Head?

[Ray Stafford]

pushing down on the water in the tank

atmospheric pressure is effectively the same on all installations.

Edit. In the UK. I'm not sure about places with really high mountains.

 

[ShaunCorbs]

atmospheric pressure is effectively the same on all installations.

yes and no i think as you get higher the air becomes less dense so this means less pressure????????

 

[Ray Stafford]

yes and no i think as you get higher the air becomes less dense so this means less pressure????????

Maybe if you went up Everest. But the air pressure in my attic and my cellar is essentially the same, but there would be a big difference in water pressure between tanks in those positions..

 

[missplumb]

atmospheric pressure is 1.013 bar which is generally why we pressurise systems to 1 bar as this is equal to atmosphere and hence the system will not be under a negative pressure at cold start.

 

[Ray Stafford]

Imagine that instead of a CWST, you had a 30m high, vertical soil pipe. It would contain about 50 gallons of water, not dissimilar to a decent water tank.

You fit a 1/2" fill valve right at the top, and a 22mm tank connector a few inches up from the bottom.

When you first opened an outlet, (provided that it was level with, or below the tank con) you would have great pressure - more or less 3 bar. But as the volume of water in the column descended (because the 22mm tank con was emptying your tall, thin CWST faster than the fill valve could replenish it,) the pressure would drop lower and lower. If you had an appliance that needed 2.5 bar to work, it would stop working fairly quickly - when just 1/6th of the water had been used.

The very lowest point it can go is the level of the tank connector, which is why I was taught to measure head from that point.

 

[townfanjon]

Atmospheric pressure is on everything , us included. If memory serves ,its 1 bar at sea level.

 

[JCSPLUMBING]

1,013.25 millibars actually

 

[plumbandplumber]

Thanks to all who replied. So, I'm taking head measurement from water surface. Glad to see a healthy debate started - have learnt even more than I thought I would.