This time of year it takes me all evening to re-hydrate after a couple of attics. Thank goodness for a working air conditioner at home! During most home inspections the living space has few surprises. The most costly ones are found in the attic, basement and HVAC systems. It's easy to spot a compressor/condenser unit that is more than 25 years old. It's more difficult to know what to do with one that's been compressing for around 15 years, which is the recommended replacement age.
Most technicians say they can keep any unit running. The issue becomes cost of maintenance, loss of efficiency, difference in efficiency with new models, effect on resale value and peace of mind.
One thing affecting maintenance cost is the price of the old R22 coolant, which is getting so scarce the price has risen tenfold. The new coolant is R410A. Also, failing cooling systems can affect other components in your home. I have seen many a disintegrating coil/pan leak onto a perfectly good furnace and ruin it.
Here's what a coil looks like that is not going to function very well.
By the way, gas furnaces last about 25 years, after which the heat exchanger probably has leaks in it.
The newer AC models have a higher efficiency, measured in SEER (Seasonal Energy Efficiency Ratio), so they cool using less electricity. Click here for SEER comparisons.
Checking the outside of a home first means I run into the compressor/condenser unit early on. An old one means a big question mark for the purchaser given the high cost of replacement. Better to have everything in good enough working order so the buyers can just move without a significant expense. As always, regular maintenance assures the best performance. Low coolant levels, and broken down insulation on the low pressure line is just a waste of electricity.
Finally, peace of mind. Having an AC unit fail in 100 degree weather is not fun and it makes one susceptible to rash spending under such duress. Here is a good source for repair or replace decisions.