Heat jumps back up when you sprint right after using a depleted cardio kick.

https://medal.tv/games/marathon/clips/mYI1CPfYPMdIYJsDf?invite=cr-MSwwMXUsNTc3MTg2MQ

As you can see in the linked video, my heat jumps back up right after using a grey cardio kick. Not sure if this applies to other cardio kick variants, but it makes the grey ones useless.