Persistent leaky water heater inlet

[M1tchsc0tt0]

Hi All,
I recently installed a new Rheem electric water heater and I can seem to get the cold supply line to seal. I am using a copper FPT soldered to the rest of the copper line. I applied loctite thread sealant with PTFE. It stays dry for about a day then seems to start leaking. I have tried different couplers and am at a loss any ideas? Do I need a new dielectric nipple?

 

[GReynolds929]

Screw it on tighter

 

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[Reach4]

I am using a copper FPT soldered to the rest of the copper line.

If it is already soldered, are you going to be able to tighten?

Good PTFE tape and pipe compound both is often good for a leak-free seal for tapered threads.

Connecting with corrugated metal couplings is required some places and a good idea in many others. Those use a gasket, so no tape for those.