Tankless pressure valve opening.

Hi all, after a few months of stress and replumbing the house using 3/4 pex for the main run branching off with 1/2 pex to faucets and toilets etc, i installed my Stiebel Tempra 29 plus ( we had the same model in our previous house and were happy with it ) It is working fine, however when we flush a toilet or turn on a tap or anything requiring cold water, after a second or so the on demand pressure relief squirts a small amount of water out ( occasionally it will light up also ) i have read that this is probably caused by kinetic energy when the cold shuts off, and the water moves back looking for the easiest way to expand which is the relief valve on the on demand. ( better that than bursting a pipe i guess ) If i half close the toilets supply so the toilets fill slower it doesn't happen, however it does for taps etc, so i believe the above could be the cause. we never had this in the old house so i am hoping someone can offer advice, i read a expansion tank would remedy this or a flow regulator. At the risk of sounding dumb is the regulator not akin to slightly closing the mains valve?
if i was to get an expansion tank, where would it go? in the crawl space? the water heater is in the kitchen corner as it was central to the house. does it just need to go on the cold supply near the water heater supply?
also could it just be the pressure relief valve is defective ?
thank you for your time.

 

The installation instructions for that unit don't in particularly call for a pressure tank.

In most areas a pressure tank is needed on a water heater by code.
This is especially important where a pressure reducing valve or backflow preventer is in use.
The pressure tank goes in the cold water line.
Typically it goes near the water heater but it just needs to be in the water line before the water heater so that it's after any type of incoming valves.

 

Thanks , that makes sense about the expansion tank ( ill admit i have no idea how they work, if they need emptying etc )
I do not suppose you have any other ideas why the valve keeps tripping?
the old house was CPVC and i am aware 1/2 pex ( B ) is smaller due to the connectors being internal not externally connected to the pipe, could that be the issue? could it be the relief valve being faulty?
the plumbing has been a royal pain in the backside, and i really thought it was all fixed after replumbing, this has taken the shine of what i thought was us starting to win in this house.

 

incase it helps anyone, i replaced the pressure regulator valve ( i got one with a gauge ) and so far so good. im guessing the old was faulty and was just letting the water flow full pressure.