People played the harp for centuries before there was air conditioning. They can stand it. Just keep it out of direct sunlight. If the house is bearable for you, it’s bearable for your harp, too. 🙂

Temperature is not nearly as big an issue as humidity. If the relative humidity is between 40% and 60% then it almost doesn’t matter what the temperature is. High humidity isn’t a problem unless it’s so high that mold is growing on it. Very low humidity is a huge problem.

Wood used to build musical instruments for the American market is dried to 5% to 8% moisture content, which makes it as stable as possible. Traditionally, and for centuries, musical instrument wood in Europe was dried to 15% moisture content. That works in a very temperate climate with small changes in relative humidity over the course of the year. But when those instruments are brought to the United States, they usually develop cracks and warp terribly. Now musical instrument wood is dried to the 5% to 8% standard worldwide.