You are confusing the output figure with AFUE or annual fuel utilization efficiency. That oldie, when fully warmed up and burning perfectly, can put out 80% of the gas being burned. BUT, AFUE will look at the losses while the beast starts, the losses as it cools down and pours heat out the flue and the stand by losses as it sits there with the flue open sucking air out of the house 24 hours a day. So AFUE on that beast is probably 55-60%.

Yes, you should have the chimney lined for the orphaned water heater, no big deal. Some 90s can put some noise out of the flue but since you are rarely sitting beside the vent in the winter, I wouldn't worry... unless you or your neighbor have a hot tub near it :)

Pumping condensate out in the winter is trouble, can you pump it somewhere in the house like a laundry tub?

I've seen the AFUE term and guessed that it was something like that, but didn't really know what all figured into it. Thanks for the explanation.

The laundry sink is ~40' away on the other side of finished space. There is a 2" cast iron stack from the kitchen sink about 20' away in the same room. Is that an reasonable option?

I wasn't thinking about the water heater being an issue in the summer, but it makes sense that any noise from the heater wouldn't be such a problem in the winter. I read one guy complaining about a water heater vent across the street from him though.

I don't see why you couldn't pump condensate all the way to the laundry sink. Our 87% boiler is a condensing type. The boiler has a condensate drain coming out of the bottom of the flue. This drain is routed to a condensate pump which hoists the water up and over my water heater, then gradually down to the washing machine drain. The boiler has been producing hot water of around 120°F lately (around 32°F outdoors). This low temperature means a lot of condensate from the flue, so the pump has been busy. I never hear the pump run, though.

As far as vent noises go, I would DEFINITELY have the air intake plumbed to the outdoors if I could do my install all over again. The air intake makes a considerable "whoosh" noise while it's running. (Of course, drawing air into the house.) The blower fan does have a slight vibration that I can hear in my kitchen, which is just on the opposite side of the wall from the boiler.

In the case of a boiler, I can connect an indirect-fired domestic water heater that the boiler services. This means one unit does two jobs by effectively making an additional zone. I don't know if this can be done economically with forced air heat.

I can't hear any noise from my boiler when I'm outside. Then again, I don't have it vented through the wall.

I just went through this same scenario. My choice was to get the two stage 80% efficient furnace (saves $500 including losing out on the tax credit), not line the furnace (saves $1500) and put the savings toward duct sealing, and sealing drafts with weatherstripping and spray foam. In an older house like mine, replacing old can lights that vent into the attic should get me a better return on investment and more savings. High efficiency (notwithstanding the emotional aspect of feeling green)is kind of lipstick on a pig.

My two cents. We'll see how it turns out.