Our (great) grandmothers used to place a bucket of liquid water in a shed or room in order to keep it frost-free.
This very ecological method makes it possible to really keep frost-free a room subjected to a negative outside temperature. The amount of water gelling releasing into the air of the room the equivalent of the latent heat of fusion of water. This heat is far from being negligible since 100L of water “contains” almost the equivalent of a liter of domestic fuel (see calculations on the link).
To know more: keep a room frost-free in an ecological and free way