As tempting as the price is, there's no way we're going the DIY route, so I've gotten a couple quotes. Both guys think the ductwork is fine (surprise). DH would really like a steam humidifier because his skin and asthma get worse in the winter, and the pad humidifier that was installed 15 years ago is falling apart and doesn't keep up anyway, so he deals with portable units on each floor which are a pain. It seems like overkill for our area (central NC), but HWMBO wants it.
The cheaper guy quoted just zoning, a couple thermostats, and adding a steam humidifier (poorly reviewed Honeywell True Steam), hoping the old gaspack lasts a while longer ($4200).
The other guy quoted a range up to $10k, with several options for HVAC replacement including single and dual stage up to 17 SEER and 95% AFUE, zoning, thermostats, and a better reviewed steam humidifier (Aprilaire 800).
So gaspacks have a max AFUE of 80%. To get more efficient heat, we'd have to install a split system, with the heat in the crawl space and the A/C unit outside. The difference in price between the gaspack and the high efficiency split system is < $500. However, it wouldn't be a good idea to just zone now and wait for the gaspack to die if we want the split system because stuff already done would have to be redone.
Do high efficiency gas furnaces blow uncomfortably cooler air? Is a two stage system all it's cracked up to be? We do plan on staying here for the foreseeable future, so paying more for efficiency and comfort is okay, but not if it's really no better.