A commenter on the previous "Natural Gas Blues" post made some good suggestions. My reply got long, so I decided to make it a new post.
Although I'd love to replace the boiler and windows, I don't think we're ready to invest $20K+ in the house right now. Our boiler is old (1983), so I'm sure we could save gas with a new one. When I asked our HVAC contractor about replacing it last year, however, he told us that it's a good model in good condition. Maybe there's more money to be made keeping the old one going as opposed to selling me a new one? If the boiler fails, I will have to look into coupling it with an indirect-fired domestic water heater as the commenter suggested. For now, I will probably just add an insulating sleeve to the heater we have. I also installed aerators yesterday to decrease our hot water usage.
The previous owner had aluminum double-pane windows installed 10-12 years ago. I don't know what gas or coatings they might have, but at least they aren't ancient (house is 85 years old). To put in the latest and greatest, we would need to replace about 25 windows on the first and second floors, plus another 15 shorter ones in the basement, which is heated. Judging from what we spent to update ten windows that he inexplicably skipped (like I said before, I'd like to smack him), we could spend $15-25K on windows alone. Window treatments with insulating qualities would probably help a lot. We installed honeycomb shades (triple-cell) in two rooms a few years ago, and I'd like to buy more of those.
The only big investment I am looking at right now is insulation. I'm sure we are under-insulated, particularly in the attic. Unfortunately, we have a SpacePak air conditioning unit up there along with a silver tarantula of tubing. That may complicate matters; we'll have to hire professionals. Obviously we should have insulated before we got the SpacePak, but we didn't think of it. You would think our general contractor would have recommended it, but alas that project manager wasn't the brightest. At least I got him to add foam board insulation in the living room and sun room when they replaced the crumbling plaster with drywall.
To be continued...