If that is what the installation directions say, I can't argue that point. However, the installation for thermal expansion tank I have allowed for it to be installed as your old one was. The main caution with any installation was to be sure there was a good support. The drawing you have with the tee would work. Mine is just a tee in the main line with the tank suspended below. The main line is strapped to floor joints. The height is of no concern.