Sounds like you have a pinhole leak at the inlet to the indirect water heating coil. These coils are made of copper and usually have cast copper or brass inlet and outlet fittings that are brazed on, and over time pinoles can develop.
I have very rarely resorted to patching such leaks by brazing or soldering (in cases where the owner was too cheap to buy a new coil or the boiler was so out of date and no replacement could be found). The best solution is to pull the coil and replace it with a new one. While you’re at it, make sure you install dielectric fittings where they are needed (if you have copper and ferrous pipe together).
NtP