Plastic Piece on inlet pipe

What is the purpose of the plastic piece with rubber disc inside the inlet and outlet pipes? This is on a new Bradford-White heater.

 

They are check valves to stop hot water migration through the pipes. They can cause issues as they can get build up inside them and cause flow problems. You could remove them if ya like.

 

It might be a dielectric union to prevent corrosion due to galvanic current between dissimilar metals. Check this article for more information:
"Using A Dielectric Union To Join Galvanized And Copper Water Supply Pipes"
http://www.hammerzone.com/archives/plumbing/systems/components/union_de/intro.htm

Mike Monett

 

The disk means it is at a minimum, a heat trap. It may also be a dielectric union, depending on what it is made of. Make sure that they are used in the proper location (hot/cold) as if swapped, they'll tend to block flow when you turn on a valve.