need help/suggestions with two story heating/cooling

All,
Looking for advice on a home heating/cooling problem, here's the story:
We have a two story house (2100 sq ft, almost equally distributed up and down) in central Texas (mild winters, somewhat hot summers). Lennox single 4 ton HVAC unit roughly 7 years old, single zone system. Summers aren't bad, although the upstairs rooms are a bit warmer than downstairs - nothing too unpleasant. Although two rooms (that face south and are over a two car garage) get pretty warm in the morning & afternoons. Winters on the other hand are completely different. The downstairs is consistently 10-15 degrees colder than upstairs. It's almost unusable on colder (30's) nights. We've tried small space heaters downstairs, but they really don't help much.
We've talked with several people about options and here are the leading three:
* replace single pane windows with dual pane * replace HVAC with a dual system, or more powerful (5 ton) with continuous fan * add damper system to current HVAC to create dual zone system
The problem is all three solutions came from people who sell that particular product, and all claim the same thing "this should solve 90% of your problem". The first two are pretty expensive, so for that money I expect more than a 90% solution. Also, I don't expect sales reps to recommend a product they don't sell, so I'm not sure I'm get a straight answer.
While money is a concern (I don't want to spend 50k to fix this) I am willing to spend a reasonable amount if it solves the problem (3-15k). We're going to stay in the house at least 10 years, but getting the system to pay for itself in energy gains is not the leading priority.
We've looked at vinyl vs. Anderson Renewal windows and love the Anderson windows, but the price is pretty steep and the sales rep was a bit odd. Vinyl through Home Depot seems to have a great warranty (haven't read the fine print), but it's still vinyl windows and I haven't been convinced they're going to last - and it still just seems wrong to have vinyl windows to me. They looked great until we saw Anderson windows.
A local company serviced our HVAC this year and when I asked the tech his advice, he seemed to think a damper system (~3k) would significantly help. We do have two returns (upstairs & downstairs), so adding a thermostat downstairs (the one we have now us currently upstairs) sounds good. He's the first person that's explained a damper system to me in a way that he sounds like he knows what he's talking about.
So there you have it - I would really appreciate feedback from someone (homeowner) who's done this and what the results were, or hearing from a general contractor on recommendations.
thanks, -andrew
twostoryhvac at gmail dot com
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Putting a door at the top or bottom of the stairs to restrict airflow will keep more of the heat on the ground floor, and more of the cool on the top floor. After that, it's just a matter of putting the heat or cold where it's needed. If you can't block the between-floors airflow, then you need some way to deliberately circulate the air between floors, which should even out the temps.
Insulating and plugging leaks would reduce your total climate-control load, but it's sounds like balancing is your current issue.
Multi-zone systems are for when you want to reduce the load further by NOT controlling the temperature in unused spaces.
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A no cost method that you can try is close the registers on the second story during the winter so that more air blows downstairs and less upstairs.
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Goedjn wrote:

I got a noticeable improvement by blocking off my downstairs *return* vents. This forces the AC to pull the hotter upstairs air and cool it off. Since you have two returns you could try this by covering the downstairs return with cardboard. If it works then you make a more attractive cover. I plan to block off my upstairs returns in the winter to improve the heat circulation.
Caution: Some of the HVAC techs have posted about the possibility of freezing your a-coil due to reduced airflow. This is supposedly not a problem with blocking a return but it is my CYA notice.
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A low cost experiment is to just run the circulating fan continiously. Be careful about closing dampers or covering returns as you can wind up with inadequate airflow thru the furnace and overheating. A good technician could possibly improve the situation by re-balancing the airflow. Anything you can do to reduce the downstairs heat loss would help some. The percentage of off time for the furnace, if any, will give you some indication as to whether the system has adequate capacity.
Don Young
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