Low boiler pressure can follow radiator bleeding, a small leak, or a fault in the heating circuit. Before topping anything up, notice whether pressure dropped once or keeps falling over time.
Check the reading when the boiler is cool
Check the manufacturer’s normal range because not every boiler uses exactly the same reference. Note the pressure when cool and how it changes during heating.
Look for signs of water loss
Radiators, valves, visible pipework, and the exterior discharge can show moisture or dripping. Pressure that falls again after correction deserves investigation for a leak.
A drop after bleeding radiators can be expected
Bleeding radiators can lower system pressure. That does not explain pressure that keeps falling for days or drops without any recent work.
Do not turn diagnosis into repeated refilling
Repeatedly topping up can hide a leak or fault and add fresh water to the system. If you do not know your boiler’s exact procedure or the pressure falls again, arrange qualified service.
Sources
- U.S. Department of Energy — Home Heating Systems — Operation, controls, efficiency, and general maintenance of home heating systems.