Just had my CH altered from cylinder in bathroom to fortic tank/cylinder in the loft. Am now getting water dripping from the vent in the air bottle. Plumber reckons its due to water boiling in system and I need a new thermostat which will need ordering. I don't think so as it happens when water in sytem is only luke warm. No hot water returning to header tank. New bottle fitted but still dripping water. Any ideas?