Realize that an instant water heater raises the water temp about 40 degrees, if your incoming water temp is in the 40-50 degree range ( a 80-90 dregree shower is not what most folks want) you will need a small 20 gal. electric to pre heat it up. Especially if you are north of the Mason-Dixon line. Ranni has a good info on this issue. And they seem to want to make sure the installer gets it right. Hope this helps.
Really wow buy a quietside then I come in at 50 and leave at 120 all day long. Sounds like a crappy product to me I will have to check if mine has an built in preheater but its all one unit. Simple stupid hook up water and gas and you are good to go I had it as hot as 160 during testing just to see.