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.
Best Answer
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.