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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 F 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.

  1. I could upgrade to a larger unit, but then you run into short-cycling issues.

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.

  • or 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?

  1. An alternate solution would be to get some portable 12,000 BTU units and just use those during the peak temperatures.

I'm not sure if that might have other side-effects.

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  • $\begingroup$ Short answer is yes, that is what a two speed or variable speed unit is supposed to do. But how bad a humidity problem do you have and what are your attic temps? If you live in a high wind and high humidity area, you may need to look at supplementary dehumidification. This can be an attractive out if the codes people won't let you install a bigger unit. $\endgroup$
    – Phil Sweet
    Feb 25 '19 at 0:42
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As Phil said in the comments, the short answer is yes regarding a two-stage system. As an added bonus, during the rest of the year besides the really hot time, the system will probably also keep the house more consistent since it will be able to operate at a lower capacity rather than cycling off. At 11 years old, however, unless you never change filters or live right on a large body of saltwater, I would hope your compressor is not already on the verge of dying off.

As a side note to some of the other details in the message, you mention the attic insulation as R-32. If that was blown in insulation, sometimes the thickness can be misleading if it seemed too much or if it is fiberglass batting, moisture will compromise it. In addition to the insulation on your walls, you should also look closely at your ductwork. If it is insulated running through a hot attic, your 3 ton AC might only be delivering 2-2.5 tons to the registers.

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The typical solution in Houston is to have two AC units for a two story house. A lower cost , efficient way to cool the upper rooms is with window units , if permitted in your location. They are relatively efficient and cheap . I have a $ 100 unit that easily cools a 13 X 13 room room addition.

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