Gas Boiler Losing Pressure with No Visible Leak: Causes and Solutions
Having to open your boiler's filling loop tap every two or three days because the pressure needle drops below 1.0 bar is a clear warning that your sealed heating system is losing fluid. If there is no puddle on your kitchen floor, where is the water going? Here are the 3 most common technical causes.
1. Deflated or Ruptured Internal Expansion Vessel
When central heating heats water from 20°C to 70°C, the water volume expands by roughly 4%. The expansion vessel contains a flexible rubber membrane with pressurized air that cushions this thermal expansion.
If the vessel has lost its air charge or the internal diaphragm is torn, water expansion causes boiler pressure to spike past 3.0 bar during heating. The 3-bar safety relief valve opens and quietly dumps excess water through the outdoor discharge pipe, leaving no indoor puddles.
When the heating cools down, system pressure plummets to 0 bar, triggering a lockout.
2. Weeping 3-Bar Safety Relief Valve (Dirty Valve Seat)
Microscopic rust grit and calcium scale can get trapped between the brass seat and rubber seal of the 3-bar safety valve, causing a continuous, invisible trickle out the external drain pipe.
3. Hidden Micro-Leaks in Radiator Valves or Underfloor Pipes
A hairline seep on a hot radiator valve packing gland or pipe union often evaporates immediately into room air due to the radiator's heat before a single drop can reach the floor.
FAQ
What is the correct pressure for a domestic boiler when cold?
Between 1.2 and 1.5 bar with the central heating turned off. If pressure drops below 0.8 bar, the boiler will lockout to protect the pump.
Is constantly refilling water harmful to my boiler?
YES. Each time you top up fresh tap water, you introduce new dissolved oxygen and calcium mineral scale into the circuit, accelerating internal radiator rust and boiler heat exchanger clogging.