If a sealed heating boiler shows low pressure, some models allow the system to be topped up through a filling key or filling loop. The exact procedure varies and should be done with the system cool, following the manual and never exceeding its recommended range.
First confirm the pressure is actually low
Read the gauge with the heating system cool and compare it with the manufacturer’s specified range, often around roughly 1–1.5 bar cold but not universally. Do not top up solely from a number remembered from another boiler.
Identify your model’s filling arrangement
There may be a flexible filling loop with one or two valves, an integrated filling key, or an automatic arrangement. If you cannot positively identify the filling controls, stop and consult the manual or a technician.
Add water slowly while watching the gauge
Open the filling control gradually and listen for water entering. Close it when the manufacturer’s cold-pressure range is reached; do not chase a high reading and never leave manual filling valves open.
If pressure falls again, diagnose the cause instead of repeatedly topping up
Visible leaks, a dripping pressure-relief discharge, or frequent pressure loss indicate a fault that needs investigation. Repeatedly topping up introduces fresh water and does not fix a leak or expansion-vessel problem.
Sources
- U.S. Department of Energy — Home Heating Systems — Operation, controls, and general maintenance of home heating systems.