Originally Posted by Evereddie
So the expansion tanks holds water, air, both? Does it matter which way it is mounted? It's purpose is to even out the water pressure and make the pump not run as much, especially when just a little dribble of water is needed, right?
Thanks for any info.
The expansion tank has a rubber bladder in the middle that separates the tank into two sections.
There is pressurized air on the Shrader side of the bladder. You set the pressure with a compressor to be less than your pump output pressure.
With no water pressure, the bladder expands like a balloon to fill the tank.
When water is introduced at a higher pressure than the bladder, the water pushes the bladder into the tank raising the pressure on the air side as water fills the "now" water side. When the pressure is equalized, the water stops entering the tank. A 2 gallon tank will hold about 1.5 gallons of water under pressure.
When running the water with city pressure, the tank acts like a shock absorber and keeps the water pressure constant under use (like a faucet opening while you are showering.
When under water pump pressure it also will supply about 1 gallon of water at full system pressure before the pressure drops enough to trigger the low pressure switch in the water pump. This is enough for several night time toilet uses before the water pump comes on and wakes everyone up.
The downside is that once it does come on, it will run till the tank is again at system pressure.
It can be mounted vertically, but you still need to access the air valve on the opposite end of the tank from the water inlet.
Here is a photo of a smaller one by Shurflo. It has a great view of the air valve. I tried it first but it is only good for a single #1 flush so I went with the larger tank.