I just purchased a 4600sf house in Indiana that is 6 years old and has the original builder grade furnace that is only rated at 88,000 btu's and 80% efficient. My previous home was 2400sf and the furnace was 120,000 btus, 80% efficient. This first months gas bill about gave me a heart attack even though I expected it to be about double what I was used to. Without doing the calculations, can anyone give me a guess on what the furnace size should be? The existing funace never shuts off and can't maintain 68* when it's 20 outside.

I doubt you`ll get anyone in this thread to make a wild guess like that. You will need to get a contracter to do a manuel J calculation on your house,, it would have to be a look-see calculation,, get several estimates, so you can compare equipment difference plus installation

Exactly what he just said.

68F on a 20F day sounds well within the design limits of a correctly sized system. I'd say your instincts are correct and the unit is too small. There's no way any of us here can size your furnace over the net and still sleep at night.

Save yourself some time and ask up front if they do Manual J for sizing the furnace. If not, move on.

Also ask if they're familiar with Manual D in designing ductwork, and have them check out that as well. With what appears like an undersized unit I'd be wondering about the ductwork.