If your home has a split HVAC system, you'll have a unit both outside of your home and inside of your home. It's generally recommended to replace both units even if only one unit fails, though it's not always necessary to do so. Here's what you need to know.
Indoor and Outdoor Units Work Together
Though you have an indoor unit and an outdoor unit (and they look very separate), they're designed to work together. They are really a single unit, they're just physically split inside and outside of your house. If you don't match them up properly, they aren't going to operate correctly. It's like two fans of different speeds working together: one may be pushing air faster than the other can vent.
Effectively, this means your system is going to cost a lot more to heat and cool, and it's going to be less comfortable because the temperature isn't always going to be evenly balanced.
It's More Affordable to Replace Both
If one part of your AC system breaks down, it's likely that the other part is also at the end of its usable lifespan. That means that it's probably going to break down within the next five years anyway. Purchasing your units as a set and installing them once will save you money compared to having to purchase them separately and pay for labor twice. It's much easier for an HVAC technician to install two units at the same time than it is for them to come back twice to install each unit on its own.
You Don't Need to Replace the Furnace
And now, for some good news: you usually don't need to replace the furnace at the same time. Air conditioners actually tend to last a shorter period of time than a furnace, so if your AC just broke down, your furnace might have another five to ten years in its lifespan. While you can buy a complete HVAC system (with both heating and cooling), you may not need to.
So does that mean that you always need to replace the indoor and outdoor units simultaneously? Not necessarily. As you can see, it's really an issue of energy efficiency. If you don't mind spending more on energy (and using more energy, which is generally bad for the environment), it is possible to replace only one unit. If your system is down right now and you can't afford to change both units, fixing one unit will make the system work again. It just won't work well.
Contact an air conditioning repair specialist for more information.