A sealed central heating system is a closed hydraulic circuit that must be held within a defined static pressure band so that water reaches the highest emitters, air is excluded, and the expansion vessel can absorb thermal expansion without tripping the safety relief valve. Repressurizing is the controlled admission of mains water into that closed circuit through a filling loop — a temporary connection guarded by two isolating valves in series, so that the potable supply and the heating water are positively separated by a double barrier once filling is complete, satisfying backflow-prevention principles. The topic sits in wet central heating within domestic building services, and the rate of pressure loss is diagnostically meaningful: slow decay over months is normal permeation and venting, whereas rapid loss indicates a circuit breach and is a fault requiring a qualified gas engineer rather than repeated refilling.
A sealed central heating system is a closed hydraulic circuit that must be held within a defined static pressure band so that water reaches the highest emitters, air is excluded, and the expansion ve…