SEC179 only applies to *NON* residential rental property. If this is residential rental property then you can't use SEC179 at all. You "may" be able to use instead the Special Depreciation Allowance. But not on a newly installed central HVAC system. Such a system is considered to be "a permanent and physical part of" the property. Therefore it's classified as residential rental real estate and depreciated over 27.5 years.
Others can argue the matter all they want and it really doesn't bother me. My own sister has already been "called on the carpet" for "this" "exact" sitiuation 10-12 years ago, so she's been there, done that, got the T-Shirt. So if you want a T-shirt too, you can probably get one a few years after you file having classified it as anything other than residential rental real estate. It's one damn expensive T-shirt too!