RH% with XL19i in South Florida

[bgaston]

I got a 5-ton XL19i installed a few months ago. I am very happy with the system and it seems to work very well. After purchasing a hygrometer, I noticed that my average RH is typically around 52-55% inside the house. I am in South Florida when the outdoor temperature has been around 92 degrees and outside RH is generally 75-90%. My system generally runs non stop from about 1pm to 8pm. Still the inside RH doesn't change.

My blower speed is set for 350CFM and I have Comfort-R enabled. I have the twe049e13 variable speed air handler match. Is this inside RH number typical?

Thanks for any feedback.

[kevinmac]

I have the same unit in S Fla, and yes it will run non stop on those very hot days on the low stage compressor. Mine does not start running continiously till about 2 pm and then starts cycling at around 8pm. This only seems to happen on those 92+ days. With only a small amount of time going to the high stage. It basically all depends on the homes insallation, and R factors.
Youir 5 ton unit can be used with the TWE049 or TWE065.
Your configuration provides 58,500BTUH, the TwE065 provides 59,000 BTUH. Not much difference. In my case I am running a larger coil in the air handler then the outside unit. I do not have the 5 ton unit.
Your configuration does provide more sensible BTUH then the larger coil.
The bottom line I have found is it's cheaper to run, I can run at a higher set temp because the humidity is very low!

[teddy bear]

What is the t-stat temp setting? With 10-12 hours of run time, you have good humidity control. Keep us posted on rainy cool weather with less run time.

[bgaston]

My thermostat temperature setting is constant at 78. It actually feels like 75. However, I was concerned that 55% RH is not low enough. Any thoughts?

[travisfl]

55% may be a ittle higher than typical for central / south Florida.

Most properly sized installations I have seen with VS air handlers here will easily achieve < 50% at 78 degrees this time of year, especially if it hasn't been raining as in this past few weeks.

[bgaston]

Seems odd then. If my systems runs constantly from 2-8pm and I only get 55%RH, what could be the problem?

[travisfl]

Any ususual moisture sources?

Bathrooms without vent fans
Interior water features
Aquariums

etc.

Is the house especially air tight?

How many return registers? One in each living space?

What are the supply and return register air temperatures after the system has been running for a while?

[Edited by travisfl on 09-19-2005 at 01:35 PM]

[bgaston]

No indoor water features although I have a big pool and spa outside.

The system seems to be sized correctly as it is not short-cycling and actually runs continuously during the hot days.

The thermostat is always set at 78. Feels like 75 indoors. Temperature at the supply on the small compressor is around 65. It is around 58 when the thermostat senses it needs a lot more cooling using the 2nd compressor.

Comfort-R running. 350CFM selected. The only time I see a drop in RH is on those rare days when the outdoor RH is very low (almost never).

Had three different companies look at the system. All of them says the unit is operating perfectly.

[travisfl]

How was the system sized originally?

[wyounger]

Sounds oversized to me. A two speed system in South Florida should probably run nonstop for much longer than 1-8pm in the middle of the summer- especially one like an XL19 with such a small first stage. I would want to see the second stage running for at least a few hours on each summer afternoon. When you get the sizing right with a two stage AC, you will very rarely ever catch the unit cycling off at all (when you're awake, at least). The only question is if it's on low or high.

There could also be a small return leak, sucking in humid air. And how confident are you in your hygrometer? It takes fairly expensive hygrometers to reliably get accurate readings; relative humidity is actually a fairly challenging measurement to make.

Finally, you say you're set for 350 cfm/ton. That's probably true on second stage, but I'd bet you're running significantly more than that on the first stage, given the difference in the discharge temps. If the RH is still a little higher than you'd like, you may want to investigate using a dehumidistat or thermidistat to kick it into lower-still flow rates when there is demand for dehumidification.

[Edited by wyounger on 09-19-2005 at 02:39 PM]

[bgaston]

Typcially, on the hottest days, it is not uncommon for the unit to run from 12noon to 10pm. That is 10 hour of non stop running. Since this is a Comfort-R unit, setting 350CFM applies to both condensors.

[travisfl]

Originally posted by travisfl
How was the system sized originally?

Where did your contractor come up with the 5 ton requirement?

[bgaston]

Big house with windows lots of windows and french doors. I live in a glass bowl practically.

[travisfl]

Originally posted by bgaston
Big house with windows lots of windows and french doors. I live in a glass bowl practically. Did the contractor do a load calculation, or did s/he guess?

http://www.proctoreng.com/articles/bigger.html

[bgaston]

sized properly.......the system can run 24 hours a day and the RH percentage will stay the same. I can only assume that is normal as everything checks out.

[travisfl]

"sized correctly" - how do you know that?

[teddy bear]

55%RH is ok. 78^F, 55%RH is a 60^F dew point. If you want lower relative humidity, slow the air flow over the cooling coil. Your coil temperature is probably +50^F. Slowing the air flow enough to lower the coil 5^F for a coil temperature of 45^F will lower your %RH to 50%RH during the high load days. The interesting part is the wet cool days with low/no cooling load and resulting %RH. Keep us posted!

[dash]

Originally posted by bgaston
sized properly.......the system can run 24 hours a day and the RH percentage will stay the same. I can only assume that is normal as everything checks out.


You might want to have them make sure it'sm setup properly,

check here. http://www.hvac-talk.com/vbb/showthread.php?threadid=84863

[bgaston]

TB....

Thanks for the info. I will keep you posted on the wet days. I actually have to keep the unit at 79 degrees sometimes because it is very cool. I won't worry about it.