NYC code calls for a vacuum relief valve to be installed to allow air to be sucked into the system during a backflow condition thereby preventing the heater from collapse due to the vacuum created by said backflow.

These are to be installed as per code but it is very rare seeing one installed on a water heater. Backflow devices are not required for water heaters, but are required on boiler installations.

BTW I have seen a 10K gal tank (chlorine sanitizer) at a water pollution control plant collapse due to vacuum, you would not believe it but it does happen.

The term hot water heater is accepted in the trade. "HWH" is the symbol on plans for a water heater.

i am a licenced plumber and the job should take anybody with decent mechanical skills about 45 min.
step one shut off the cold water supply to the water heater located on top,
step two shut off the gas valve to the water heater down below,
step three remove the earth quake straps,
step four, disconnect the pop off line at the top or side,
step five, disconnect the water supply lines hot and cold from the top of water heater,
step six, connect water hose to out let at bottom and drain water heater,
step seven, place a long strap under water heater leg at rear of water heater, lean the water heater against your body and lift the straps and the water heater wil lift easy, set it onthe ground and follow the steps in reverse to install,
if the is any copper lines to cut for removal you can remove all the fitting on your old water heater and reuse them with out sweating the copper lines, just use a shark bite fitting or install 3/4 flex lines for water heaters.

The back flow preventer is installed on a hydronic (hot water boiler) heating system. It's sole purpose is to prevent boiler water from the heating system to migrate into the cold water supply to the house.
If you didn't have a problem with water hammer I wouldn't bother with an expansion tank.
If you do the job yourself be very careful with the gas connections. Use a proper thread sealer (not plumber's teflon tape), and before lighting the pilot as others have said use a mixture of water and dish soap and use on the fittings to check for gas leaks.

Thanks for all the comments, water heater installed and all is good!

Wayne C.

A heater that produces hot water can be called a "hot water heater" and still be grammatically correct.
Adjectives.

What is that?
It's a heater.
What kind of heater? A heater that produces hot water.
Oh, thanks.

Un-bunch em before you get a rash ;)

This is 100% correct. It may not sound amazing, or be preferred but it is correct.