You do realize you resurrected a 12 year old thread, right?
You don’t mention the type of harp, but above average humidity will slacken (and drop pitch) on both nylon and gut trebles, which leaves the wires more taut.
Does the house have humidity control? How much are they moving sharp? How long will you be away? Are you turning the air conditioning (or heat if you are in southern hemisphere!) entirely off?
If you want to slack off the whole harp half a semitone, it won’t hurt anything. It will just take some time to pull back up and settle in when you return.