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.

dave a little history first. Rudd/Rheem produced the first successful modulating furnace. they were quite successful. one problem. it only had a rating of 94.7% efficiency. so when the Fed Govt Standard for the tax credit was 95%, they got clobbered in the marketplace for those wanting the tax credit. I am not certain if their updated model was ever released. a modulating furnace correctly sized would be a good choice for your area's winter climate. I have to say this and you won't like it but you need to hear it. Why cripple a nice furnace with the wrong thermostat? it's simply a bad decision and takes away from the features and full functionality of the furnace. If this was my decision, I would hold off until getting a price on the York/Luxaire modulating furnace that has 97% efficiency. IMO

huskerb what size is your home? what size AC and furnace are you replacing? I think an 80% eff furnace for your area is a poor choice whether paired with AC or HP. New HVAC does not overcome poor airflow-that indicates poor ductwork design/sizing. You should request a thorough evaluation in that area. You know your home. Point out the specific problem areas and your concerns.Put it in writing! And I strongly recommend that you seek other quotes/brands like Carrier/Bryant. That upgrade price from a 4 ton XR13 AC to a 4 ton XL15i HP is absurd. IMO

Do you have a crawlspace? Why the ductwork install in attic? The system you were quoted is a nice sgl stage system. If you have deep pockets, want the latest and greatest with all the bells and whistles, then get an upgrade price on full communicating system. I believe Trane offers 3 different models that include two var speed condensers, their newest introduction. You would go from the TAM7 or TEM6 to the communicating TAM8 air handler. Do I think it's worth it, I do not. If an upgrade to system quoted, I would consider the two stage XR17 condenser. IMO