The union near a water heater can join piping to the unit and allow service access. Discoloration around it can result from an old drip, a current leak, or corrosion at the fitting. A fresh wet spot is more useful to report than the color alone.
Look for the highest wet point
Without touching hot fittings, look for moisture above the union as well as at the joint. Water from another connection can run down and leave deposits on the union. Share a photo and note whether moisture appears during heating or while the heater sits idle.
Ask which connection needs work
A technician can check the fitting type, its compatibility with adjacent piping, and whether it seals. If they recommend replacement, ask whether they will check the connections on both sides when the heater returns to service.
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