I'm in process of replacing my water heater (gas). I've got the new
one out of the carton and it's got all the normal stuff plus something
extra: a bag containing two 3/4" galvanized steel nipples. Each is 2"
long with a blue plastic lining running the length of the inside and
protruding about 1/16" at each end. If I screwed one down on the dip
tube (which seems to be the same color plastic) effectively the water
wouldn't touch the galvanized nipple. I don't know why I'd want this
on the hot side though.

But wait, there's more. At one end of each nipple attached to the blue
plastic lining is a (also) blue plastic ring bringing the outlet
diameter down to 3/8". Inside the ring is a black rubber flap, very
thin, and attached only on one side. Except that it's rubber and
there's a small gap around the black rubber and I can blow the flap
out of the way in either direction, it could be a check valve. There's
no directional arrows on the nipple.

The only reference in the manual is on the repair parts schematic
where these nipples are described as: "Nipple w/Heat Traps". In the
installation section the diagram shows an arrow pointing to the hot
and cold inlets with the description: "3/4" Threaded Nipple". Nothing
about heat traps. The impression is that you could use any 3/4"
nipple.

Does anyone have any idea of the function of these special nipples and
which way they go in. I'm a little worried that the rubber flap is so
flimsy that it'll break off and end up blocking some faucet. Maybe
it's a partially disconnected washer (the hole hasn't been properly
punched out) but why would they reduce the size to 3/8"?

If no one has any good ideas, screw them. I'll just put back the 3/4"
brass nipples I removed from the old heater.