I just went to a seminar at the local supply house. Turns out there are some issues with using pipe dope to make connections - can effect the flow sensor and the flow regulator (controls water flow). Make sure you use teflon tape, not pipe dope.
If you are experiencing temperature variations, it's a setup issue or a flow sensor issue or possibly a gas pressure issue. Read the setup sheet at the link below.
They handed out a startup instruction sheet which I've posted.
If you are going to run a continuous pump, make sure you have an aquastat that runs the pump. It should be set 10 degrees below the heaters set point or heater'll start short cycling as it reaches set point. This will significantly shorten life and increase repairs and gas use.
These buffer tank units address the issues people have with tankless in an elegant and very efficient manner - but they are NOT a replacement for using an indirect connected to a Prestige ModCon.