Did this guy want to put the compressor in the attic, or just the air handler?
I'd want a lot more info.
Did he give you a price? (Putting vents throughout a house that has none has got to be expensive.)
What type of heat do you have now?
I'd get a second and a third professional opinion.

The last house I rented was a 102-yr old house, 2 stories plus attic, 3,000 sq ft, that the landlord had central air/heat added to. (The house was in VA's Northern Neck in a village called Morattico, about 75 miles east from Richmond and 80 miles north of Norfolk.) The installer ran the air ducts through the old closets (which took up probably 50% of the room in each one...but they were fairly small anyway.) when he could. He also ran some ductwork down along some interior walls, after which a drywaller came in and enclosed them (separate contractor). In the attic were the air handlers with the filters, and the drain pipes to allow condensate to flow out. The landlord had two complete systems put in - 1 for downstairs and 1 for upstairs. The downstairs system used propane for the heating and the upstairs system used electric for the heating. The two condensor/coil units (? - I know electronics, not A/C systems - sorry!) were outside the house. After the two systems were installed, the landlord had the pulped paper-type insulation blown into the attic beneath the floorboards. The only ventilation in the attic was the vents in the window openings, one of which had an exhaust fan on it.

I didn't move into the place until after the systems were installed, so I can't really compare "before" and "after" attic temperatures. I can say that the attic didn't get extremely hot in the summer with the units running in the attic. The exhaust fan didn't kick on too often that I can recall. The attic was a little warmer in the winter, which I attributed to the heating systems and the insulation. Then again, the walls of this house had NO insulation in them and the interior surfaces were plaster, so I felt the cold wind!

The landlord told me it cost about $10,000 for both systems to be installed plus the blown-in insulation (all done by 1 company). The landlord did this to help the house sell...I moved in during July 1999 and they sold the house in September 2000. I hope this information is of use to you. Jim D/Heathsville, VA

They did this on a episode of this old house. The house was hot water - radiant heat so there was no duct work. The compressor and coil were located outside. thats where all your heat is disapated. The part inside is whats normally in your furnace, a fan and a coil. There would be condensation on the coil that would have to be sent somewhere just like in a regular furnace. I thought it was a neat installation. They just ran flexible insulated ducting through the attic that looked about 5 inch round and I think they had about 3 inch round openings in the ceiling that looked alot like recessed lights. That way you get cold air sinking from the warmest (high) part down. I would think it would work well. You would want to install attic insulation though.

How to insulate HVAC units installed in residential attic space

My electric furnace and A/C combination unit was installed in our attic space. The unit is not well insulated, 1" to 1-1/2" in thickness. I believe it should be housed in an insulated compartment or wrapped in some way to retain the heat in winter and cooling in the summer. Please confirm best practice and method.

Nosakcin

Hello
Working on a project for client one building has 2 units habitable area is at second floor (first floor is just for access to second level) each unit second floor is 900 SF.

Client likes to have 2 A/C units (one for each unit) my question what models are out there and what are the specs sheets. Engineer needs to calculate loads and also I like to know clearances needed to accommodate unit at attic. Project is new construction. I'm sure clients will look for inexpensive units.

I'm believe client likes to have these units at attic space to safe kept in place.

Thank you for your help.