Water Heater Expansion Tank/Vacuum Breaker?

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...
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Jerry Napoliano wrote:

Either the DET-5 (smallest) or the DET-15 will work. The size of tank is dictated by the volume of water being heated, in this case there is very little volume.

No. There apparently already is some check/backflow valve, which is preventing flow back out to the street.

No, just TEE the exp tank off the Cold inlet; it can hang downward. Jim

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What is your normal pressure? I can't see see the after-heating driving it up all that much, so I suspect you are starting out too high. Have you checked it?
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No, I actually haven't. I have a pressure gauge, but no place to screw it into. Suggestions? Install a fitting?
Thanks to Speedy Jim, too for the quick answers..

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Jerry Napoliano wrote:

Put the TEE in for the exp tank. You can get a TEE with a 3/4 female pipe thread on the side outlet. The tank will screw right in. Before you put the tank in, screw a gauge in there. Jim
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I have put mine on the drain of the water heater, but maybe you don't have a drain. In that case an outdoor faucet is fine.
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