A used HVAC system value to write off on a rental property

Can you clarify EXACTLY what you are doing that want to know the value?

I think you are asking about the Expense Election under Reg. 1.263(a) promulgated in 2014.    Right?   I suggest you hire an old-timer HVAC person and have them give you a written estimate of the cost to install that kind of system at the time it was installed. Under that Regulation, you can expense certain property rather than depreciating it. The betterment, alteration or replacement property must cost less than $2,500 or must cost less than 30% of that "component's" original cost (or qualify under another provision in the Reg that doesn't seem to apply here). If the replacement cost doesn't meet one of those tests, the replacement property must be depreciated on its own as a new property. Either way, the original cost (good luck with that) and accumulated depreciation of that old HVAC unit, if scrapped or retired, may need to be dealt with in your depreciation schedule-this may result in a write-off of the undepreciated amount of the old unit. NOTE: Under the Regulation, the HVAC system is one of the 5 or so "components" addressed and defined in the Regulation and that defined building component includes: the duct-work, air handlers, electrical sensors and activators in addition to the heating and A/C units themselves and their filters, fluids, drainage, etc. So, if the cost of the new unit is less than 30% of that component's original cost, you can expense it under the Reg. If you elect to expense it, the taxpayer must make the election on their tax return. Additionally, this is probably a change of their accounting policy, but under the Regulation, there is no need to file a 3115 for this change but taxpayer must have a written accounting policy that complies with the Regulation.  If you can get the parts and installation cost individually listed on the invoice, and if some or all of the listed items are under $2,500, those items qualify as an expense under the Regulation. Then, if the remaining item that costs more than $2,500 is less than 30% of the component's original cost, it too would qualify for expensing under Reg. 1.263(a). If the invoice is just the total price, you'll need to get that original cost of the entire component from the old-timer and see if it is less than 30%. If so, you can elect to expense it. You can also use the common sense method ... for example, if you have 5 outside AC units and you replace one of them at a cost of $15,000 -- you only replaced 1 out of 5 or 20% and the cost qualifies for expensing ... and that doesn't include the rest of the system. 

Depending on the size of the equipment(BTUs? Square footage?) I would do this:

1) Take several dice in hand(4, 5  or 6, most likely);

2) Roll them;

3) Depending on the number of dice, you will get a value between $1,111 to $6,666 for four dice; $11,111 to $66,666 for five dice, and $111,111 to $666,666 for six dice;

4) Depreciate.