In most cases it's cheaper to replace the furnace and AC together. Replacing a furnace alone usually costs about $2,000 to $5,000. When you add a furnace during an air conditioner replacement, it should cost less. However, if your furnace is less than 10 years old, it may make sense to replace it solo. There's a good chance it should last until your AC needs to be replaced. A furnace should last 20-30 years and AC systems should be replaced every 10-15 years. If your furnace is 15+ years old you should replace the furnace and AC together. In addition, if installation is complicated or you want to avoid HVAC repairs or replacements for a long time you should replace the furnace and AC together.