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Heating & Air Conditioning

Honeywell/Trane system dehumidification help, please!

14 years ago

I recently had a Trane XL15i heat pump (4TWX5024A1000A) installed in a new building along with a 4TEE3F31B1 air handler. After reading some earlier posts here, I went with the Honeywell VisionPro IAQ and EIM thermostat. Controlling humidity is very important for me; this is my newly built guitar making workshop, and I need to keep RH in the 40-50% range. 45% would be ideal. I'm located in Northern California.

The installer did a clean looking job, but said he hadn't installed the IAQ before. Before he left he showed me that the system was working on both heating and cooling; it still is. But today for the first time I tried setting the IAQ on dehumidify, figuring it would alternate between heating and low-fan-speed cooling. I went to the "Dehumidify" screen and set RH for 45%. I checked (and left alone) the default temperature range setting, which allows temperatures to drop 3Âº below the set temp. while dehumidifying. I put the fan and heat/cool settings on "Auto" (previously I had only run it on "Heat", and things worked fine). I figured first it would heat up to 64Âº and then the system would switch over to low fan cooling to get the humidity to drop.

Never happened. It just went on heat, and maintained my temperature as if I had never set the dehumidification control. It read 59% RH when I started up the system, which came down to 55% just from the warming of the air. The system never went on cool or dehumidify.

I looked at the dip switches on the air handler. #5 is on and #6 is off, which should give a 90 second ramp up (the advice I read here was to let the IAQ control low fan speed rather than let the air handler do the long ramp up, so that seems OK). Then I opened the EIM. I noticed that the installer's manual for the EIM on page 4 shows a "typical hookup of variable speed blower for dehumidification at low speed." In the diagram, there is a jumper from R, RC, and RH going to D1 on the contact block below, and what looks like a connection from M2 on that block to the "low speed fan." There are no wires on my EIM going to to D1 or to M2, or anywhere else on that lower block. Could this be why the dehumidify setting isn't working? If not, any suggestions?

I'm sure I can get the installer to come back out, but as I said, he was not experienced with the Honeywell system. So I thought I should first ask the pros here.

Help would be greatly appreciated. Thanks!

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Comments (2)

turningfool
Original Author
14 years ago
Just to clarify, 64Âº is my heat setting. I'm moving around a lot and that is comfortable for me.
creek_side
14 years ago
A lot of installers do not understand how to set these systems up. The IAQ can dehumidify by slowing the blower on a variable speed air handler equipped with a connection for that purpose.

The IAQ's EM must be wired to the AH correctly for this to happen. The IAQ must also be programmed correctly, and the AH may also have to be set up correctly (don't know about a Trane).

And here is the big one, it only work when the system is in the air conditioning mode and is actually actively cooling. To dehumidify while heating, you would need a dedicated dehumidifier. The IAQ will also control that, if it is set up and wired properly.

The dehumidify by slowing the blower bit works by shifting the system's latent/sensible cooling characteristics. In the dehumidify mode, it biases it toward latent at some expense to the sensible.

If you don't understand the terms latent and sensible as used in the HVAC industry, you will probably need to do some reading up on the subject to make sense of this post.
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