Cold Air Return placement

Hi,
During the summer months, the second story of my house is significantly hotter than the lower levels. The house is filled with vaulted ceilings throughout, and there is no attic.
We had air conditioning put in when we moved in last year, and we've replaced the windows that get the afternoon sun. However, its still regularly 5 to 10 degrees hotter upstairs. (I've tried closing the registers downstairs, so that only the upper registers were working, but what winds up happening then is the house NEVER gets cool enough...the AC runs constantly. )
The cold air return for the central heating (and now cooling) for the upstairs is located at the top of the stairs, but at the bottom of the wall. So, even though its upstairs, the cold air return is at the bottom of a 10 foot high wall. I have verified that the return is just between two studs, and, as far as the wall is concerned, there is no duct work - the studs act as a channel for the return air. (I imagine that it hits duct work once it reachs the floor somewhere.)
I have a theory that putting ANOTHER return register at the TOP of the wall would make a significant difference in the efficiency of our cooling system during the summer months (provided that I can easily close off the lower return register.) It just seems to me that having a register as high as possible is more likely to recycle the hottest air back into cold air.
How much improvement am I likely to see? Is this going to make a 1 degree difference, or is it likely to make a much more significant change? I know that the house wasn't really designed with AC in mind so the duct work on the upper levels isn't as big as it should be, but shouldn't this change still make a significant difference?
I'd really like to know, since my wife and I would rather not put another hole in the wall unless its worth it. The more detail on this answer, the better, since we REALLY want to be educated about what we're doing HVAC-wise.
On a side note, many of the (non vaulted ceiling) houses in our area have whole house fans, that suck hot air into the attic, and then out of a vent in the attic. I don't have an attic, since all of my ceilings are open. Is it possible to put a whole house fan in that performs the same purpose, but in a _vaulted_ ceiling? In other words, one that just sucks the air right out of the house and out of a (properly weatherproofed) hole in the roof?
Any other suggestions as to what we can do to make the temperatures more even throughout the house? Any quick fixes that I'm missing? - Thanks for all of your help,
Todd
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some thoughts from buffalo ny: first my comments and a great link at the end: do you need a bigger cooling capacity? probably, since you say the house never gets cool no matter what. do you need a system repair or recharging? maybe. was the system ever cold last summer? do you also need to improve the airflow in the hvac system? yes there is always room for improvement but it may not solve this by itself. the hvac must be redesigned to your home based on your heating and cooling loads, so why not have the installing company come out and pay them for a system checkup on a warm day? WHO calculated the load? WHAT is wrong? - is insufficient comfort. WHERE do you live?- determines climate factors, new orleans and las vegas both have hot summers but completely different humidity to deal with for example. WHEN do you want this done? -probably yesterday. summer temperatures have already arrived when the fans start vanishing from the walmart shelves. WHY not set up a budget for this upgrade or repair. Sometimes doing the job correctly costs more than you wanted to spend last year. other: HOW: air is lazy and takes shortcuts. if the wind isn't blowing naturally, you will need to move the air around mechanically. if the outside air is desirable in terms of temperature and humidity compared to inside air, of course a fan ventilation system would be most efficient and least costly. but to be effective and comfortable, ventilation of only desirable fresh air needs to be automated into your hvac system. hidden items include costs, insulation, reflective window coatings, awnings, shade trees, roof color, and noise factors of fans.
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free professional help for your home, wonderful detailed answers are ready to read at: http://www.buildingscience.com/resources /
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Todd wrote:

You have the right idea concerning relocating the Return air to a higher place on the wall.
Vaulted ceiling produce more cubic feet of air to deal with for both heating and cooling. In the winter the heat goes way above your living space, they are bad for efficient heating and cooling.
For the air conditioning mode, you will want the air to stratify so the heat is above you in each room; it then acts as an insulating barrier to the exchange of temperature differences.
I do not know how your home is laid out, but you do not want the return at the top of the stairwell because you want the pull the warm air from the top floor and about 7-feet up on the side of the wall. Allow the upper area air to stratify.
Make certain that you have plenty of return area from the upper floor rooms. http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_systems.html
First, do a heat-gain on your home and do everything you can to reduce it! Then size the duct system, both supply and returns to meet the btuh requirements of the heat-gain results per room.
I might be tempted to modify the vaulted ceilings and have a separate heating and cooling system for the two floors. If possible, have doorways to close off the stairwell from the upper & lower floors.
You should experience a lot of improvement in the cooling mode! - udarrell - Darrell
--
Air Conditioning\'s Affordable Path to the "Human Comfort Zone Goal"
http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html
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Ok...so here's the next question. So the wall where the return resides is in a common area shared by 3 bedrooms and a bathroom. Again, I have vaulted ceilings throughout all of these rooms, all though this common area is walled off from the rest of the rooms, and those rooms are accessed by doors, obviously.
The doors are standard doors, and there are no other openings on those walls to allow hot air that has risen in those bedrooms to get to a return air register in the common area. Does this make sense?
So...this begs the question...how HIGH do I want the new return register on that wall? Do I want it as high as possible? (10+ feet) or do I want it just barely higher than the height of the doors in the common area? (7+ feet.)
I have ceiling fans in two of the three bedrooms, and plan on putting in a third. Does this help in the answer at all?
- Thanks,
Todd
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