I have a Myson Velaire oil boiler which appears to be working fine, as does the heating, however, water from the expansion tank is disappearing at a rate of a tank every 2 days. Normally the water in the tank is 'topped up' once a year, now I am topping it up every other day. I have checked for leaks in the rads, there is no sign of water or damp in the house, I am at a loss as to why my system is using so much water. The Grundfoss pump was replaced 2 months ago, 3 rads were taken off when we redecorated. Apart from air in the system, which was dealt with, all appeared to be OK. I would appreciate any ideas or advice on this.