Some background: I own a 1600 square foot house in Madison, WI. When we bought it it was set up as a two-flat, upstairs and downstairs. Each floor is heated with a boiler and cast iron radiators. The current setup is a pair of Bryant (?) boilers, 112K BTU in and 93K BTU out. I have no idea how old they are, as they came with the house (so > 5 years) and we have no documentation.

Recently, we added insulation to the walls (3.5" cellulose, dense-pack) and attic (10"). We've been steadily replacing windows but there are a large number of original windows. We plastic them in the winter. Before this there was no wall insulation and minimal (2-3") in the attic.

Since everything I've seen says that our boiler setup is now massively overkill, and we definitely get nowhere near 100% utilization even in the middle of winter, I'd like to make this system more efficient. However, the budget is _really_ tight right now as my wife is out of work. So I'm looking to make do with what I have. My two options are as follows, and I'd like advice on which one to select.

Option 1: rework the system to use a single boiler. If the system was oversized to begin with (pre-insulation) then a single boiler should be able to handle the load easily.

Option 2: purchase a used boiler and switch to it. This just came up. The replacement is a <brand unknown for now> 140K in, 90% efficient, PVC vented unit, 7 years old. The PVC venting is definitely attractive, although it would not mean we could seal the chimney up as we still have a gas water heater. The 90% is more attractive. Unit cost for that is a cheap $600.

I'm also looking into an intellisense unit. However, budget is a factor.

Which option will be the more efficient? Any other things I should look at?