Have an HVAC system for nine years, and suddenly the compressor stops working. With just nine years of use, you could probably choose to have the repair without giving a second thought to replacing the entire unit, and wisely so. However, think about if the system were 16 years old. When the system's age starts approaching 20 years, it's time to start leaning towards replacing the unit instead of repairing it. Sure the repair could fix that problem, but there’s a great likelihood that another costly part might break a few months later. 

If a system is being added to the home for the first time, most of the items noted in the previous question and answer may require installing the new system. Besides the equipment, the most significant component is ductwork. The ductwork can be made of either metal or fiberglass. The ductwork needs to be properly sized to deliver the right amount of air to each room. A central air conditioning system uses supply and returns ducts. The supply duct is attached to the furnace or air handler's outlet and delivers air to individual zones in your home. 

Smaller systems do not mean less power. The smallest split systems available are 1.5 tons, which is really not that small. Second of all, 1.5-ton air handlers are rated to 0.5 IWC external static pressure just like 2 and 2.5-ton systems. If that sounds like gibberish, it means 1.5 ton systems have the same “power” to push air through long runs as larger systems. The blower motor is smaller only because it’s pushing less air, just like a motorcycle has a smaller engine than a car but can still accelerate as quickly. We have seen 1.5 ton systems used in 1500+square feet 2-story homes. If you can’t get air to a 900 square foot apartment, you have a duct sizing issue, which would be a problem no matter the air handler's size.