Augmenting Existing Propane Forced Hot Air System

A much more comprehensive energy heating fuel comparison calculator is at: http://www.eia.doe.gov/neic/experts/heatcalc.xls

It uses geographic location for determining heat pump operating efficiency. Be sure to look at all four of the sheets. Many, many links to energy price forecasts, heat content in fuels and equipment efficiency, etc. A wealth of information. Worried about your investment in coal burning equipment? Just compare projected coal price increases to other fuels. It's all there just analyze away.