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Old 07-13-2010, 03:39 PM   #1
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Temp & Humid control for AC?

I have my room (window) AC unit wired to an HVAC thermostat on the wall.
I have it set to maintain a temperature of 83 at night, which was quite comfortable when the humidity wasn't as high as it has been recently.

I understand what the temperature humidity index (THI) is, and the fact that at a higher humidity, I will need a lower temperature to remain comfortable.

So, I was wondering if there was such a thing as a temperature and humidity controller for controlling indoor air conditioning (and heating).
I did a Google search, and came up with greenhouse controls, which are very expensive.

So, the product exists (no patent for me ).
Since I am not going to spend $500 or more on a comfort controller, I thought maybe I would wire a de-humidistat in parallel with my thermostat, so that a constant humidity could be maintained under a wider variety of temperature and humidity. If the temperature is very warm, but the humidity is low, then the AC would be controlled by the thermostat alone. But if the humidity is high, and the temperature not so hot, then the AC would run longer to lower the humidity to a more comfortable level.
But then I realized that it's not so simple.
First, I need to know what the temperature is before I decide to reduce the humidity. I won't be comfortable if the temperature is 65 degrees and the humidity is 55%; a further reduction in humidity would make me less comfortable.
If I wanted to reduce the humidity from 60% to 40%, I would probably need to run the AC unit continuously, which would result in the temp being lowered to 60.

So, I guess the greenhouse controller is really the only thing that would do what I want; and even then, those controllers are designed for a different purpose, and would probably not work in the temperature and humidity range I am looking for.

What I really need to do is install both temperature and humidity sensors to my PC, then write a program to calculate the THI, and allow me to set the level I want.

In the end, it is an interesting "science fair" project, but would not have much practical use.
It would be much easier to guesstimate the desired temperature based on the relative humidity, and set the thermostat to run the temperature down a few degrees at night.
This is contrary to what is normally done with HVAC (for cooling), isn't it?

So, what do people who have whole house HVAC do? Just keep the temperature as low as they need it, then use blankets if they get too chilly?

I am trying to save energy and money on cooling, so I am trying to set the thermostat at or slightly higher than my comfort zone.
Actually, my "perfect" system would check the outdoor as well as the indoor temperature and humidity, and if conditions were right, would shut off the AC and turn on a fan that would bring in air from outside. I actually prefer the fan over the AC when it's cool and not too humid at night.

Any ideas?





Last edited by Ultrarunner2017; 07-13-2010 at 04:02 PM.
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Old 07-13-2010, 05:00 PM   #2
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My thermostat is a Lennox touch screen. Controls temp, humidity and has an outdoor sensor. I believe honeywell IAQ stats do the same. Without the ability to slow the blower all the fancy stats are going to do for you is overcool by 2-3 degrees .


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Old 07-13-2010, 08:45 PM   #3
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I don't think that will work on my unit, since it's only a single room window. Your system most likely includes humidifier and dehumidifier as well as heat and AC. I guess I'm looking for something that can't be done. My query was more of a curiosity than something I would actually expect to accomplish. FW

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