Most of my experience is with oil fired units but I have replaced a few gas units and all were replaced due to heat exchanger failure. How old is the unit you have now? Gas / LP fired furnaces are not typically designed with much of a life expectancy. The heat exchangers fail over time and flue gasses start to mix with the air supply that is routed through the home. There’s also not a great deal of adjustability with the burner on gas units either. With some units, you can’t adjust anything. If you are serious about replacing your furnace, take a look at Lochinvar. They make some extremely efficient units approaching 100%. Their gas fired Knight Boilers are amazing. Their water heaters are pretty slick too.
