For your health and safety, have your heating system checked by a competent professional at the beginning of each heating and cooling season. Check your filters monthly. Replace or clean them as needed. When selecting a heating and air conditioning system, it's important to not only think about the initial purchasing cost, but also about how much it will cost to operate over the years. That's why a high efficiency comfort system makes so much sense. Leaving your thermostat fan switch in the ON position so it runs constantly actually helps filter the air in your home. And, because the air is constantly moving, you get a more even temperature and better humidity control. Cover the ground in your crawl space with plastic to keep unwanted moisture from coming up into your home from the ground underneath.