There are a couple things I would look into. First, that air handler will come factory set for an airflow suitable for 3.5 tons, so it should be adjusted for both the high and low stages of your 3 ton unit (4TWX6036 is a 3 ton). Second, is Comfort-R enabled? It's a slow ramp-up profile that will help remove more humidity during the cycle.

If the requirement for your home was 2.8 tons, the 3 ton isn't oversized.

I have a 4ton American Standard 16 (same as the Trane XL16i) in an old poorly insulated 1600sqft home with lots of windows in Tampa,FL and it runs in 1st stage most of the time keeping a steady 48% humidity level.

I would not consider 65% acceptable.

Comfort-R is enabled on mine and when they installed they made sure to lower the CFM to the proper range for my ducting from the factory settings.

My thermostat is an Omnistat 2 from HAI http://www.homeauto.com/Products/Omnistat/Omnistat2Products.asp which seems to be pretty intelligent about running first stage as much as possible, it can control humidity as well, but I just have comfort-r doing it for now.

Here is a link that might be useful: HAI Omnistat2

If I set my humidity setting lower than 60%, the blower will run at slow speed too long for me. When it runs at slow speed, the compressor is running too long, and you can kiss your SEER rating bye-bye.
60% is very comfortable to me, in a 70% climate.
I've read some RELIABLE sources on the web say to not set the humidity less than 60%.

Mold growth is well supported at 65% RH. Some molds will grow at 60%. The indoor RH should be kept below 60%. This information is readily available on-line.

I am amazed that a Florida HVAC contractor would claim the 65% RH is normal. That is an invitation for mold problems.

From what I understand ASHRAE Standard 55-1992 Thermal Environmental Conditions for Human Occupancy state 30-60% with below 50% being ideal to control mold and dust mites.

With the AM/Trane 16 I believe the blower runs at 80% speed on with the compressor on 1st stage, which helps in keeping SEER up while still lowering air speed and maintain low humidity levels.

JHarrel is correct. Check the separate source linked below.

One thing I believe it does not say: Cheap RH meters can be very inaccurate. You should verify the calibration and record the error, so that you know how to interpret the reading.

Instructions are available on the web. I just don't remember where I saw them.

Here is a link that might be useful: EPA On Mold

Comfort-R and airflow are configured by setting the "dipswitches" on the circuit board. The airflow should be configured for a 3 ton unit, and the unit, when running in low stage, will send a signal to the air handler to run at a reduced speed. At least that's how I understand its operation, and if any pro knows it to be different please correct. The newer communicating XL16i may have a slightly different operation, but that would not apply in this case.

Have you checked to see if the air handler is draining the condensate off adequately? From your description, it sounds like you may have a drip pan full of water sitting in the air handler.

"Have you checked to see if the air handler is draining the condensate off adequately?"

With the compressor running and the air handler on high speed (by lowering the thermostat to 74 from 78), I got about a gallon or more an hour of condensate water out of the drainage pipe. I am not sure how to check inside the air handler to see if it is not draining properly. If there is more water sitting inside the unit then I would suppose that this is getting recycled back into the house as additional internal humidity when the unit is on slow speed and no compressor running.

"Comfort-R and airflow are configured by setting the "dipswitches" on the circuit board".
I have the manuals for all the units in this system and saw pages of exhaustive dip switch settings. I will need to look over these and see if I can determine if they are set to the specs you are suggesting.

And as a test last night, I cooled the house down to 74 overnight and then turned the A/C off in the morning. It took about 8 hours until mid afternoon to get up to 78 with humidity remaining constant at about 60%. Outside it was at around 90 in the afternoon as usual -- which tells me I have do have a well insulated and sealed house without external air (and moisture) intrusion. So I think it is more of a techical A/C equipement problem than an environmental problem.

If the air handler drip pan doens't drain completely for some reason that you can't overcome, you may have to make sure the fan only runs when the compressor is running.

karstranger

first off, the XL16i has been a problematic mdl and was a poor choice for your climate and best dehumidification properties. your particular mdl/system does not have "dehumidify on demand" feature which would be a plus for your climate.

now, a couple of suggestions.

1.if home is built on a crawlspze, it should have a vapor barrier.

2.do not run fan "on" in AC mode.

3. Trane installers are definitely lax and some not properly trained on correct settings for comfort-R and dip switches. This needs to be checked by someone who knows what they are doing. I do not recommend that you do this unless you are familiar with these switches and settings.

4.the suggestion that your blower is set on high speed is wrong. I would change the setting to low(350 CFMs/ton), run several days and monitor RH as well as any freezeup at air handler.

IMO

Thanks Tigerdunes for the info. I am going to meet with my HVAC installer on Thursday to talk about how best to decrease the airflow while the compressor is on to allow sufficient water to be condensed from interior air -- with goal to keep my internal humidity below 60% (or more).

Re your comments 1-4:

1. No crawlspace, house is 9 ft elevated on stilts. I do have a vapor barrier on bottom of R-38 batting in floor.

2. I never run fan in "ON" mode, however, it runs a whole lot on slow speed without compressor running in "Auto" and/or "Cool" mode -- which I think may increase the humidity like "ON" mode as noted by contributors in this forum thread. However I must solve the more major problem of not removing sufficient humidity when the compressor IS running on, first. Then address this if high humidity is still an issue.

3. I agree that attemping to re-program dipswitch settings, even with the manual, is foolhardy for a person not trained in HVAC and Trane eqpt. in particular.

4. Will attempt to get air handler air speed to be more correctly matched to dehumidification needs. Especially in light of your and ryanhughs comments: "First, that air handler will come factory set for an airflow suitable for 3.5 tons, so it should be adjusted for both the high and low stages of your 3 ton unit (4TWX6036 is a 3 ton). Second, is Comfort-R enabled?"

Thanks again to all for your continuing assistance. I will post how this does (hopefully!) get resolved.
KR

Hi folks,

Thanks for your advice. With the help of members of this forum, the A/C tech and I have fixed this problem. We (he) turned down the CFM of the air handler to match a 3 ton unit (as noted above it came from the factory set for 3.5 tons and was installed this way). That helped remove more humidity, but still the humidity went back up when the air handler was running on low speed. Then the A/C guy figured out that the signal from the air handler on slow speed to the compressor was not getting to the compressor to turn it on. He replaced a board and the rectifier that powered it, and YEA!, the compressor now runs most of the time when the air handler is on slow. Indoor humidity levels are typically from 47 to 55% at all times. Thanks again so much for the help and direction so I could pointedly tell the A/C tech basically what to do.
Now, I have a fairly simple task of cleaning up a minor amount of mold and back to fishing !!

karstanger

glad your humidity situation has improved and thanks for providing the update.

TD

I'm glad you got it fixed. Being persistent can pay off. For future readers, I don't care how fancy the equipment, once the compressor goes off, after about one minute at best, the blower should be OFF. If not, that is dumping humidity back into the living space.