I'm a professional plumber and the plumbing industry in general does not require or suggest annual inspections. I had a new American Standard Freedom 90 - Comfort R system installed in my home 2 years ago. The installation looked very good with metal ductwork, turning vanes etc. The system performs very well but I received a letter from the installing company suggesting twice a year maintenance. How often should this system be checked? Couldn't they do the furnace and A/C maintenance just once a year? How often in the absence of obvious problems should this be done?