Unfortunately, a new furnace in a commercial rental property would not qualify for any Economic Stimulus depreciation treatment.
Heating and air conditioning are considered structural components of a building, which do not qualify for Section 179 (nor does rental property in general).
Special (Bonus) Depreciation requires that an asset have a useful life of 20 years or less. Non-residential real property has a useful life of 39 years.
For those reasons, the new furnace would be depreciated over the same class life as the commercial rental building itself.