We sell are generally two types of heater.
"Flow-Thru" - large diameter tube with either a pressure switch or flow sensor detection such as M7. These will operate above 15gpm in most cases but more commonly over 20gpm.
"Low-Flow" - small 3/4" or 1" tubing generally. These will often be paired with a small circulation pump such as a Laing or a Tiny Might and operate at around 6gpm to 10gpm.
Most common flow detection for these is a reed flow switch but some use a pressure switch instead.