In a post below, I raised some questions on humidity control, and one option was for slowing the blower speed and adding a TXV device. I'm starting a new post to focus on the TXV device.
I discussed the problem with the installer today, and he was in favor of lowering the blower speed, but was unsure of the benefits of the TXV.
My understanding is that by regulating superheat, one effect of the TXV is that the coil cools down faster and humidity removal starts earlier in the cycle. Are there other benefits of the TXV in regards to dehumidification.
Also the installer said that the indoor coil is 0.5 tons larger. In my mind a TXV should help this situation even more. Would downsizing the indoor coil also help.
The primary funtion of the thermostatic expansion valve (TXV) is to control superheat. The main benefit of it is the fact that with a correct charge and a correct installation of the valve you use a higher percentage of your coil for heat transfer under all conditions. For humidity control, the .5 ton coil is a better choice than down sizing the coil due to the larger surface area of the coil. In order to remove the Humidity (latent heat) you must keep the coil at a temperature lower that the dew point of the air that is being introduced into the coil surface area, and you need an airflow that is correct to allow enough time for the air to "give up" it's humidity, your installer was correct in his recommendation to lower the blower speed to accomplish this. I would also recommend a TXV on any residential installation.
FPN 1 :If the system is oversized for the cooling load the "on" time of the system may not allow enough time for the air to remove the correct amount of latent heat, this may be overcome by setting the thermostat at a lower temperature to allow for the longer run time that may be required.
I think it makes for a more even temp and humidity control thru out the changing outside and inside temps. in regards to the I.D. coil downsizing, that would give less coil surface to collect the moisture . I don't think 1/2 a ton will make that much differance.