There's no getting around it: Replacing your entire heating and cooling system is a big, expensive project. But if your system is more than two decades old, the new one should easily pay for itself in three to seven years. In the long term it can save you more money than any other home improvement project.
Annual savings : As much as 25% to 50% on your fuel bills.
Up-front cost : $3,000 to $6,000 for a new furnace and air conditioning unit, including installation.
Tax credit : $300 for central AC system; $300 for water heater; $150 for furnace or broiler (credit cannot exceed $500).
Quick Fix : Air Filters (costing about $30 a year) could save you about $100 a year if you change them every three months, according to the U.S. Green Building Council. Chose a pleated model - it'll do a better job.