I was thinking about this for some time and her's what I came up with:
The base needs to be both equiped with heaters aswell as radiators.
Heaters are used at the start of the game, during coldstorms, accidents and during unusual big heat drainage.
Radiators should be used later in the game, when your base has estabilished stable conditions (air mix, temperature and preasure).
At start the base is exposed to martian air. The base has the air's composition, heat and preasure as on Mars. Every equipement must have the same temperature as outside the base. When you seal the hatch, the base will still have that temperature, and calculating the total mass and volume, it would take weeks to fully heat the base with human+machine excess heat only. Therefore - we heat it up with a heater coil to stable conditions. When those conditions are archieved, the heaters are turned to "STANDBY MODE" or "OFF" since the temperature will rise without them. Now the problem is that the heat will slowly but continually raise. COHAB activates passive and active radiators. If not instaled or breaked, the temp will rise and rise and rise, untill it will be to hot.
Some heat is lost during normal activities like Hyper said. Some lost in the airlock, and the other part can be lost in the air-circulation process (air recuperator prevents that).
IDEA: Each equipement while activated drains power and returns some heat like a heater coil.
The same with humans. When more astronauts they are, the faster will the module get heat from them.