I recently installed a Takagi Flash-Jr tankless water heater.
Although it is working better than I ever expected, I'm now getting
pretty significant water hammer when my toilets finish filling, and
there is a good amount of discharge from the pressure valve.

After doing some research, it looks like I'll need to put an expansion
tank on the cold water inlet to the tankless heater, but I've also
seen suggestions to install a backflow preventer, and a vacuum
breaker.

My questions...

1. How do I size the expansion tank? HD has a variety of models from
Watts - DET-15 DET-30, ET-15, etc. I live in a condo, 1/2" supply.

2. Do I need a vacuum breaker/backflow preventer?

3. Do any/all of these items have to be mounted higher than the water
heater?

Thanks all...