I am completing the section on Energy Efficient Home Improvements. We installed a furnace powered by natural gas in 2023, but there isn't a line item for it.

[AnnetteB6]

On the page with the title, Enter your qualified energy property costs, use the third box down the page.  

It is labeled Water boilers (natural gas, propane, or oil water), but it is the correct place to enter the cost of your natural gas furnace.  If you click the Learn More link beside that entry, you should see an explanation stating that furnaces are included in this box.