Hot water feed sucking in air

[topol]

Some help please. Whenever I open the bath hot taps I can hear air being sucked into the hot feed thru the h/w tank vent pipe (the one that has an open end back into the c/w tank). This happens as soon as I turn the taps on - or the power shower for that matter.

The c/w is used faster than the mains feed so after a short time (too short when Im in the shower...) the hot water stops. Theres still loads in the h/w tank - if I turn the taps off for a few minutes I can get more h/w, until the same thing happens.

Any ideas ????

 

[MANDATE]

It sounds to me like the cold water is coming from the tank( whereas mine is direct of the mains supply). This is done so the pressure of the cold and hot are the same.

The hot water leaves from the top of the cylinder normally being pushed out by the pressure created by the height of the cold water tank.
Now i'm thinking what would be the result if there was a restriction in the pipe feeding cold water to the bottom of the cylinder.
The water in the expansion/overflow pipe would provide the same pressure, but because of the very small volume it would last only seconds and the level would also drop. If you use a pump to suck the water from the cylinder it just amplifies the situation. It can't get enough water if the water is not replaced at a fast enough rate so it takes whatever is available which includes the bit that is in the expansion/overflow pipe.
To add to it, If the cold water is also coming from the tank the level in the tank is probablely dropping (including the expansion pipe) making the pressure less and less.
Well thats my theory, but I'm not a plumber.
Finding a simple solution is not easy. Remove restriction maybe?
Stop the air entering the expansion pipe whilst still allowing water to escape? ( spring loaded valve). Bigger water tank?
Lets see what the plumbers think!

 

[AlanE]

NEVER EVER put any obstruction on the vent pipe.

Has system always been like this or have you done ANYTHING just prior to fault developing?

Alan