Natural gas furnaces provide the warmest heat. Substantial saving can be achieved by using 2 stage gas furnaces.  When a thermostat calls for heat only 65% of the available heat is turned on. If the furnace can heat the home in 10 minutes or less the other 35% never comes on. Imagine how much energy that could save. Did you know furnaces are oversized except on the coldest days?  The majority of the time Oklahoma heating days are moderate and only require lower heating capacities. Natural gas furnaces are available in 80% - 90% afue models.

See related topic variable speed