Solar roof fan info can be found at a # of sites. One I looked up www.rewci.com sells a unit for $339. I also saw one for $309. Cutting a hole in the roof and installing the unit correctly ( no water leaks) should be worth 1/2 a day for a mechanic. I would think the price you received is ok, but the size of the attic may need more than one fan. Look at the sites, they will help you make a decision.

Thanks guys. Let me see if I can provide a clearer picture of what I'm working with. The house is over 100 years old--a brick row house in the center of a block. Most walls and all the ceilings on the third floor are plaster, with lots of lumps and bumps and a few small cracks, but mostly solid. There are no ceiling moldings. To call what is above this floor an attic is a an exaggeration. It is a space that ranges from about 3 foot tall at the front of the house to about half that at the back of the house (roughly 40-feet on that floor...first and second floors have a bigger footprint and do not need insulating between living space and roof.) There is a central air (newish high-velocity, narrow tube) system that snakes around the "attic". The blower is installed at the one access point there had been. This system serves ONLY the third floor and has two outlets in each of three small bedrooms and one in the tiny bathroom. The system serving the first and second floors is in a second floor closet. The insulation guys tell me that they can blow insulation in but will need to cut at least one new mansized hole in the ceiling to get in--possibly two, one toward either end. There are no recessed lights on the floor, and only three overhead lights at all. All other lights are wall brackets, wired from the floor up, rather than from the ceiling down. Of the three overheads..one was put in new by me, so isn't K&T. One the previous owner says was moved by them, so isn't K&T (But may connect in to K&T at the meeting of wall and ceiling. And one is the bathroom overhead, which can easily be checked. My electrician replaced the fixture there, so he should know what's above it. The roof is asphalt and fairly new (say 5 years old?) so, I'm not inclined to want to replace it anytime soon. I've gotten a quote for $950 to cut an access whole and blow in insulation. I'd have to pay someone else separately to fix up the access panel afterwards, probably adding another few hundred. Most neighbors don't even try to do much of anything with ventilation in their "attics", as they are all scared witless of cutting holes in their roofs. But I have found info on the Brooklyn Brownstoner blog about successful attic and whole house fan installations and one neighbor loves the whole house fan he has which sits in a sort of plastic cove on the roof. Unfortunately, he bought it with the house and has no idea who installed it. I don't intend to move in the near future, or even the more distant future. But I would like to make the third floor more habitable in the summer, especially since at some point in the next two years or so, I'd like to turn it into a separate apartment and rent it. Honestly, I seem to be finding such conflicting information and I'm so dubious about how knowledgeable folks you hire to do this stuff are about old and "out of the norm" houses that I really, really, do appreciate any and all help I can get here on this forum. Thanks,

Negative pressure will occur if the air flow being pulled out of the house by the fan is greater than the air being supplied by the open windows. This means the air pressure inside the house is less then the air pressure outside. If you have appliances using combustion (hot water heater, fireplace, etc.) the fumes which would normally be rising up through the chimney will reverse direction and flow into the house. This could lead to carbon monoxide poisoning. Whole house fans were popular before the days of central AC. I become concerned when someone wants to switch between the two. It is very easy to switch on the fan and forget to open the windows. You have to be careful.

This is not a duct. You are simply replacing the fan/motor assembly. The shroud (cap) keeps the weather out but is not a duct. After replacing the assembly make sure the screen surrounding the top of the shroud is in place to keep the critters out. The louvers you are talking about are the gable vents. These are meant to be an exit for air coming into the attic from the sofit vents. They have fallen out of favor with newer construction as ridge vents, which go all the way across the peak of the roof are more eficient. These setups normally provide all the passive ventilation needed for an attic but I suspect a previous owner installed the power vent because of insufficient ventilation. Make sure your soffit vents (located in the flat area underneath your gutters are not blocked.