Does the need to vent a gas water heater get factored into the
equation? Having learned a bit about water heaters in the last few
days, I see that the vent from my current heater (4") goes off to the
chimney. But, unlike the one from my furnace, this one has nothing to
stop warm air from exiting all the time. (The furnace has a shield
that opens and closes as the furnace cycles. In fact, one cold winter
day I found out that the furnace wouldn't fire because a bird had
flown into and blocked that mechanism. I soon had that cage thing
installed on the top of the chimney.)

Since the vent is not sealed to the water heater but sort of sits on
top with an opening of about an inch around it, it seems that warm
house air is free to move out and cold air is free to come in.

I'm guessing that the air movement is pretty static unless the heater
is on. The vent is at the bottom of the chimney and so is protected
from direct winds. Still, after insulating everything in the house to
hold down heating costs, a direct passage in and out is a little
disconcerting.

As an aside, I'm putting in a Carbon Monoxide detector. I never
bothered before because the furnace has an automatic CO shutoff and I
didn't think of the water heater as a potential source for CO; I
thought of it more as the vent from the electric dryer. That thinking
has changed. Obviously it is no where near as dangerous as the
furnace, but not to be taken lightly either.

Hmm. There is another thought. I wonder if I'd save money by
installing a gas dryer rather than an electric one? Probably not
enough to justify replacing a working dryer for over $600.