Jerry Napoliano wrote:

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.

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.

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

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


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

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

Thanks all...