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My house is heated by hot water, and it's about 1100 square feet on
one floor, with open stairway leading to the basement of about 900
square feet. There's an open floor plan, except that the two
bedrooms and the bathroom have conventional doors.

I'm most concerned about sleeping comfortably at night, though
obviously it would be nice to keep the living areas below 80° during
the day. My bedroom is small, so you'd think a window a/c would make
sense. But the window is 39" wide, which means an a/c small enough
for the room is too small for the window. Security isn't a major
concern, but it's a concern. And even in summer we do have some
pleasant nights, but with an a/c unit in the window I can't get the
breezes.

I've seen advertisements recently for ductless air conditioning,
which I gather is a unit mounted on the exterior wall of the building
with pipeline to the heat pump outside:

http://www.thisoldhouse.com/toh/askt...192764,00.html

Does it even make sense for me to consider this? Are there any
"gotchas" I should look out for if I talk to a/c people?

The mini splits can be very efficient and save on the electric bill.
These use all sorts of advanced technology to make them more efficient.
Certain brands are more efficient than others, so shop around and
compare before buying. I think Mitsubishi might be one of the more
efficient types?

But the best mini split system can be even MORE energy saving. That is
one model I saw where you can have up to 4 separate inside units (one in
each separate room) which all connect to one outside unit. So just turn
on the inside unit(s) in the room(s) you want to cool or heat. Then the
outside unit ramps up/down depending on how many units inside are on and
the cooling or heating demand for that unit. Then you are just
cooling/heating the room(s) you are using, not the whole house.

For example if just a little cooling is needed, everything runs slower
including the outside unit.

Whole house and window A/C units are full blast and on/off cycles (less
efficient).

And you can the the kind which allows for 4 inside units, but just
install one inside unit for now. Add more units later.

For heating, the heat pumps on these new mini splits will now work to
very low outside temperatures. Another improvement.