First concern is that the coolant level was low. Most systems do not need topping up, so I would doublecheck for any leaks or signs of overheating. When you say the fan kicks, in are you referring to the engine fan or the heater fan? If it's the heater fan and its not blowing hot air (when the engine is at the correct temperature) it could be a number of things, like the heater matrix leaking (maybe thats why you are topping up the water).

Also, trace the heater pipes that come from the engine to the heater to see if they are hot. If not, then the system is not allowing water from the main engine block to the heater matrix. Be wary that in some cases low engine water levels will result in the heater not working correctly, so I would suggest that you monitor your water levels closely over a number of days to make sure that you have no leaks.