the feed/expansion tank for my immersion cylinder keeps overflowing. I have isolated the cold water supply but have noticed the 22mm pipe feeding the cylinder is warm at the base of the tank. Also noticed "swirling" patterns coming from said pipe back into the tank. As the heating system is a gravity one, and uses a coil inside the tank, can I check for coil leaks by simply turning off the pump? or is there anything else that could be causing this?