Are you sure it only does it while heating? Have you run the pump for a good hour or so without the heater going?
Originally I thought mine was also only when it was heating, then realized it was the heat exchanger leaking (since that's the only thing that would be leaking inside the heater).
If it only happens when the heater is "on," it is likely condensation. That shouldn't happen with a new heater. The condensation issue with pool heaters was solved years ago. Warranty.
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