A master bedroom/closet/bath addition (slab) has its own AC unit, and we are taking at least five gallons of water per 24 hours with a portable dehumidifier. No exaggeration.

We have reinsulated walls and ceiling, and added ductwork to send some of the air conditioning to other parts of the house (to make the AC less efficient and run more). Covered the vents that draw from humid outdoor air (shhh). No help. Third or fourth HVAC expert is here and wants to move the two air returns from the floor up to the top of the wall. The registers are all on the ceiling.

Will this really make that much difference? Is there a way to keep both air returns, and switch in winter? I think the room would get cold if heat goes in and out near the ceiling.

Help! We've already spent a bundle on this problem. Thanks ~

A master bedroom/closet/bath addition (slab) has its own AC unit, and we are taking at least five gallons of water per 24 hours with a portable dehumidifier. No exaggeration.

We have reinsulated walls and ceiling, and added ductwork to send some of the air conditioning to other parts of the house (to make the AC less efficient and run more). Covered the vents that draw from humid outdoor air (shhh). No help. Third or fourth HVAC expert is here and wants to move the two air returns from the floor up to the top of the wall. The registers are all on the ceiling.

Will this really make that much difference? Is there a way to keep both air returns, and switch in winter? I think the room would get cold if heat goes in and out near the ceiling.

Help! We've already spent a bundle on this problem. Thanks ~

Oh, and we also lowered the fan speed. It's a 2.5 ton unit (I think I said that right).

Comfort wise, moving the returns will make a big difference.

However, I would add, not remove. What you need is what you suggested. Returns that are switchable or have the ability to be turned off and on, so you can have cold air returns in winter and hot air returns in summer.

A return is where the furnace/AC takes air from. A register is where it is exhausted.

Typically you would want warm air to be placed under a window.

Did you insulate the slab?
Are any ducts in the slab?

There are no duct in the slab, and I don't know if it is insulated (the addition was already here), but the hardwood floor has a thick plywood insert where rug covers it and it's dry.

We can't figure out where all that water is coming from.
BTW, the addition is about 30x30 ft.

The 2.5 ton is just for the bedroom/bath/closet? How long of a cycle does it run? Is it off and on frequently? Is it on continuously?

Yep. I know, it's too big.

It runs frequently enough, and we tried to make it less efficient by running two additional ducts/registers to the other parts of the house. It does not run continuously. There is more cooling potential, we can turn the themostat down further (it's normally set at 72-74) but then all we get is a freezing, damp cave! And risk mold from the condensation.

Was going to mention the dehumidifier but then reread that you already have that going. How long has the unit been in? Any chance you are looking to replace it in the near future with something smaller?. :)
Is the humidifier portable or installed?

We just spent $6500 on a new one for the main house, and not really clear what good that would do anyway. This unit is only 10 years old.
It's a portable unit, a big one and it works pretty good but I don't want to be empying it twice a day forever and I don't trust the hoses.

For fun, I'll leave you with this article about humidity control in a hospital setting: http://www.trane.com/commercial/uploads/newsroom/June2006ASHRAE-Murphy.pdf

Just wondering of you can use Re-heat or a dessicant wheel to control humidity?

The control strategy will be way more complex.

We just spent $6500 on a new one for the main house, and not really clear what good that would do anyway. This unit is only 10 years old.
It's a portable unit, a big one and it works pretty good but I don't want to be empying it twice a day forever and I don't trust the hoses.

This last post of mine wasn't very clear, let me try again. I'm told 2.5ton unit is the smallest, so the best we could do was make it more inefficient by lowering the fan speed and sending some of the cooled air off to other parts of the house. I don't think, at least it has never been suggested, that there is anything wrong with the unit itself so that's why I'm not sure what good it would do to replace it with another. It's only 10 years old, it's made by Trane.

The dehumidifier we have been using is a portable unit, has a 17 pint tank and 50 pint/day capacity. (FYI, Frigidaire makes the best one, in my experience). It's noisier than a fan so we turn it off at night, usually, then on again first thing. The gauge shows humidity in the room reaches 80% if we don't keep it on as much as possible. Where is this water coming from? Geez.

Could you have an underground spring under the slab?

HVAC people say AC that works right will remove 5 gallons/hour from the air, so they think putting adding an alternate return up high for summer will work... we'll see.