Green plumbing is plumbing designed to be the most energy efficient and at the same time conserve the most water. This boils down to the hot water plumbing since the cold water side does not consume, lose, or waste energy or water. Typical products associated with such plumbing systems are tankless water heaters and hot water demand systems.
Tankless water heaters are seen as green products these days, but you have to be careful about opting for tankless. Due to the capacity limitations and temperature rise limitations on tankless heaters, they generally best used as point of use water heaters. In other words they would be located at the fixture or fixtures where the hot water is to be used.
Point of use water heaters eliminate that long pipe run, virtually eliminating the energy and water wastage attributable to the distribution piping.
If point-of-use water heating is not feasible with your plumbing layout, then consider a hot water demand pumping system. A hot water demand system is a pumping system similar to a standard hot water circulating system, except the pump only comes on when you demand hot water by pressing a button.
Pressing the button causes a small pump mounted under the fixture furthest from the water heater to begin pumping the hot water from the water heater to the fixture at high speed, turning off when it detects the arrival of hot water.
Then when you turn on the fixture you have instant hot water and no water was run down the drain. Demand systems only consume about $1 to $2 per year in electricity to run. Some models are under $200 retail, and are very easy to install, they hook up with hoses. They save you time, water, and money.
Make your plumbing system green, add insulation if possible, and use plumbing products like point-of-use water heaters, and use a hot water demand systems if you can't use a point of use tankless water heater.
If you have long pipe runs, loop your hot water piping from fixture to fixture and install a demand system at the end of the line.