so, apparently, my total Furnace heat output is rated at 210k btu. 120,000 for my bigger one, 90,000 for smaller one.
for a 2200 sq ft house. 39 years old furnace, still kicking hard. doesn't even seem that inefficient. my gas bill's only 200 a month so i'm not really in rush to replace it.
asked for a quote from national energy chain to replace both, and they came with 7700ish to replace 2 with 2 smaller ones at 92%, or 1 larger 110k btu at 96% for 7800ish.
not sure whether that's good deal as it's only the first quote. will check with other furnace companies.