There is basically the cooling capacity of the system and air flow to
deliver the cooling to each room. Determining this can be quite complex
depending on local weather, your windows, insulation, size of rooms and
house, etc.

Problems can be too little cooling capacity, too much cooling capacity, too
little cooling in some rooms and too much in others, and the
upstairs/downstairs thing... Upstairs is too hot and downstairs it too
cold. (In Phoenix, west facing rooms with large windows seem to never cool
down in the afternoons, but may be too cold in the mornings...)

If there is too little capacity/air flow, the house will never cool down.
Too much and it will freeze you out of your home. Just right will keep all
rooms at the same temperature.

They may be able to just add on to your existing system or they may need to
add to or modify your duct system to add more air flow.

They can do calculations and a report on what is needed. A sample report is
at the link below.

I guess the best thing to do is to call for estimates from several
different companies and see what each says. (May want to take notes.) Then
ask back here if you have questions about what they said. Also may want to
ask on alt.hvac

Note that you can also get a humidity control, electronic air filter, HEPA
air filter for allergies, and temperature controls for each room (or
separate temperature controls for upstairs/downstairs).

Temperature controls for each room requires installing electric air
controls in the vents to each room.

Sample report...
http://www.hvac-and-hvac-software.co...tailreport.asp


"Vmcw" wrote in message
Hi!

We have a townhouse in Flagstaff, AZ, as a second home. It's two
stories,
about 1600 square feet, has a gas furnace in the attic. It's about a
year old
and the furnace has had very little use as it's basically our summer
home.

Although most folks in Flagstaff don't have air conditioning, we're
thinking of
adding it for the few days in the summer when it gets uncomfortable. If
you
don't know the climate, it's relatively low humidity and rarely gets
above 90
degrees. A day in the mid-90s is considered a major heat wave, and then
it
will drop down to below 60 at night. We're not talking about
Phoenix-style
weather, obviously, but we're getting spoiled as we age and do want to
add the
air.

I'm just asking for general advice, as I don't want to be totally
ignorant when
getting bids from contractors. What do I need? Capacity? Brands to
look for?
Brands to avoid? What should it cost? Is there someplace I can read up
about
this?

Thanks in advance!

Tom M.