Help - new AC causes low humidity?

[likemike]

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.

[TURTLE]

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 You will start getting the correct amount of Humidity withdrawn from the air as it should be and you will start getting a very low humidity level in the home.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

[Joseph Meehan]

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?

[Greg O]

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

[Mike Jak]

Sounds like your system works really well. If you like higher humidity get Aprilaire humidifier installed ...

Cheers, M P.S. I don't sell nor affiliated in any way with em.

[Stormin Mormon]

"We can't help it if the old one was ineffective"

[Stormin Mormon]

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.

[David Efflandt]

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?

[Jud McCranie]

Why is that a problem? 50% humidity inside is too high. My house has around 40% humidity inside. My car is about 20% with the A/C on, and it feels great.

[nicksanspam]

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

[bumtracks]

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.

[nicksanspam]

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

[m Ransley]

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.

[likemike]

Thank you all for the replies. Now I understrand that the new AC works well above it is supposed to do. I will ask my parents to run the fan more often. And I am glad to answer some of the questions ...

My mom is a patient who is sensitive to low humidity. She doesn't feel comfortable with 20 RH as others usually do.

The humidity is measured by a digital meter which is in one of the bedroom. It is not the warmest part of the home, not the coolest part either. My mom uses it all year long and it is fairly accurate. We can see it raise quickly when we turn on her little humidifier in the room.

[Jud McCranie]

You could use a humidifier. Or probably anything that makes the A/C run less should make the humidity higher, like setting the thermostat higher and using ceiling fans to keep it comfortable.

[m Ransley]

You wont know if it is accurate unless you test it to a known source, try outdoors with a local present weather forcast.

[Joseph Meehan]

As someone suggested do measure the outdoor humidity as a check of the calibration of your meter. I have found that meters of all kinds measuring humidity are not very accurate, although that one should seem to be very far off or it is really low. Sorry I don't have any really good, well proven, ideas for a solution. I hope someone has some better ideas. I just have a hard time imagining how it is doing it. The use of a higher speed setting and running the fan after the compressor shuts off are at lest ideas that may help.

[udarrell]

NO 14 SEER A/C is going to get the humidity that low even if it runs all the time. As the humidity level goes down, the ratio of condensation goes way down, too! His humidity gauge is way off! High SEER systems are less efficient at bringing down humidity due to the higher E-Coil temperatures.

[Tekkie]

David Efflandt posted for all of us....

8% humidity???? Point me to the NWS page that would prove this!

[David Efflandt]

I thought maybe I heard the radio wrong, but found it on NOAA ($2 for a report). Fortunately I was on a jet from SNA to ORD (Chicago) by then:

Latitude:, 33° 41' Longitude:, -117° 52' Station Elev. (ft):, 49 (not far from the sea) Time Conversion:, +8 Date:, 09/05/2004 SID:, SNA Station (Type; Name, State):, ASOS; "SANTA ANA, CA"

1553, , 36.1, -2.8, 16.2, 8 ,29.75,6,012 , .00

ie, 3:53PM 36.1C (97F), -2.8C dew pt, 16.2C wet bulb, 8% rel humidity

Note that this was after it was 31.1C, 36% 9/4, and 23.9C, 55% 9/3.