Down side of a too large AC unit.

Have a small house, 650 Sq Ft. Needs about 2 tons of AC. Found a cheap 3 ton unit. (roof unit) So what exactly is the down side of too much cooling? BTW: This is in Tucson, usually dry air.
Al
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The unit short cycles. Your comfort level is better if the air handler goes on, stays on, and dries and cools the air. A large unit drops the temperature quickly but does not dry the air (perhaps not as large an issue in Tucson) and turns itself off because the temperature is satisfied, at least where the thermostat is located. You could even out the ups and downs by leaving the fan on continuous run.
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Short cycles Less dehumidification More money to operate Shortened equipment life Equipment requires larger ducting Larger temperature swings throughout home Requires more airflow which could lead to a noisy system
You can get it done cheap or you can do it right. Doing it right is cheaper in the long run!!!!
What's low utility bills and your personal comfort worth to you?
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Should have mentioned: The too large 3 ton AC unit is three phase. I have three phase power, so that's not a problem. I'm thinking the larger three phase unit might cost less to operate than a 2 ton single phase unit. Plus, three phase units are much more reliable. No capacitors, no start relay for the compressor. Did some looking and I can not find a 2 ton three phase unit anywhere. Since the 3 ton unit is three phase, there is no market for it and I can get it for next to nothing.
In Tucson we have a lot of 15% and less humidity, except when we're in the monsoon season like now. It's 52% right now:(
Al
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Sounds like you have your mind already made up. So 'by all means', knock yourself out!!!!
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Big Al wrote: ...

How do you figure the 3T unit could be less expensive to run than the 2T unit? You're going to need a specific amount of cooling from either unit and that can only come from a fixed amount of energy, the only difference would be the relative effective SEER of the units. Lower current on each of three wires still adds up to higher current on two. It takes the same total Watts/T of cooling given identical efficiencies.
Others have already noted the cycling from the comfort standpoint, the cycling also would be a potential early failure mode if excessive. For the size of the house, if it's at all tight and decently insulated, 3T sounds _way_ oversized even in Tucson.
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Big Al wrote:

While 2 ton to 3 ton sounds minor, it will use 50 percent more power than a 2 ton. You can run a single phase on the cheaper 3 phase power, and so there goes any savings.
The thing is probably old, so 10 or less EER, while a newer 2 ton would be better.
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The house has both a single and three phase meter. Three phase is one cent more per KWH than single phase. But the single phase rate changes during the year, different rate in summer.
I'm going to shop for a two ton unit and see what I find. Some guy recommended a couple window units because the house is so small. The house has crank open steel sash windows, so there is another problem.
Thanks for the help.
Al
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I would pay 600 dollars for low utility bills, but I don't think I can go much higher than that.
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Part of the answer depends on how cool you like to have the place. If you want to keep it at 72 degrees then it may work out OK. But if you plan on keeping it at 82 it's probably not going to get enough humidity out of the air. In my experience here in Phx I have never had a house that had a big enough AC unit to adequately cool things in the middle of the summer. The AC often runs continually from about 1 in the afternoon until 10 at night just to keep the temp in the high 70's. My rule of thumb is whatever the "experts" claim is the right sized unit should be upped by half a ton.
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On Tue, 07 Aug 2007 20:42:01 -0700, Big Al wrote:

Back in 1969 my dad had a 3 ton gas fired water chiller cooling system installed in our new ranch house that could have easily done with a 2 ton unit. I remember the house to be ice cold and clammy. Definitely not overall comfortable. And in the basement where my room was I cold leave lunch meat and cold cuts out on my dresser for a week and they would remain fresh. Like others said it isn't a wise idea but if your overall humidity is low you may fare better results especially if you can run your blower for longer periods.
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I would suggest a capacity control for your AC unit. If you know how to work on air conditioners you can do it all by your self. If you know a good AC tech who has a bit more knowledge than the normal bozos who just slap them in, he/she /it could help you. Here is a link to one manufacturer of the devices: http://www.rawal.com / APR controls work very well and I think it would take care of your problem of over capacity.
[8~{} Uncle Monster
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