Has anyone tried to use a fan speed controller to control the speed of the fan of their central air conditioner?
Iām neither an electrician nor an HVAC technician, but Iām not sure how that would work 
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What do you use to control the fan in your central air system now?
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I would be careful doing this. Modulating the speed of the supply fan without controlling the associated compressor could lead to problems such as freezing coils and/or high humidity levels.
That is why on zoned damper controlled systems the supply duct static pressure is controlled from a bypass damper to allow constant flow across the cooling coil.
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In theory, with the older multi-speed blower fans, I suppose it is possible to use a series of relays such that a different speed gets selected.
But the new ECM motors are variable speed controlled directly by the air-handler control board, so that wouldn't work.
In my pre-automation days, I would manually configure the blower for low-speed operation in the summer to promote dehumidification, and higher speed operation during the winter.
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Most new central air conditioners can have a fan speed control of auto and on. Using the Go Control AC driver, I can set my fan to circulate or on at any time. But controlling the speed other than that would take additional electronics that may void any warranty and may not be safe.
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Agreed.
I meant the old non-ECM multi-speed blowers like this one. My old HVAC setup had one of these, and I would manually set it to the lowest speed in the summer (to optimize dehumidification) and a higher speed in the winter.
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As other have mentioned, the air handler controller generally controls the fan speed. In some cases, there may be a little opportunity to influence how it sets it. Some air conditioners have a dehumidify mode that uses a slower fan speed. Some thermostats may set that mode, but most don't. Most thermostats either don't measure humidity or measure it and display it without doing anything with the information. Most thermostat are at least able to choose auto or off fan settings that determine whether the fan is off between heating or cooling calls or not.
Exactly!
The humidity inside the house is running about 70%. I want to slow down the air speed over the coils to increase dehumidification.
That's high alright. What kind of air-handler do you have? ECM blower or not? How many speeds is the blower?
Have you tried manually switching it to a slower speed?
Also make sure the blower fan is not constantly on.
And, the most common reason this happens is that your AC system is oversized.
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This old house says you need a dehumidifier for this time of year and a humidifier for August. I used to live in San Antonio. Hot and humid in April through June, then hot and dry. But - it is highly worth it. It is a great city to live in.
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With this high humidity you might consider having a qualified AC tech check your system out to make sure it is working properly, One thing you can do is check the temperature of the supply air at a diffuser when the system is in full cooling. The supply air temperature should be in the range of 55 to 60 deg F. Any higher and it will not be dehumidfying properly. But the AC tech can measure your refrigerant pressures to verify proper operation.
I do agree with @aaiyar that high humidity is often the result of an oversized system because the system cycles too much and does not run long enough to dehumidify the space.
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Just down from the attic. We have a Trane XT80, which has a variable speed fan. Apparently, the unit was installed with a low-cost thermostat with no advanced capability.
The plot thickens.
@Hal3 - also look for a 18-20 degree difference between the return register and the supply register closest to the air-handler while the system is operational.
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The smarts are in the air-handler control board. This was one of the models I considered 3 years ago.
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Yes assuming the system is working properly a thermostat upgrade might be helpful.
By the way I do live in San Antonio and it is hot and humid here now. Not as bad as New Orleans where I am originally from. 
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Where I'm at!
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Geaux Saints!
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Well, this is escalating quickly.
This thermostat costs $540 plus, probably, installation by a professional. I'm not even sure this is what I need or if it will solve the problem.
I just want to slow down the fan.
