It is the rapidity of the change, rather than the warmth, that becomes the threat. As moisture creeps into the inner cellular structure it evenly expands. If your instrument is always in a wet environment, it should go through its initial expansion and then stabilize. Your kitchen table, lacking the even distribution of undamaged cells, will sometimes crack as the woods attempt to distribute the stresses associated with internal expansion. The other thing to keep in mind is that air conditioners are not good for woods in the same way they are not good for your skin.
Wood can lose so much moisture that it becomes brittle.
If you need to humidify the air in your home for the sake of your tissues, the same will be true of your harp. Again, the key is moderation and consistency. If you are moving your instrument from a wet environment to a dry one, loosen the strings a half step in tone. The harp will naturally stretch the strings and retain pressure as the soundboard falls. When going from warm to cold or cold to warm, your instrument needs to be insulated.