How to repressurise a boiler — safe, step-by-step guide

How to repressurise a boiler: a clear, safe guide

Knowing how to repressurise a boiler can save time and keep your heating running reliably. This guide explains why pressure falls, how to check the gauge, and a safe step-by-step method for repressurising most modern sealed systems — especially combi boilers. Read this before you start, and contact a qualified engineer if you are unsure.

Why boiler pressure matters

Boiler pressure keeps water circulating through radiators and the central heating system. If pressure is too low the boiler may lock out, radiators might not heat evenly, and the appliance can display fault codes. Conversely, over‑pressurising can strain components. Most domestic boilers indicate pressure on a simple gauge marked in bars; the manufacturer’s manual states the correct target for your model.

When to check the pressure

• Check the gauge when the system is cold for a baseline reading.
• Make a note of the pressure immediately after the boiler has been switched off and cooled — this gives the most reliable reading.
• If the pressure only drops after running the heating or after bleeding radiators, the cause may be air release or a leak.

Step-by-step: how to repressurise a boiler safely

The following steps cover the common procedure for sealed-system and combi boilers with a filling loop. Always refer to your boiler manual for model-specific instructions.

1. Check the manufacturer’s recommended pressure. Most domestic boilers display a target pressure on the gauge or in the manual. Make sure you know the cold pressure you need before adding water.
2. Turn the boiler off and let the system cool. Work on the system when cold to avoid scalding and to obtain an accurate pressure reading.
3. Locate the filling loop. On many combi boilers this is a flexible braided hose with two valves or a single lever between the mains supply and the central heating return. It is usually beneath the boiler casing; consult the manual if you’re unsure.
4. Open the isolating valves slowly. Open the mains valve(s) on the filling loop just enough to allow water into the system and watch the pressure gauge. Do this gradually — you are topping up until the gauge reaches the recommended level.
5. Close the valves and secure the loop. Once the correct pressure is reached, close the valve(s) fully and ensure the filling loop is returned to its stored position if it is a removable type.
6. Bleed radiators if necessary. If you rebuffed the system after bleeding radiators, you may have introduced or released air. Bleed radiators to remove trapped air and then recheck the boiler pressure, topping up if it has dropped again.
7. Check for leaks and reset. Inspect visible pipework, radiator valves and the pressure relief discharge pipe for signs of leakage or corrosion. If the boiler displayed a lock‑out, clear any fault codes according to the manual once pressure is correct.

Common causes of pressure loss

• Minor leaks in radiators, pipework or valves.
• Pressure relief valve (PRV) discharge due to overheating or a fault.
• Air introduced during bleeding which temporarily lowers pressure.
• Faulty expansion vessel or loss of its pre‑charge — a more technical fault that needs an engineer.
• Corrosion or worn seals on older systems.

Safety and troubleshooting tips

• Never force valves or overtighten components. Work gently and use the correct tools.
• Do not attempt internal repairs to the boiler or mains gas components yourself. Electrical isolation and gas work should only be carried out by a Gas Safe‑registered engineer.
• If the pressure drops repeatedly after repressurising, do not keep topping up without finding and addressing the cause — this can hide a significant leak or a failing component.
• Keep the boiler manual to hand and follow manufacturer guidance for your particular model.

When to call a professional

Contact a qualified heating engineer if:

• Pressure keeps falling after you have checked for leaks and repressurised the system.
• There are signs of water damage, visible leaks or dampness around pipework.
• The boiler shows persistent fault codes or repeatedly locks out.
• You suspect a faulty expansion vessel, PRV or other internal component.

Regular servicing helps prevent pressure problems and identifies faults early. If you’d like professional maintenance or a diagnostic visit, see our boiler servicing page at Boiler servicing or get in touch via our contact page.

Need help now?

If you prefer a technician to check your system, Sigma Plumbing & Heating offers experienced engineers who can diagnose pressure loss, repair leaks and carry out full servicing. Contact Sigma Plumbing & Heating on 07939012055 to arrange a visit or request a quote online through our contact page.