Do lots of research ( like you are doing?) and be very carefull. If you start making steam instead of hot water your house can vanish.
Didn't that thing come with instructions?
I made my own shell and tube heat exchanger out of 1' and 3/4' copper pipe. Works ok just make it as long as you can. Mine is 40'' and I wish it was longer.
I have a diagram on how it should be done that I got from a co that mfg's a system, I made my own. if you have acess to a fax machine email me and I will send it to you. Its a very simple system that preheats the water that goes to your exhisting hot water tank