I have an apartment on the 4th floor. Water is pumped up. My problem is that if I turn off the water at the stop cock to the electric unvented hot water cylinder ( as I'm told to ), when I return to the flat and turn on the stop cock, I hear a few gallons of water rushing into the cylinder, and there are air locks at the taps. Where did the water go to? I have risked not turning the water off because I am alarmed. Not a good idea in this weather. The builder brought the plumber to see it,and claim the system is working properly. Any ideas please ?