Symbolism, Imagery, Allegory
Given that the story is set in a coal-mining town and the protagonist's husband is a miner, it might not surprise you to find numerous references to fire and heat. More often than not, heat seems to get associated with life. For example, in describing the environment in the house as the family waited for Walt to come home, the narrator notes that:
The kitchen was small and full of firelight; red coals piled glowing up the chimney mouth. All the life of the room seemed in the white, warm hearth and the steel fender reflecting the red fire. (1.36)
The association of life and heat comes back again in full force later, when Elizabeth is reflecting on her life with Walter once the "heat of living" has gone out of it (2.128). She mentions that the parlor in which she lays out her husband is quite chilly. As her husband's interim resting place and a veritable icebox, then, it provides a stark point of contrast with the warmth and life of the house as presented early in the novel.
It's worth mentioning that Walter's body remains covered in coal dust when he finally arrives home, one reminder of "hotter" things and times. However, given that coal dust is basically just ash, it seems like even this reference is more about coldness and death than the heat of life that has passed out of the Bates house at the end of this tale.