Jack The Sailor: No! a glass doesn't "Wax empty", but I believe that love can "Wax cold" (i.e. grow cold). Whereas the glass's contents would wane, Wax Cold seems acceptable. Describing ones reaction to the weather, it is accepted usage to say I'm getting cold, or getting colder.
By the same system, whilst it is right to say that leaving the door open of a heated house 'lets the warmth/heat out', it is also common to say that someone is 'letting the cold in'. The transfer of heat (from a scientific point of view) is always from a 'hotter' to a 'colder' body. But that is not always the apparent effect.