What is the difference between a well expansion tank and a water heater/boiler thermal expansion tank?
Had an inspection where a 2 gallon H20-TO brand (maroon colored that are usually used for well systems) expansion tank was installed on the water heater. Usually they say thermal expansion tank, whereas this one did not (mod #T2; Date code - 14810765).
Can someone please enlighten me.
Thanks.
If there is a difference it would be in temperature and or pressure rating.
BTW-Well “expansion” tanks are predominantly blue in my area.
I would think the main difference is the size. From my expericence, a water heater expansion tank is not nearly as large as a typical water well/pump tank.
The water heater expansion tank may also need a higher pressure rating, although it should never have to exceed the pressure of the TPR valve.
Hi. Joshua, I will try to be a better plumber than an electrician here to help you on this one. I do know my hammer a little better though. 
In some ways they are alot similar, but provide a different function. The water pressure tank stores enough water so the well pump does not cycle everytime you open the faucet.
Here is the hydronic expansion tank.
http://www.radiantheat.net/image_manager/15xpansion.jpg
Expansion tanks are required on systems which are closed and recirculate the same fluid over and over. The expansion tank uses an air cushion to allow for the natural expansion of liquids as temperature rises. As liquids heat up in closed loop systems, the liquid needs room to expand or system pressure can increase beyond recommended operating levels. Expansion tanks can be as simple as a cylindrical tank with the inlet and outlet below the lowest liquid level expected. Expansion tanks in smaller systems can have a pressurized bladder to allow for expansion. This unit has the available volume of 2 gallons. Expansion varies depending on amount and type of liquid.
Here is the Well Water Pressure tank;
http://www.hammerzone.com/archives/plumbing/well/pressure_tank/pbwta47b.jpg
What Is A Pressure Tank? Why Do You Need It?
When a house is built in an area that is not served by a municipal water works, a private well system is used.
The well pumps used in most systems are quite powerful, often capable of delivering more gallons of water per minute than any single faucet could draw.
Everybody I have spoken to in the plumbing or water-well business says that well pumps can burn out prematurely if they are made to start and stop more than necessary.
By using a holding tank, the water can be used in a leisurely manner over a long time, and the pump will run briefly to refill the tank.
A pressure tank can be as simple as a big metal chamber with one hole in the bottom for water to flow into.
As the pump pushes water in, the air in the tank is compressed.
At some point, called the cut-out pressure, a switch interrupts the power to the pump and no more water enters the tank. Since there is compressed air above the water, there is enough force acting on the liquid to push it out of the tank, through the pipes, and all the way upstairs (possibly several flights of stairs) to the faucet.
But… there is a flaw with this simple design. Air can be dissolved in water, believe it or not.
Eventually a simple tank with compressed air above the water will experience a loss of this compressed air. So the air space above the water gets smaller day by day.
What you notice is that the pump will run for very short periods, perhaps 5 or 10 seconds, and do this frequently whenever water is being drawn.
A proper well pressure tank does not use a simple empty steel can, it uses a flexible bladder or bag inside the tank, with the bottom of the bag pointing to the bottom of the tank. The bladder occupies a portion of the total tank volume. The steel tank contains compressed air.
Hope this helps you a little bit.
Marcel :)