Variable speed would help in both situations. You are asking the cooling system to dehumidify with very short run times. In the cooler seasons, it has short runs because it is cool, When you are not there it does not run much because you want it hot inside. The least expensive way to solve the hot weather problem could well be to lower the t-stat. I don't know for sure. Someone would have to do some calculations. That will also help in the cooler weather, but won't solve the problem. You will always have periods of low cooling demand when it is humid.

Variable speed is almost like having a system with 1/3 the capacity available and is biased to humidity removal when necessary. You might get to the same place with a window unit or a centrally-located mini split. Again, you will still have periods of high humidity, but fewer of them and pushed more into cooler days in the cool seasons. Yes, a dehumidifier will fix it. A freestanding unit is a cheap fix. Many people would not like the noise while occupied, but when you are not there, it might work. Whole house units are more expensive and more efficient. Here is a somewhat dated case study that you might find useful. Current high end variable speed systems may perform better than the ones described in it. You could also check Florida's land grant university for more current studies as well as other land grant universities in hot, humid states (LSU for example).

Conditioning Air in the Humid South—Creating Comfort and Controlling Cost

Research Report - 0214

November-2002

Building Science Corporation

Thank you for the reference. I found the article very informative. I do not want to put a freestanding dehumidifier in the attic or in a closet. Drainage will be problematic and these units are not generally designed for 24/7 usage. It appears as if the only solution is to install the whole house dehumidifier. I was hoping for some feedback on these systems.

The case study is a little dated so it is probable that the VS systems are better at humidity control than they used to be. I was surprised at the relatively poor performance of the VS system in the case study. My mini splits often haul the RH down to the low 40s on warm, humid days. In the nights in between or with low sun load, it can creep up to 50%. I have a leaky home. One big problem that I have seen is that the VS compressor systems are marketed as dehumidifiers. People swallow that very easily and get disappointed. The fact is, any cooling system in a humid environment reaches a limit at which it can no longer keep the living space as dry as you like it. What that point is, is dependent on a lot of variables including leakiness of the envelope outdoor humidity, characteristics of the cooling equipment, and rate at which humidity is generated by the people (and houseplants).

I have to disagree with your perception that the freestanding dehumidifiers are not designed for 24/7 use. Continuous use is what they are built for. They are noisy, but if you are not there, no one will notice. They have a much lower upfront cost than a central dehumidifier. The advantages of a more expensive, ducted unit are quiet and higher efficiency (usually measured in liters/watt).

I have no direct experience with whole-house dehumidifiers. My impression is that they work well. These days, desiccant wheel systems fired with gas for cost efficiency, might be possible. I don't know if they've made it yet, but if I were in the market, I'd check. at http://hvac-talk.com there is a member, teddybear, that posts a lot about domestic dehumidifiers.

If you found that paper useful, you might check this one out. It has more data, and is more up to date.

"Residential Dehumidification Systems Research for Hot-Humid Climates"

A.F. Rudd, J.W. Lstiburek, P. Eng, and K. Ueno

The Florida Solar Energy Center has a lot of good information as well about HVAC, roofing, and other+ construction issues in hot, humid climates. They've spread way beyond their name. Louisiana State University does too.