Just a small suggestion, but from the description it sounds like the heatpacks in the game are based on the type that can be reused once they have been heated up, which reverses the chemical reaction and returns the contents of the heatpack to it's liquid state. Would it be possible that after using a heatpack, an "inert" heatpack is left in its place, which could then be heated back up at a fire to be used again at a later date?