We have an 18-month old oil-fired (Firefox) combi and 3 weeks ago it lost pressure.
Now it loses pressure when not on (rest state) from 1.5 bar to zero in 4 hours.
We have had the boiler engineer out who says the boiler is fine. He isolated boiler and pressure tested to 3 bar for an hour.
We replaced the secondary Expansion Vessel as a precaution.
We have no visual central heating leak signs and all pipework goes through roof, not floor.
We isolated the central heating circuit and pressure tested to 3 bar for 1-hour. No pressure drop.
So, no leak from boiler. No leak from central heating circuit. No pressure relief valve activation and no water from condenser discharge.
Is it possible for an internal leak to occur across the heat exchanger when in a "rest" state the higher pressure can force water into the water circuit and then be discharged when a tap is turned on?
