Hey Richard, I didnt know what the heck you were talking about either…What I THINK you are trying to find is just RECIRCULATING PUMP!
all it does is circulate hot water through the pipes so that when you turn on the shower in the morning it is hot already, no having to wait for the shower to get warm enough to get in.
They sometimes have timers, and sometiimes they will be run off a thermostat..(aquastat)
It may be kind of expensive to have a pump at the heater itself, because a plumber has to run a line all the way to the other end of the house.
There is a circulating pump that you can have installed under the bathroom sink for very cheap. they work pretty good! the plumbing is VERY simple to hook up, but you have to have an electrician install a GFI outlet under your sink.
I’ll try and find the literature and tell you the name of the pump i’m talking about.