"Gas fired warm air furnace", now that's interesting. I've never heard of a cool air furnace, hehehe. Like my neighbor always said, there's no such thing as a hot water heater, else you wouldn't need the heater.
Anyhow, you are looking at about a 2 to 2 1/2 ton unit for 1100 sq ft. Suggestion, find a contractor who carries Carrier, call them out and have them look at your existing system. They may be able to add the air conditioning evaporator coil, which is the inside coil, and then the outside condensor coil, to your existing furnace/fan setup. If the house never had air conditioning, the duct work may not be sized the best for air conditioning but may be ok.
I mention Carrier for a few reasons:
- I have a lot of experience using and maintaining Carrier and other A/C products
- Carrier's new line of 'split systems', those which have separate inside fan and coil and outside coil, have variable speed fan motors and use a Thermidistat instead of a thermostat. This gives you the ability to dehumidify during the cooling season even when the temperature in the house is cool and also other good features. Other good brands have similar capabilities
- Stick with a good brand like Carrier, Trane, Rheem
You should be able to find a contractor who can do it all but they still may sub some of it out if any new duct work is needed.
Are your ducts insulated? You will need insulated ducts because the condensation will be bad news otherwise. You will have mold everywhere if you don't insulate. With a house the age of yours and it only ever centrally heated, there may be some retrofitting of ducts and other things you don't anticipate. Just make sure to get 3 bids from 3 reputable contractors who carry name brands like mentioned above. Also, your contract with the contractor should spell out everything to be done. For instance, if they have to insulate ducts, specify minimum
1 1/2 inch thick insulation installed without compressing the insulating material, ie, you want the 1 1/2 inches. I got screwed on my new home because I trusted the contractor after talking to him and didn't spell out things like the insulation and duct size/shape. The people who did the duct insulating wrapped it with 2" insulation, which was a good thickness, but the wrapped it so tight, the insulation is only about 3/4 " thick. My ducts are in the crawl space and sweat in the summer because of this. So spell everything out.
Your evaporator, inside coil, needs to be plumbed so the condensate lines are installed per code. Typically the primary goes into your plumbing drain system and the secondary/overflow drips outside the house.
Make sure they insulate the larger suction line completely, from exit of the evaporator to inlet of the condensor and the insulation needs to be glued at seams with rubber cement. Pipes need to be adequately supported.
Make sure you get a 13 seer or greater rating unit, they do pay for themselves. Look up seer on the internet for more info.
Good luck