I am considering a Ultra 230 boiler and in looking over the documentation I have three questions:

1. What the heck is a "condensing" boiler?

2. Will this boiler system work with cast iron radiators?

3. The piping diagram shows a 12" long connection atop the boiler between the input and output water lines. What keeps this connection from "shorting out" the boiler loop? The note says "don't exceed 12".