I consulted w/ another contractor yesterday and he too was against the power vent so I have not installed it. As to attic cross ventilation, there are 4 gables on the house w/ windows approx. 2' x 2.5'. If that is enough "makeup" air, as a lay person I would think that the power vent would pull primarily through those windows and not any small crevices in the ceiling. I.e. there would not be a huge negative pressure in the attic.
Alternatively, what about turbine vents? The windows are about 1' off the attic floor and there is no high escape for hot air.
I don't know where you guys are coming from re sizing. If a contractor here put 1 ton/1000 sf systems in houses I assure you he would not be in business long. Seriously.
The amount of free air space needed to limit the attic depressurization with active ventilation is substantial. Turbine vents can severely depressurize the attic too. You have no idea just how large the effective hole is in the ceiling, but if history is any guide it's a heluva lot larger than even your wildest imagination would deem reasonable. The leakage paths are not always obvious- it takes a blower door (usually with IR imaging) to fully sleuth out all the thermal bypass air paths. You could fix 1001 small leaks and still be missing a major path that renders those efforts nearly useless. Unless the house has already undergone a round of blower-door directed air sealing the combined path is likely to be approching the size of an open window (!).
Stop thinking of attic ventilation as a solution to your cooling load issues- fix the real cooling load problems, such as air leakage in the building envelope (which are already aggravated by having the ducts and air handler outside the pressure boundary of the conditioned space).
If you want to DIY it, start with the most obvious large leaks, such as register boots that haven't been can-foamed to the ceiling gypsum, plumbing stack chances that aren't sealed. If you have a masonry chimney there is likely to be a square foot or two of leak due to code-required clearances to combustibles, that can be air-sealed with sheet metal + fire-rated caulk. Any recessed lighting fixtures need to be boxed-over (with 3" clearance to the box) and can foamed to the gypsum. It goes on, and on , and on, and it's more tedious than actually difficult. All of this comes WELL before weatherstripping the attic hatch/doorway.
A primer on air sealing an attic lives here.
When you've fixed all of the stuff you can find, a large (preferably reversible) window fan intentionally pressurizing/depressurizing the attic and a smoke-pencil (or a cobweb- thread, used as an air motion detector) will likely find you a bunch more.
Partition walls with missing or leaking top plates are a common less-obvious path, with air entering the partition wall cavities from loose or unsealed kick board trim, electrical & plumbing penetrations, etc. Some of these are easy to fix, some not so much, but you have to at least find them to be able to fix them.
Venting the roof with a soffit-to ridge venting scheme will lower the roof deck temps a few degrees (it won't fix your cooling load problem, but at least it won't hurt). A ridge vent will also depressurize an attic, unless the free air space of the soffit vents are at least 25-50% larger than that of the ridge vent, so don't just slap on a ridge vent without the soffit vents, or you could make the problem worse rather than infinitessimally better.
Regarding system sizing, I know of an
HVAC contractor in central Florida doing geothermal heat pump systems who regularly ends up at load/conditioned space ratio ratios smaller than a ton per 1000', often closing in on a ton per 1500'. This is in reasonably air-sealed houses with sealed-conditioned attics, with as little as R20 open cell foam under the roof deck, usually retrofitted as part of the process. He is able to sell them the roof deck insulation & air sealing for less than the marginal increase in cost of the larger GSHP system it would take to manage the larger loads.. (He uses a theatrical smoke generator to verify that the attic is fully air sealed before the foam installers break down. Plumes of smoke makes spotting the leaks pretty easy.) That's probably not a cost effective solution for you, but there's no way a 4 ton system should be failing to keep up with the load. The fact that a good part of the system is outside the conditioned space would probably keep you from being able to get by with a 2-tonner.
There's another contractor/blogger in Atlanta GA who likes to measure stuff, including the cooling load of his own house
as measured by the duty cycle of his existing sytem. (Strangely, his old air-leaky condo still came in at about a ton per 1000' at 1% outside design temps comparable to yours.)
Old school HVAC hacks typically specify a ton per 500' on older housing (which is roughly your ratio), a ton per 750' for newer construction and usually end up oversizing it by quite a bit, even with the ducts in the attic.
A true professional would actually run the Manual-J load numbers, but merely being a licensed HVAC contractor doesn't mean they even know how that's done. (It does mean they should know how to properly charge & test the system, the regulations on handling refrigerants, and should have at least a clue as how to design & install ducts, existence proofs of truly atrocious duct systems notwithstanding.)
