Originally Posted by 150EH
The container will capture a lot more heat in the direct sunlight and it is constant, a cup of warm water started cooling right away and then when you added the bottle to the cup it cooled by more than 50% in the first 30 seconds. I pretty sure the sunlight is your problem.
That make sense. If Someone have a spare heater, they should try this experiment and let us know how it turn out.