We bought a 2400 sqft house (in Michigan, built in 1925) a year and a half ago, and have been steadily upgrading the energy use of the house. We've added attic insulation, blown cellulose in the walls, foam in the rim joists, insulated the 1/2 above ground basement walls, and worked on some air sealing with a blower door test (we're still not good, but not awful). We have lots of old windows that I'm in the process of rehabbing, but that's going to be a lifetime job.
The house has an old boiler that's working fine (no CO, 77% combustion efficiency), so I don't know if I can ever make a rational financial decision to upgrade to a modern boiler. However, there are a few things I'd like to change and I hoped I could get some input.
Some stats- boiler is 225 kBtu input, 180 kBtu output. I think it used to be gravity fed (old expansion tank in attic and 3" mains in the basement), but it's been retrofited with a 1/6 hp circulator and one input and one output. It's all on one zone with one thermostat, and there aren't any thermostatic valves or outdoor reset. Each radiator has a unique branch off the mains and unique return to the master return. I calculated the EDR of all the radiators in the house, and I got 75k on the main floor and 53k on the second floor.
I haven't calculated the heat load of the house, but I can if that makes a big difference. The house is comfortable and uniformly heated, but there are two main things I want to change-
The pipes in the basement are about a foot below the ceiling. I'd like to eventually finish the basement, so I'd like to replace everything with a manifold and Al-Pex in the homerun configuration. I also want to replace my circulating pump, since it uses a lot of electricity.
Without changing the boiler, does it make sense to go to a ECM Grundfos Alpha, add thermostatic valves, and seriously reduce the water volume by running everything in 3/4 or 5/8" Pex? I'm pretty sure the boiler is oversized, so would a bypass, outdoor reset, or ??? help?
Finally, the additional complication is would any of your suggestions change if I ever want to swap in a mod/con boiler with indirect hot water?
Thanks for your advice.