If the battery is full and you are doing some heavy regen (like I do, pull out of the garage and immediately go down a steep hill), the engine will start to provide some compression braking. It's not very effective in my opinion, but that's the way they've programmed it. Since the battery is full, there's nowhere for the regen energy to go, so they fire up the ICE to help with the deceleration. But yes, if the HVAC is calling for cabin heat the ICE will also start, as the PHEV doesn't have any sort of resistive heat, nor a heat pump.
2019 Kia Niro PHEV EX Premium
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