What Pressure To Set Expansion Tank?

[Bob Rohr]

The tank sizer at Wessels site is handy as it shows the difference in a plain steel tank and a diaphragm or bladder tank.

It's important to know a plain steel tank will need to be 75- 80% larger than a diaphragm tank, as there is no pre-charge available. Here is an example of a system with 2200 gallons capacity.

It's best to use these calculators, not rule of thumb %'s, especially if you are planning to buy a large diaphragm tank.

www.wessel.com

 

[Beagle Dad]

Since liquids are not compressible like gases are, a hydronic system must be under the influence of the pressure in the expansion tank. For this to happen when the system is cold the water must be slightly filling the expansion tank. As the system heats up and expands further the expansion tank takes on more water, the air compresses and the water pressure rises. As soon as the expansion tank has no water in it the system pressure will drop to a pressure equal to the height (head) of the system since the weight of the water is the only force left in the system to create pressure. Expansion tank selections take this initial volume into account. The Amtrol site has a good tank sizing tool.

The 80% is just the 12airpsi/15water psi, similar to the recommendation on a well system where the pressure switch has a recommendation to charge the well tank 3psi less than the cut-in pressure.

(broken link removed to http://www.amtrol.com/support/extrol_com_sizing.html)