@Carl wrote:
.....There's no doubt that a "new" unit will fetch a higher price for the property, than you might get with a 10 year old unit still there....
That is absolutely not a settled issued @Carl and is in doubt.
Prospective purchasers expect to get a central A/C unit (if most properties in the area have one). A brand new unit may result in a faster sale but, typically, will not command a higher price for the property.
I have been through this literally dozens of times with certified property appraisers and they never fail to appraise comparable (and at times almost identical) properties with new (or newer) mechanicals at roughly the same price as properties with older units. The new units may add some value but it is negligible (and the increase in value is nowhere near the cost of the new unit).
Regardless, "added value" is not a litmus test for qualification as an "improvement".
