Most thermal expansion tanks manufacturers will call for the tank to be installed in the vertical position and properly supported. They type of support required for the expansion tank will vary with the piping in the house.
Copper supply lines can support a standard 2.1 gallon expansion tank. However you should ensure the copper tubing is connected properly.
Pex supply lines make for an easy addition to the expansion tank. Be weary, without additional support you could have issues in the event the expansion tank fills with water. A copper t is usually the best way to setup proper bracing.
An expansion tank can fail within the first two years if not calibrated properly. In order to calibrate a thermal expansion tank you must have a water pressure gauge and air pressure gauge. The expansion tank pressure must be equal to your water pressure or it will fail quickly.
One of the biggest drawbacks to thermal expansion tanks is that they usually don’t last as long as the water heater they are designed to protect. A typical lifespan for a thermal expansion tank is 5-7 years. As regular maintenance, an expansion should be inspected every year.
Expansion tanks come in a variety of sizes and warranties. A typical expansion tank will have a 1,5, or 6 year warranty.
If an expansion tanks fails it will fill with water. You can usually determine if the expansion tank is full or not by the feel. You can also release some air out of the valve stem to see if water is released.