler itself.
Can I repressurise a boiler myself?
Yes — repressurising via the filling loop is a homeowner job and takes about two minutes. But if the pressure keeps dropping, you have a leak or a failed expansion vessel that needs a Gas Safe engineer.
Why does my boiler lose pressure after I top it up?
Water is leaving the system — through a micro-leak, a passing PRV, or a failed expansion vessel. Topping up gets the heating on again but won't fix the cause; an engineer needs to find and repair the leak.