Vent/Overflow pipes - how do they work?

[mk2phil]

After thinking about this for a little while I have 3 questions regarding my open vented system:

1. the feed for my gravity HW curcuit comes up from the floor below and tee's off into the the HW tank. The pipe then continues to go up into the loft and vents over the CW tank. Why doesn't the water continue to push upwards and out of the vent when the HW is on? Seems to me that there are 2 routes for the water to take yet it only goes into the tank.

2. Again, for my domestic HW the pipe comes out of the top of the HW tank and tee's into a vertical pipe. Why when you turn a tap on doesn't the water flow both out of the tap AND up and out of the vent pipe? What stops it?

3. If the system is open with vents going to the tanks in the loft, how does the system know when to stop filling? If its not a closed system why doesn't the system continue to fill up and out of the vent pipes? Surely water will always go where it can?

I realise these are probably plumbing basics, but any explanation would be appreciated, Cheers.

 

[stem]

I don't think you understand how the system works. The pipe you refer to is not the feed, it's the pipe that supplies water to the taps as it goes down, and the vent going up.

The "feed" is the cold water main that supplies the tank via the ballcock. This ballcock is what stops the water going in when the tank is full.

Gravity stops the water comming out of the vent, because is higher than the level of water in the tank below it.

Here's a diagram.

http://www.gasman.fsbusiness.co.uk/system_basics.htm