Tankless water heater leaking after installing new expansion tank

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whoswho

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I have a tankless water heater with an expansion tank. I noticed the expansion tank began leaking (rusty hole on side of expansion tank) so I ordered a replacement tank. I chose the exact make, model and size tank as the old one.

Shortly after the installation, the pipe joints (cold water inlet) leading to the tankless water heater are leaking. I did not touch these joints at all during the installation and the joints were not leaking before.

What could be causing this and how can I fix it?

Also, I did some research online which says tankless water heaters do not need expansion tanks unless there’s a tank storing water (which I don’t have). So I have no idea why mine was installed with one. Would really appreciate any help here. Thanks!
 
Correct you do not need the expansion tank as a tankless only heats what is being used, no thermal expansion. Who did the install, maybe they were unaware. Maybe some pics of the joints that are leaking
 
Home was new construction and the tankless water heater came installed this way.

Below are pictures of the old leaking tank, new tank, and leaking inlet joints circled in red after installing the new tank.



I thought about tightening the joints, but a bit hesitant as they weren't leaking before. How should I go about fixing this?
 
There are rubber seals in those connections.

I’d try to tighten them a little.
 
Correct you do not need the expansion tank as a tankless only heats what is being used, no thermal expansion. Who did the install, maybe they were unaware. Maybe some pics of the joints that are leaking
So that tank in the pic is not needed? I thought it was some pressure relief thing if anything happened or what not?
 

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