fixmyplumbing

If the filling loop has been removed and capped so you are sure the combi boiler is not filling from the mains the heat exchanger is the only remaining cause. Combi boilers have a heat exchanger that takes the heat from the boiler and transfers it to the cold water coming from the water mains.  Modern heat exchangers are made from very thin stainless steel in the form of flat plates.If the steel fails either through corrosion or manufacturing faults the mains water can mix with central heating water.  The hole that is caused by the corrosion/fault is normally very small so the pressure rise is generally over several hours or even days.

The heat exchangers can’t be repaired so have to be replaced. This isn’t a DIY job so should only be done by a registered boiler engineer.