I have problems maintaining the pressure on my Vokera linea 24. No leaks at the radiators. I have 2 pipes (plus flue) going out to my external wall, one has a valve at the end of it( I think this will be a pressure valveand also used for draining) the other does not have any valve and is dripping(slowly)....Can anyone tell me what this pipe is for and why it is dripping....please