Humidistat

I read that if you set the T-stat to 70 and the H-stat to what you *perceive* to be a comfort level, the AC only runs to achieve the relative humidity set on the H-stat. It went on to say that it helps to keep the unit from cycling as often as it normally would, thereby saving electricity and still be comfortable. The THI here for a week has been 110 - 115, and it just got me to wondering. As inexpensive as the H-stats are I thought it would be worth a try - or at least the time it takes to do all this typing. :-) Opinions appreciated! TIA - - - - - - - - - - - - - - - - - - - Easy's gettin' harder everyday
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1D10T wrote:

either the Temperature or the Humidity set points are Not met, the AC contactor will stay energized & the AC unit will stay running. Especially if the system is somewhat oversized, it is a good way to go. - udarrell
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Wiring the humidistat and thermostat in parallel should theoretically work, but.............
most humdistats are designed to *********close********* their contacts when the humidity falls below a specific set-point (so as to energize a humidifier) and therefore, will *******do the opposite******* of what the original poster intended******* if wired in parallel as recommended.
What is actually needed is a humidistat which closes it contacts when the humidity level *****rises***** above a specific set-point,. thereby energizing the air conditioning when there is too much humidity.
Using a conventional humidistat as the prior reply indicates will ***NOT WORK*** as claimed, and, in fact, will turn on the a/c and keep it on as the air gets drier.......
Smarty

http://www.udarrell.com/principled_adjudication_disputes_administration_justice.html
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And if the system only ran to reach the desired humidity level, why would you need the thermostat at all? One would think that in a period when you have a temp humidity index of 110-115, which sounds like a jungle, that any reasonable AC system would have dropped the humidity to a comfortable level before reaching 70 deg., which sounds mighty coool. If you want to save money and time, just raise the thermostat to 75 or higher, which should still feel like a freezer if you really have a 110-115 THI outside.
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Smarty wrote:

Mode Humidifier Humidistat.")

wired parallel, the circuit would stay closed until the Setpoints of both sensible temp & % RH were met. In the Heating mode the Humidistat contacts would work the opposite way & close to bring the humidifier on when the RH got too low. Because Heat & Cool are separate circuits', each separate circuit can be "wired in parallel to do different or opposite things. The Humidity controller in each separate circuit is engineered to perform differently, in the Heating Mode the Humidistat contacts are open until the low %RH setpoint is reached & then the contacts close bringing on the humidifier. The "Cooling Mode Dehumidification controller & circuitry" works the opposite.
If they are wired in series, as soon as one of the two setpoints' is reached the control circuit to the contactor in the cooling AC mode would open shutting it off before the other setpoint was reached. The entire purpose is to keep the AC running until both comfort level setpoints are reached. The setpoints of the individual RM TH & the dehumidistat can be set at a point where the individual wants each one for their desired comfort in either the Cooling or Heating Modes.
There should also be a differential settings on both setpoints so the controller can eliminate short cycling, especially of the AC in the Cooling Mode. http://www.udarrell.com/air-conditioning-contractors-seer-eer-sensible-latent-heat.html
- udarrell

separate Heating & Cooling Modes will be wired. (See Above.) - udarrell

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udarrell,
This original poster's question was very simple and asked how a humidistat could possibly be used to save air conditioning cost / lower electric consumption by cycling the a/c based on humidity as well as temperature. The incorrect reply someone offered was that a parallel connection of humidistat and thermostat was all that was needed, and the lower of the two setpoints would then properly control the air conditioner.
I raised the concern in my post that merely connecting the two controls in parallel would not work. This is very simply because a standard humidistat closes its' contacts when the humidity drops, and thus, in a parallel arrangement someone else suggested, switches in the exact opposite sense from the desired opening of its' contacts. As the humidity dropped, the humidistat would continuously call for more cooling, and the a/c/ would run on wasting huge electricity and over cooling in the process.
The subsequent "improvement" to this approach offered by someone else was to then add a relay and power source, to invert the humidistat switch closure, so that dropping humidity which closed the humidistat would then open a set of relay contacts. This would indeed correct the original design mistake originally made by whomever offered it.
I replied to offer the suggestion that a Honeywell dehumidifier control such as the one I cited at Amazon would switch correctly, and avoid the need for a relay or additional power altogether. In fact, this type of dehumidification control is designed specifically to switch a system "OFF" when the humidity level is lowered, exactly what is needed in this case.
This was NEVER a discussion of room heating thermostats, or had any reference whatsoever to what could or should be done in a more complex system with a furnace, humidifier, etc. You are certainly correct that ****IF**** the original question were to be asked as to how to configure a thermostat and humidistat for all season control of furnace, humidifier, air conditioning, etc., then a different approach would most certainly be warranted.
This was, however, not the question which was originally asked. Nor were the replies intended to answer this different question.
Smarty
question and discussion never asked about heating. It asked about how to control air conditioning using both a thermostat and a humidistat.

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Smarty wrote:

purposes. I apologize to you Smarty, I only view posts that have not been read. He misstated when he said humidistat instead of a dehumidistat, or Thermidistat. I would only use a control designed to accomplish the intended purpose. It's okay Smarty, it simply appears there were a lot of misstating a humidistat when they may have meant de-humidistat. - udarrell

The original poster probably meant dehumidistat. Some call the combo a thermidistat, etc. - udarrell
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I have some $5 Navy surplus humidistats with both NO and NC terminals. Some line-voltage thermostats do too. For constant-temp dehumidification, we might put one $80 AC in a window and one in the basement and wire the window AC to the close-on-rise contact of Grainger's $16 2E158 SPDT thermostat and the other to the close-on-fall contact and run the common to Grainger's $60 2E574 (Honeywell H46C1166) "dehumidification control."
Nick
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wrote:

It occurs to me that there would be 2 ways to connect the h-stat.
1) In parallal with the t-stat, so that the ac will run until both standards are met. I'd have to review the thermostat wiring diagram and how the AC works, or just wait until somene says, to know if this is an easy thing to do, but it seems like it might be the best.
2) In series so that if either the temp or humidity is low enough the AC turns off. I think this would be harder to do, and not what people want anyhow.
If you use method 1, I have the feeling that one stat or the other will dominate things. Once in a while the other stat will keep the AC on, but after that it will be the first one to rise, the humidity or the temperature, that will determine when the AC goes back on again.
Maybe this is usually the temp and that is why most ac run only on t-stats. ???? I'm just speculating.

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wrote:

To add to my post: To compensate for what Smarty says, you would need to put in a 24 volt relay in the h-stat that would turn the load off when the relay is on, and turn the load on when the relay is off.
By load, I mean the wires from the ac that go to the t-stat and which would have gone directly to the h-stat if it weren't backwards from you want.
By the relay on or off, I mean a simple relay that has a 24v power supply and is turned on when the the H-stat says it should be, and off when the h-stat says it should be. On is when the contacts in the H-stat are closed and off is when they are open.
If they don't sell them with this relay as an attachment, it would still be easy to get one and easy to do. I say 24 volts because there are loads and loads of 24 volt relays and that is the voltage supplied to the t-stat to begin with. You don't need single pole single throw, but get DPDT anyhow for a dollar more. You'll find other uses for the other parts. Maybe you'll want to turn an indicator light on when you turn the AC off, or whatever.
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Instead of adding a power supply and relay to invert the humidistat control, another simpler approach would be to use a wall control which "closes" on rising humidity such as this one which are designed for the application requested by the original post:
(Amazon.com product link shortened)
Smarty

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wrote:

I suggested what I did because of your post at xx:12 o'clock which said that a conventional humidistat would not work, and implied that the reply previous to that post suggested a conventional humidistat.
You said what was needed was the opposite, but didn't actually say they made such things.
So I was saying that if, as I gleaned from your post, they don't make the kind of humidistat that you say is needed, then you can add a relay to use the kind they do make. I think the relay would only cost about 4 dollars.

So this one would work in parallel with a thermostat, as udarrell suggested, no other parts needed? Great.
I guess to simplify this, For use when it is too humid, be sure to get a humidistat for a dehumidifier and not one for a humidifier.

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Yes, a "dehumidistat" (not a commonly used term or commonly found item) would work without the need for extra power or relay, and your solution does indeed allow a standard "humidistat" to be used as well. Both are workable solutions.
Smarty

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Ignore Smarty. Just buy the right kind, eg Honeywell's H46C1166...
http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?ItemId11632220
Nick
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Nick,
If you "ignore Smarty" and use the original parallel combination of t-stat and h-stat which was incorrectly recommended, you wind up with a system that switches the a/c off as the humidity rises, and thus does exactly the opposite of what is required.
My suggestion was (and is) to avoid adding an extra relay and power source by choosing the correct type of control, namely, the Honeywell I recommended or something equivalent to it. A parallel connection in this case will work exactly as expected, namely to turn on the a/c when either the temperature or the humidity level called for additional refrigeration.
I thus do not understand your recommendation to "ignore Smarty".
Could you please clarify?
Thanks,
Smarty
wrote:

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While everyone is engineering away, am I the only one wondering why anyone would be worrying about a humidistat solution for a 110-115THI outside environment, with the insidethermostat set to 70? In that environment, any properly functioning AC system is going to run enough (like all the time) to keep the temp under control that it will reduce the humidity, without any help from a humidistat. And why you;d need a set temp of 70 in that jungle environment is beyond me. When it's humid and 85 outside, I set mine to about 76 and it feels nice and cool. And that nowhere near the 115THI level.
There are high end thermostats that include humidity control, which is what you want instead of some half assed glued together widget. But I think they are targeted at systems that include a variable speed blower. So, if the system gets close to the set temp, but the humidity is still too high, it will run the blower at slow speed so it takes more moisture out before reaching the desired temp. That makes sense. Putting a humidistat in parallel with the thermostat does not. How are you going to feel if the room temp goes down to 62 to get the desired humidity?
In short, I think the OP is looking for a solution to a problem that doesn't exist.
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snipped-for-privacy@optonline.net wrote:

oversized to achieve the application goals. Also, if the lower blower speeds will achieve a tolerable CFM airflow, one can use relays to change to a lower blower speed tap even with a regular multiple speed motor. Where there are humidity problems the Entire System Design has to address that problem to work properly.

Yes, in some applications they also slow the blower down until a %RH setpoint is reached. - udarrell
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Nick,
If you "ignore Smarty" and use the original parallel combination of t-stat and h-stat which was incorrectly recommended, you wind up with a system that switches the a/c off as the humidity rises, and thus does exactly the opposite of what is required.
My suggestion was (and is) to avoid adding an extra relay and power source by choosing the correct type of control, namely, the Honeywell I recommended or something equivalent to it. A parallel connection in this case will work exactly as expected, namely to turn on the a/c when either the temperature or the humidity level called for additional refrigeration.
I thus do not understand your recommendation to "ignore Smarty".
Could you please clarify?
Thanks,
Smarty

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