What the pressure gauge is telling you
Many sealed heating systems have a pressure gauge. If the reading is low, the boiler may stop operating or show a fault. The correct range depends on the appliance, so use the manufacturer’s instructions rather than relying on a universal number.
One drop or repeated pressure loss?
A single change may have a straightforward explanation, but repeated pressure loss needs investigation. Possible causes can include a leak, a pressure-relief issue, an expansion vessel or another system fault. Repeatedly topping up the system can hide the symptom without fixing the cause.
When to ask for help
If you can see water around the boiler or pipework, the pressure keeps falling, or the boiler will not reset normally, call for advice. Do not remove the boiler casing. If you smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999.