I have a Ideal Logic 30 Combi boiler. I noticed that every time when I have the central heating turned on, the boiler leaks out water from underneath the boiler. It doesn't leak out a lot of water and it's not like a constant leak, but it leaks at least once every time I have the central heating turned on. What's strange is that the boiler appears fine and doesn't leak when I am just using hot water. What could be wrong with it?