Water pressure

What is the formula to figure pressure from water. I have an expansion tank I was thinking of mounting above my boiler. It would have about 7' rise from top of boiler to tank. how many gallons would it take to make 12lb pressure. The tank can hold 125 gallons.

Im not really sure if this helps but....
Water pressure in a column (static at sea level etc)
..is based on hieght of column.

..at roughly 62 pounds per cubic foot (weight of water)
A column of just over 2 feet high will produce a pressure off 1 psi at the bottom (about .433 psi per foot)

... like I said I don't know if this will help... I would think your setup is going to be dependent on the incoming PSI of the water supply

So what pressure would 50 gallons at 7' drop make. I think it is 7.5 gallons/cubic ft.

To answer your question - The amount of gallons in the tank is irrelevant, it is all about height. From memory, I believe you get 0.43 psi for every foot of water height. But that doesn't matter much...because you should be able to hook up the tank at your proposed location, and once it is connected to the boiler...open the valve and add water to the boiler until there is 12 psi in the system...the tank will have a large air cushion in the top, and the temperature should remain very stable.

Then can I assume that it would not matter if the tank was higher than the boiler or not. The tank does not have a bladder just a steel tank.

It should be higher than the boiler, and piped so any air bubbles can go up into the tank...but for $100 for a big Extrol tank, I don't think it is worth the effort.

What are you currently using for an expansion tank?

A extroll with a hole in the bladder.

Phil May wrote:A extroll with a hole in the bladder.

I bet that doesn't work as well as you'd like.