The Heat Monitor is a device that helped the planet's inhabitants monitor their thermal energy level. In some ways, the Heat Monitor replaced the Oxygen Meter from the first two Dead Space games. Only this time, Oxygen is plentiful, but an environmental hazard monitoring device is still being used.
The only instance of needing to keep an eye on your body temperature is Chapter 8: Off The Grid, just after the shuttle crashed onto the planet's surface. Isaac's (And Carver's in co-op) helmet began to malfunction and left his head open to the hostile environment. Once the players reached the Suit Kiosk and switched suits in Chapter 9: Onward, the heat monitor is no longer thrown into play. Despite the protection against the climate that the suit offered to the rest of their bodies, Isaac and Carver will die if they are exposed to the cold environment for too long. The monitor tracked their body heat as they wandered around topside. The heat depleted as they are exposed, but their suits will slowly warm them back up once they are out of the elements.