Why is water occasionaly coming out of pipes above the boiler?

Bolier Ideal Classic. there are three pipes coming out the top of the boiler one must be the water in, one the water out and the 3rd seems to be a drain outlet with a plug same as a radiater. one must be allowing water to escape enough to soften the wood boxing them in also the kitchen rad is always cold. System is 12 years old commissioned in 95