Hello. Long-time lurker, first-time poster.

I have a question about the piping of the water feed into a residential boiler (a two-pipe, hot-water system with cast-iron radiators; not steam).

I had a plumber install a hot water heater for me a couple of months ago. Its turned out that there's some things that he did that were completely wrong. (I'll post some pictures when I get a chance.)

Something that I just noticed is that as a result of how he piped things, the water feed pipe that would be used to fill up the boiler comes off of one of the hot water lines that comes OUT of the hot water heater. Before that plumber, the boiler feed pipe came off of the cold water line before it fed water IN to the hot water heater.

Does it make a difference if the water feed into the boiler is off of a hot water pipe or a cold water pipe? Have I been wasting gas needlessly heating water that goes into the boiler? Are there some rules or guidelines for the best ways to layout the boiler feed pipe?