strange A/C experience

Our home is in the desert, where we cool about 9 months of the year. When we bought the place, it had two A/C split systems- a 5 ton and a 3 ton. There were no labels, but a friend who is familiar with the subdivision and builder told me that they were built-up no-name units with a SEER or <10 and no TXV. Our electric bill last year for this time of year was >$500 and the temps were about like this year. With that in mind, I had a good, reputable A/C company bid on installing SEER 13 units with TXV's. The price was just over $9k, installed.
Our home is an "open" design with a second story opening out to a loft area. It is impossible to keep the cool air upstairs from flowing over the railing to the downstairs. Strangely (to me) the 3 ton unit serviced the upstairs and the 5 ton the downstairs, which is just 25% larger in area. The A/C company did calculations and measurements, and told me that a 2-1/2 ton would be just fine for upstairs and that we did need a 5-ton for the lower level, so I agreed.
The installation was done about two months ago and I was pleased with the quality of the workmanship I saw. They even returned on a very hot day to check the Freon levels.
Now for the strange part: The downstairs unit seldom runs at all. This is what I would expect with the cooled upstairs air flowing down over the railing. Both intakes are upstairs- the one for the downstairs is in the ceiling right over the railing, and the upstairs one is in the middle of an interior hallway. Our electric bill for the first really hot month was $250, which naturally includes all lighting and about 10 hours a day on our pool's pump. That places it at about $250 less than last year.
The upstairs 2-1/2 ton unit runs about 75% of the time on a 110f day and the upstairs is more comfortable than it was last year. The downstairs is equally comfortable, which is a change from last year as well. I am extremely happy with the results of the switch, but now wonder if there was a miscalculation on needing the 5-ton unit for the downstairs. Without knowing the math, from what I've seen so far, a far smaller unit would probably work just as well, if not better, because the 5-ton unit goes sometimes 1-2 days without even running. . . and then runs for under 10 minutes before switching off.
Any thoughts?
Nobody
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nobody wrote:

Maybe, but it is not possible to say from here. Someone really should have done the numbers and calculations to determine the size needed.
You may want to consider that if the second floor unit fails the other one should be able to keep you alive until the upstairs unit is repaired, so there is at least that plus.

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Joseph Meehan

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especially cooling 9 months out of the year. I think the 13 and under units are not being made anymore and you are getting the end run units. You did not mention how big the house is or when it was built or the insulating material and construction. A company should be able to do the calculations for your house and recommend the correct units. As you are in teh desert I would guess the humidity is not worth worring about as it is probably low. In the area I live (North Carolina) the units need to be sized to run alot to take the humidity out of the air in the summer. It will feel much cooler with a lower humidity than it would with a high humidity.
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Ralph Mowery wrote:

adequate access to water, evap coolers work REALLY well at lower cost than 14-19 seer AC
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Most definitely they run at a lower cost than Refrigerated air. However they do use a lot more water per year which in the desert, can be a real scarcity. As well, I would not say they work "REALLY" well. They work but ultimately I have found with mine, the temp will only come down around 15 degrees which makes for a pretty darned hot house at 105 degrees plus. Humidity plays the biggest role in whether they work well so after a rain, they don't work worth a darn. Cheers, cc
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I think 13 SEER are still being manufactured. As I understand it from 3 A/C contractor's, they are still selling off the 10 and 12 Seer units and 13 SEER are the lowest versions still being made. For me a 13 SEER is probably the best I should go with as we cool for perhaps 4 months of the year and even then, only during the day. Cheers, cc
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Yep, you are correct. The 13 is the lowest rating still allowed to be made. For you that may be the best for the cost vrs the payback. For the fellow cooling for 9 months and in a very hot area I would think the payback for a higher seer unit would be much beter.
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nobody wrote:

The cold air is heavier and always flows to the lower levels. By far the largest heatload is on the top floor and that is where the larger of the two units should be. This is merely speculative: I would probably have put a 3-ton system upstairs and a 2 or 2.5-ton equipped system downstairs depending on how one prorates the heatloads of the various areas. - udarrell - Darrell
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Based on how the units run (runtime versus off time) it's obvious that 2 1/2 tons is keeping you comfortable. Congrats on the power bill. Sounds like your unit will pay for itself in a few years.
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Christopher A. Young
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