Stop in for a cup of coffee

toolmanmike said
Yep Nat. gas isn't as good as LP FYI One cubic foot (ft3) of natural gas contains 1,030 BTUs and one cubic foot of propane gas contains 2,516 BTUs, giving propane about twice the energy content of natural gas.

But, the furnace is orificed to the same BTU output. 100,000 BTUs is 100,000 BTUs regardles of the type of gas.