Boiler Pressure Keeps Dropping? A Top-Up Isn’t the Whole Answer

Most combi and system boilers like to sit at roughly 1 to 1.5 bar when the heating is cold. It will rise a little when the radiators warm up — that is normal. If it falls below about 0.5 bar many boilers lock out to protect themselves.

Your boiler manual shows the exact figures for your model; it is worth keeping it with the boiler.

Most homes have a silver braided hose or a built-in key under the boiler. With the heating off, open both valves slowly, watch the gauge rise to around 1.2 bar, then close them firmly. If the gauge keeps rising after you close them, a valve is passing and needs attention.

If you are topping up more than once every few months, look for one of these:

• A weeping radiator valve — check for green or white crusting under each radiator.
• Recently bled radiators — releasing air also lowers pressure, so a top-up after bleeding is normal.
• A dripping pressure relief pipe — a small copper pipe leaving through the outside wall near the boiler.
• A tired expansion vessel inside the boiler, which makes pressure swing high when hot and low when cold.
• A hidden leak under floorboards, common in the 1950s–60s houses around Shard End and Hodge Hill where pipes run in screed.

If the pressure falls quickly, the relief pipe is dripping, or you notice damp patches on ceilings or floors, book a Gas Safe engineer. Work inside the boiler casing must only be done by a registered engineer.