I don't know much about this but I've not heard of a recirc system on an on demand water heater before. And I'll be damned if I'd feed the electric company enough to pump 3/4 gpm constantly and then fire my demand heater just to not have to wait a minute for hot water! But then I'm an old dude and have tons of patience and wouldn't spend all the extra money for the plumbing, pump and the rest. IOWs, I think it's a dumb idea and possibly not doable. Why didn't the plumber do it?
Quaity Water Associates