Some people say a hot water return on the hot water lines of a house saves energy. A hot water return is where the hot water supply pipe (for example to the sink) actually loops back to the boiler so that hot water is constantly flowing in the pipe and ready to be used. This means there is always hot water on demand when you turn on the faucet, which means you don’t waste money and water waiting for the water to get hot.
I found this very good article on return lines and circulators.
Although the article does say that circulators are good, they only work really well when they involve a circulator pump that is manually turned on. Meaning they need to be activated by a switch (in the bathroom for example) a little while before you plan on using the water. Sounds way too complicated and basically stupid.
I’ve basically that in terms of energy conservation it isn’t worth having a hot water return. The water saved does not warrant the energy wasted (in hot water, up front costs and electricity). For comfort of course it is good to have immediate hot water but not for energy savings.