Use a water pressure gauge to check the water pressure in your house. You can usually screw it on the laundry tub faucet like you would a garden hose then turn on cold water and read the pressure you want your expansion tank to have the same pressure as the house pressure. To read the tank pressure use an air pressure gauge you can use the gauge that you use on a car tire. Fill or release tank air pressure to match house water pressure.
Everbilt sizing chart
In the old days they weren't needed because when the heated water in the house expands, it could push the excess back into the water main. But new codes require that water meters incorporate check-valves to keep water in the house from going back into the main and possibly contaminating the public water supply. As a result, especially when you start up a water heater with a full tank of cold water, and no faucets open in the house, the pressure can climb high enough to (hopefully) open the T&P valve on the water heater or blow out sink hook-up hoses or any other weak part in the system. A T&P valve that seems to often be seeping is more likely caused by the lack of an expansion tank. Very often the water district will "upgrade" the meters without telling their customers that they had better install expansion tanks.
It will be on the box. look before you buy
Yes, it can be installed horizontally.
Since almost every residential system has a check valve between street and house your water heater irrespective of type is a closed loop system. If you have a recirculation pump/loop on your heater you should definately have an expansion tank. Many cities now require the tank no matter what. More and more Tankless heaters have recirc pumps built in. Theres a 99% chance (give or take a percent or ten) your system is closed loop if residential.
Took me less than 10 minutes to install. Draining the water from the lines took longer than the install.
yes there is pressure. you will have to turn off the water main and drain the pressure at a sink.
yes mine is installed that way. However I ran a metal support line to a ceiling joist for added support.