Humidifiers and Dehumidifiers - Advice required for humidifier option

I've done a lot of searching, but still have some questions, especially after talking to a local Hvac company.

Background: 20% humidity during winter. Dallas location, 3300sqft, 1st floor/2nd floor mostly open to eachother. 2 furnaces, 1 hardly ever runs, both in attic.
Carrier 58STA Furnace/AC system, HORIZONTAL installation.
Outlet nearby, water line close enough.

Issues: The metal box just past the blower unit is only about 2 1/2ft square or so, with round/soft ducting going out. 'Looks' like enough space on one side of AC box to mount a unit.

Possible Issue: HVAC company said, 'probably' not a carrier coil in box, but a cheap other brand that 'probably' takes up the entire metal 'box'. Said would need to replace with a plenum?/bigger box to be able to mount humidifier.
Suggested $2100 to change out coil with more efficient one, then $1400 to install an aprilaire steam(?) unit, plus a charge to run water line from source..

My question, Can I mount a humidifier to this small box with the coil inside? Will it rust out the coils? What unit can be installed in this horizontal orientation? If I can't use the current 'box' setup, can I build a new vertical style? is this a DIY thing? I'm pretty handy :)

I don't think the attic ever gets to freezing. I know other homes in the area have humidifiers in the attic, everybody's house have attic HVAC units here.

I saw a post that the spray/steam systems can rust out the metal really fast, but I've also seen posts from a moderator here recommending a spray system. Is the rust problem only if you put it on the return side?

I was thinking about the HE360A available at HomeDepot, but the only drain I have is a pipe out the eave of the roof for when the AC breaks or something. I don't want constant water draining out of it, so I really need a system that doesn't 'normally' spit out excess water.

Thank for your time and advise! Mike

Over the years We have sold them all here. So dont even think steam are spray kind of humidifier there at all. Id say look into a Aprilaire 700 is the one to go with . There on the plenum. Rust on the coil???? Think about it . That coil is wet all summer long from the AC.

The Aprilaire 700 indicates it can't be mounted in a horizontal system. Something about airflow from left-to-right doesn't work with the design.

I didn't understand your comment about steam/spray?

Yeah, I figured the same about coils being wet, just didn't know what the other post was referring to about rusting problems.

The Aprilaire 700 indicates it can't be mounted in a horizontal system. Something about airflow from left-to-right doesn't work with the design.
Correct, the air flow interfers with the how the air is drawn into the system over the pad. You could do it, but may not perform as you'd like it.



I didn't understand your comment about steam/spray?

Spray is water dropplets can lead to larger water dropplets, and will cause water damge to duct work.. Steam is steam, and very small chance of any damage. I would look into steam to advoid any water issue as you stated.

I would like to see a photo of your lay out.. Do you have a furance in the basement?