Got iPhone 6 and 6+ same time about 1 month ago. Both share similar settings.
My iPhone 6+ no heat issue. But my wife's iphone 6 tend to heat up when surfing and using other apps.
IMHO there's a problem with her phone since hers tend to heat up and not mine. In fact I am more of an intensive user than her.
Went to A.Lab yesterday at Plaza Sing. CSO told me its normal for the phone to get warm after surfing. But I don't get that issue with my 6+. Anyway he reset the settings and ask me to monitor. If it gets hot again, I should bring it back to have it check thoroughly.
My thought. What if it doesn't heat up after resetting the settings? If I set the phone back to previous settings and it heats up again? So this resetting doesn't really solve the issue.
May just go back to insist on replacement. After all it's a new phone.