After talking to a Turbo Tax CPA, I found the issue. Credits for energy efficient furnaces are entered on the "Tell us "How Much You Paid for Energy-Saving Improvements" screen. TurboTax has the furnace fan ("Main Air Circulating Fan") as a separate entry field with a max credit of $50 on 2021 taxes - it's the 2nd to last line on the screen. The furnace itself ("Furnace and hot water boiler") is a separate entry field with a max credit of $150 on 2021 taxes - it's the last line on the screen. So regardless of how much you spent on both the fan and furnace, the maximum credit is $200 for the 2021 tax return.
