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The upper floor of my house has a split A/C, three-ton. When the temperature gets up above about 95 Fahrenheit, the unit can't keep the upstairs cool. That means that for a few weeks a year, my kids' bedrooms are still almost 80 when it's time to put them to bed. That's not really acceptable. Multiple technicians say there's nothing wrong with the unit. I understand that A/C units are sized to keep you comfortable 99% of the year, so it looks like I'm just struggling with that 1%.

My attic has an approximate R value of 32. Not super, but the calculators I've seen indicate that upgrading to 49 won't pay for itself in less than twenty years. So I don't think insulation is the main issue.

The house is about eleven years old, so I expect the compressor to require replacement any minute. I could upgrade to a larger unit, but then you run into short-cycling issues.

But what if I got a larger unit with a two-stage compressor? For example, I could get a Goodman GSX14 three-ton as a direct replacement, but I could also get a GSXC16 four-ton. Would the ability to run at half-speed compensate for the short-cycling concerns, and allow me to have a higher peak rating without the downside of lost dehumidification?

An alternate solution would be to get some portable 12,000 BTU units and just use those during the peak temperatures. But I'm not sure if that might have other side-effects.

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During the AC season close off outlets that go to the cooler area of your house, and be sure the other outlets are wide open. Put in a brand new, low restriction furnace air filter. Keep curtains closed all day on the sunny side of the house.
After these steps you may use a portable on the lower floor if you need it. As long as the portable unit has 2 air ducts going outside, it will have no adverse effect on your central system.
Also, be sure to have the central AC set so that it comes on well before noon. If you wait until it feels like it's getting too warm ... then the system will never catch up.

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It depends on the size of the inside units, with split systems I usually use larger inside units that are needed, this allows for each zone to do its job, downstairs units won't require as much cooling in the summer but upstairs units need more, the reverse is usually true in the winter. With mini splits each zone only heats or cools when there is a demand so if the compressor was undersized but the inside units were properly sized a larger outside unit may do the job and make your home comfort just right. If the inside units are not large enough it would be a waste of $ to put in a big outside unit.

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I suggest adding a room size window unit to each room ( such as 6,000 BTU). They are inexpensive , simple to install and should be no problem for your electric system. Larger units like 12,000 would put a large stress on your electrics. When I lived in Houston I had 2 AC units ; about 4 ton whole house and a 2 ton upstairs only. You could add a similar central unit for upstairs but it would be substantially more expensive than the small room units. I would not mess with your existing central unit by closing off vents, etc. Currently my central unit is fine but my spouse likes the bedroom extra cool so I added a window unit in the bedroom rather than lowering the temperature of the whole house.

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