September is an excellent time to take stock of your HVAC system before the cool season begins in earnest. If nothing else, you should change your filters and hire a pro to inspect and tune up your system, at a cost of around $80 to $100, before turning your heater on. However, if you're ready to make a big investment, this is also a good time to think about upgrading your system with zoned, multistage or smart systems. These improvements make a big difference in energy efficiency and ...