Help - new AC causes low humidity?

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Googled the net but couldn't find an answer.
My parents had their 20 years old AC system replaced recently. It includes both indoor and outdoor units with two UV lamps. The units are 4-ton American Standard of 14 SEER.
They are in Houston, Texas and the humidity is relatively high, above 50% in the summer. However, it drops to 20% while running the new AC. There was no problem with the old AC. The AC dealer couldn't answer the questioin. It says UV lamps shouldn't lower the humidity, either.
Anybody has similar experience? Thanks.
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This is Turtle.
WOW, this one is a hard one to answer here.
Well , Here Goes ! When you install correctly a good piece of equipment and everything is done right. You will start getting the correct amount of Humidity withdrawn from the air as it should be and you will start getting a very low humidity level in the home.
Wait a minute here and let me rest some here.
For a System to start at 50%+RH and drop it to 20%RH while running is a very finely tuned system and you should be proud of it. Half the hvac customers out there wish they could have what you have !
We can't help it that the old system was not doing it's job and not properly dehumidifing the house. Also new properly installed hvac system do work better than old ones with problems.
TURTLE
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"We can't help it if the old one was ineffective"
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Christopher A. Young
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likemike wrote:

In almost any place in the world, that system would be perfect. Most of the time the problem is the system is too large or other issues cause it to not remove enough moisture.
I guess you are going to need to wait until someone has a better idea, but I am thinking that if the fan can be speeded up, that should cause a higher humidity.
Of course it also comes to mind the question of what the humidity really is. How is it being measured? How sure are you of that measurement? Is it being measured in the warmest part of the home?
--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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Sounds like it was sized and installed properly, where the old unit was not. Running the fan continuously may raise the humidity slightly as it will evaporate some of the condensate off the "A" coil and condensate pan. Seems funny when you read of some concerns of poor humidity removal with high SEER units! Greg
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Sounds like your system works really well. If you like higher humidity get Aprilaire humidifier installed ...
http://www.aprilaire.com/product.asp?ID81E7EEA99441FAA8855FE4844DB11F&categoryID6F78145781484A8A7C756B87F43AB3
Cheers, M P.S. I don't sell nor affiliated in any way with em.
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Yes, a good AC lowers the humidity. It feels cooler, and you can set the thermostat up a couple degrees. Enjoy it. Doing what it's supposed to.
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Surprised nobody mention "why" low humidity is a good thing for AC. It means that you can turn up the thermostat, use less electricity, and still be comfortable.
I was recently in California and it was 97F with 8% humidity. Do you think 85F at 100% humidity would be more or less comfortable?
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In an airtight house. If it's 97.2 F outdoors (the July average daily max in Houston) with w = 0.0172 and a 2400 ft^2 house has 400 Btu/h of thermal conductance and 0.5 ACH (160 cfm) of air leaks, which uses more AC power?
1) 79.0 F indoors with 20% RH
Pa = 0.2e^(17.863-9621/(460+79) = 0.2027 "Hg and w = 0.62198/(29.921/Pa-1) = 0.00424, so latent load = 1000x160x60x0.075(0.0172-w) = 9331 Btu/h, and sensible load = (97.2-79)400 = 7280 Btu/h, totaling 16.6K Btu/h, or
2) 77.6 indoors with 50% RH
Pa = 0.2e^(17.863-9621/(460+77.6) = 0.4837 "Hg and w = 0.62198/(29.921/Pa-1) = 0.01022, so latent load = 1000x160x60x0.075(0.0172-w) = 5026 Btu/h, and sensible load = (97.2-77.6)400 = 7840 Btu/h, totaling 12.9K Btu/h...
The ASHRAE 55-2004 standard says 79 F at 20% RH and 77.6 at 50% are both perfectly comfortable (Y = 0.)

ASHRAE 55-2004 says an average human would find 97 F with 8% RH "too hot" (Y = 3.08, with 99% of people dissatisfied), and 85 F with 100% RH would be "too warm" (Y = 1.84, with 69% dissatisfied.)
Nick
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On 22 Sep 2004 02:31:11 -0400, snipped-for-privacy@ece.villanova.edu wrote:

Assuming a room temp of 75F, your supply air temp leaving the evap coil would have to be 26F or less in order to give you 20% RH. I don't think so. Much more likely you need better instruments to measure your RH.
Gary
http://www.techmethod.com
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On Mon, 27 Sep 2004 12:20:28 GMT, snipped-for-privacy@gatecom.com (Gary R. Lloyd) wrote:

Let me clarify this, so there is no misunderstanding:
It didn't happen. No way in hell. The humidity indicator is WRONG.
Gary
http://www.techmethod.com
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Agreed, but why 26 F? The dew point of 75 F (535 R) air at 20% RH is about 535/(1-535ln(0.2)/9621) = 491 R or 31 F.

Maybe the coil is under freezestat control.
Nick
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On 27 Sep 2004 09:39:23 -0400, snipped-for-privacy@ece.villanova.edu wrote:

Yep, I stand corrected. I misread my psych chart. Although the SA temp would need to be somewhat below dewpoint to balance against latent load and/or bypass, so it probably isn't that far off.

Is it technically possible? Yep. With an A/C unit? Very highly unlikely. The airflow would need to be very low, in which case it wouldn't handle any appreciable heat load.
Realistically, it just didn't happen.
Gary
http://www.techmethod.com
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[original post is likely clipped to save bandwidth] On Mon, 27 Sep 2004 14:25:53 GMT, snipped-for-privacy@gatecom.com (Gary R. Lloyd) wrote:

Indirect RH measurement may well be the culprit. As you (Gary) said, it didn't happen. What was not mentioned is what was used to measure the RH!
Calibration of indirect measurement hygrometers is not easy to maintain and some are just junk near extremes or with age. Was a good old wet bulb used? They are not too vulnerable to aging, though salt build up on the wick can induce some error.
Even lab quality dew point hygrometers need maintenance to be accurate. One of the classic laboratory methods is optical detection of fog on a chilled mirror. Fail to keep the mirror very clean mucks them up.
I suspect a piece of bad equipment was used to measure it.
gerry
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Water freezes at 32. An interesting coincidence.

After some sort of control failure...

Maybe an iced up coil on a mild day...
Nick
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Gary R. Lloyd posted for all of us....

Hey Gary, welcome back!! Missed you.
--
Tekkie

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Oops. Make that 31F.

Thanks, Tekkie.
Every once in a while I drop by and skim through a few thousand new posts as time permits. And of course, the one I choose to respond to I misread my psych chart and get the dewpoint wrong... LOL
Gary
http://www.techmethod.com
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85F at near 100% humidity is frequent here in sunny west central Florida , that's fairly comfortable outdoors, not too hot/humid until you have to reel in a fighter and then while sippin on your tea then you say it sure is humid today and add some more ice or celebrating the catch by pulling out a slimy cooler beer from underneath it.
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So you wipe the fish guts off the beer and drink it and feel fine... :-)
A fan helps at 85 F and 100% RH. It makes 97 F and 8% worse, since skin is about 92 F, but we can make it comfortable by evaporating some water. Air's specific heat is 0.24 Btu/F-lb, so cooling a pound of 97 F air to 80 takes 0.24(97-80) = 4.1 Btu, ie 0.0041 pounds of water. With no water, Pa = 0.08e^(17.863-9621/(460+97) = 0.1443 "Hg, so w = 0.62198/(29.921/Pa-1) = 0.00301. With the water, w = 0.00711, well within the ASHRAE comfort zone.
Nick
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So everyone here answers WOW it is so low to go to 20%. I say what BS , in Texas to start at 50% and go to 20. It is more likely starting at 80 going to 50% The point is I will bet anything you do not have a good humidistat. At 20 % you would be making Static everywhere, but im sure that is not happenibg, right. Digital humidistats are usualy more accurate. Analog need to be calibrated every year acording to Taylor Instruments, a leading maker of thermometers and humidistats you see at every box store and Ace. A few months ago at Ace I went to buy a Taylor humidistat. Every one of the square temp-humidistats were 30 % - THIRTY % Off. The crap that is sold to idiots like you that believe what you buy is accurate is amazing. Get a few good digital, and a calibrateable analog, calibrate it, and come back when you really know whats up. First put your humidistat outside in the shade and listen to a weather update to find out present humidity. I will bet you are 30% off.
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