Corollary Question to AC Humidity
 

Understand about the AC kicking on to lower humidity, but now that heat is running, The humdify light is actionable. What happens to raise the humidity if it gets too low?

A humidifier can be added to the system. It will evaporate moisture into the air to raise the relative humidity. You will see these devices up north. All houses "breathe" to some extent. If the outside air is very cold then it's absolute moisture content is very low. Once that dry air is "breathed" into the house and heated, the relative humidity will be very low. A humidifier will bring the relative humidity up to more comfortable levels. They really aren't needed here.

The only way to raise humidity is to add moisture directly to the air. You can buy a humidifier that is a small box attached to the HVAC unit. It will spray a mist of water into the supply air plenum as needed. A lot of houses in the north have them, but they are not usually needed in Florida.

So even though the humidity setting on the thermostat can be changed, you are saying it does nothing unless a humidifier is added to the system?

Yes, that is correct if the system is in heat mode.

Correct, if your thermostat is set to the heating mode. But, you can also set the thermostat to both the heat and cool mode, in which case the unit will automatically change between heating and cooling as needed. Note that of you are in the heat and cool mode, most thermostats will require the heat setting to be at least 4 degrees below the cool setting.

Note that a cooling coil does remove H2O from the air stream but it also cools the air so that when the room cools down the denominator of the RH equation becomes smaller since colder air has less capacity to hold water vapor.

Ideally one would slow the fan speed down so that a higher percentage of the energy of the compressor is used for dehumidification rather than cooling the air. But if you slow it down too much you will cause ice to form on the coil.

That is why variable speed fans with the right controls are so much better than the standard AC system.

What is sensible cooling vs latent cooling?

The proportion that is used for purposely cooling the air is called sensible cooling capacity. Any proportion of the cooling capacity inadvertently used to dehumidify the air is called latent cooling capacity.

The compressor would kick in and that means the ac will come on to remove the humidity. You can set your humidistat to 60% and in a humid environment you don’t want to have it higher than that. If you want to add humidity, simply purchase a humidifier. In the 5 years I’ve lived here I have never thought the humidity in my home was too low.

Dryness
 

We've felt that the air is very dry causing dry mouth at night and coughing. We've tried a humidifier but it doesn't help. What am I missing when you say usually not needed in Florida?

I would suggest getting a quote for a central humidifier. It is a small box attached to your HVAC air handling unit. It has a water line connection and it is controlled by your thermostat. They function much better than a portable unit.

We never get really cold here and therefore never see really dry air, in an absolute moisture sense. Up north, when very dry outside air is heated to 70F it results in single digit relative humidities, which many people find uncomfortable. I had a humidifier on my HVAC system up north but once I moved south I never felt the need. Your needs sound like they are different.