As posted previously, it could be just doing it's job. I've noticed when first firing up after the heater's been completely cold, there is a major pressure increase and it would make ours drip through the relief valve.
Now I just turn off the pump, crack open a hot water valve at the sink, and fire the heater- letting the expansion drip at the sink.
Once the heater is up to temp and just maintaining, it's never a problem.
Gary