On the page Enter Your Qualified Energy Costs use the box labeled Water boilers for the cost of the furnace. The box needs to be re-labeled. If you click on the Learn More link it will describe -
Enter the amounts you paid for a natural gas, propane, or oil furnace or hot water boilers...
That is the correct location to enter the furnace cost as it will flow to the Form 5695 Line 24a for Enter the cost of natural gas, propane, or oil furnace or hot water boilers