I replaced a furnace in my rental property. Would this count as an Asset, Improvement?

[MaryK4]

Replacement of the furnace in your residential rental property: is generally a restoration to your building property because it's for the replacement of a major component or substantial structural part of the building's HVAC system.

It is a separate asset with a new placed-in-service date and are in the same class of property as the residential rental property to which they're attached.  Generally, you depreciate over a recovery period of 27.5 years using the straight line method of depreciation and a mid-month convention as residential rental property.

There is a Safe Harbor Election for Small Taxpayers whose average annual gross receipts is less than or equal to $10,000,000. You may elect to not capitalize amounts paid for repairs, maintenance, or improvements of certain eligible building property if the total amounts paid during the taxable year for such activities don't exceed certain dollar limitations. 

[Carl]

Replacing a furnace is a property improvement that typically gets capitalized and depreciated over time. However, if you paid a total of less than $2,500 (including installation labor, delivery, etc) then you may have the option to just expense the asset under safe harbor de-minimus. If this interest you, the rules can be complex. But you can read a synopsis of those rules at https://www.irs.gov/businesses/small-businesses-self-employed/tangible-property-final-regulations