I keep space heaters, window air conditioners, and some fans ready along with an extra water heater.

I have a house for sale, rented for the last 5 years, in Seminole County, Lake brantley school district, 3/2, new roof, new septic, new air conditioner and, has had some cosmetic upgrades.

Rather than replacing all of my thermostats I have door hangar signs that say: "If the door is open the Air Conditioner will turn off!"

The HVAC we are quote for is a Trane XR14 3 ton air conditioner and Trane XR80 80,000 BTU gas furnace with 4 ton drive.Joe, the 2nd opinion said there could be rust in the heater exchanger which could ruin the furnace, which is why they recommended to replace both AC and furnace.Nicky, the original AC was installed before we purchased the house. 

With total closing costs and paying off the loan balance on an air conditioner that leaves me around $30k.

No formal stats...and nothing newer than 30 years old...the difference between 30 years and 100 years old hasn't been significant.We've done several roofs and replaced lots of air conditioners.  

Yes, there are automations that can help you, but no automation takes care of a brand new bee hive under the front deck, or the air conditioner that broke down when it was 95 degrees out. 

The property has all new mechanicals and roof, but was built in 1910 and the air conditioners along the back of the house are actually across the property line on a vacant lot.

Within reason, extend that across all appliances... an old and loud energy inefficient air conditioner isn't really going to hurt you (beyond statistical noise), and a new energy efficient one that's quiet isn't really going to help you (beyond statistical noise).

@Whitney Tutt air conditioner theft is not uncommon.