Why a boiler keeps losing pressure, how to top it up safely, and when to call a Gas Safe engineer. Call 020 8050 9933, open 24/7.
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A combi or system boiler that keeps dropping below its normal pressure is telling you something. Topping it up is a simple job, but if the pressure falls again within days, there is a leak or a fault that needs an engineer.
Most sealed-system boilers sit at about 1.0 to 1.5 bar when the heating is cold, and rise a little when it is on. Check your boiler's manual for the exact range. A gauge reading below 1 bar usually means the boiler will lock out or run poorly.
A leaking radiator valve or pipe, a recently bled radiator, a faulty pressure relief valve, or an expansion vessel that has lost its charge. Slow leaks can be hard to see, so look for damp patches, stains and drips around radiators, under floors and at the boiler itself.
Switch the heating off and let the system cool. Check the filling loop is connected, open both valves slowly and watch the gauge until it reaches the recommended pressure. Close both valves fully and, where you can, remove the loop. Only follow the steps in your manual.
If you have to top up more than once in a few weeks, stop topping up and book a visit. Repeated topping up can hide a leak that is damaging floors or walls, and adds fresh water that speeds up corrosion inside the system.
Never open the boiler casing, adjust gas parts or disconnect pipework yourself. Gas and boiler work must be done by a Gas Safe registered engineer. If you smell gas, leave the property, open doors and windows, and call the National Gas Emergency Service on 0800 111 999.
Yes, if your manual shows a filling loop and you follow the steps with the heating off and cool. If pressure keeps dropping, book an engineer instead.
Boiler repair (labour only) is typically £90 - £150 including VAT, plus the call-out fee. Parts are extra.
A fault with the expansion vessel or filling loop can push pressure up. If it rises above about 3 bar or water drips from the relief pipe, switch the boiler off and call an engineer.