I have two dielectric unions (hot and cold) above the boiler.
The leak is from top of the nut of both dielectric union.

Close the cold Shut off valve to the boiler drain some water from the tank, open a hot faucet, disconnect both the hot and cold lines clean the dielectric union threads apply 4-5 turns of Teflon tape and tighten the nut hope your problem will be resolved, good luck.

John

In addition to John's advice: the leak can be from the threaded connection itself or from the plastic isolator sleeve. If the sleeve is deteriorated and leaks, best would be replace the entire union as these isolator sleeves are not sold separately

Milo