500

Thank you for the non-zoomed in photos. I would get a Aprilaire Model 500, and install it on the right side of the return next to the condensate pump (keep drain fitting above pump, so install it up as far as you can). Then run the 6" bypass duct up the side and cut it in at the top right of your supply plenum. Put the control on the front left of the return plenum. Pipe to hot water. I think I see a water heater with PEX in the background, so you can use a SharkBite Tee Stop Valve to easily add the 1/4" supply line. Or tee it off and do a straight stop (depends on plumbing skills). A fan powered unit just isn't going to fit in this location. The combination of piping to hot water and using the digital control with blower activation should be sufficient with the Model 500 in your apartment. I strongly recommend installing the outdoor temp sensor if you can manage it.

Digital so you have blower activation and outdoor temperature sensor. The manual controls are sort of like the stone ages to me.

That is larger house, so you'll need an AprilAire 600 or 700 series humidifier. The type you get also depends on how much space you have on your furnace supply and return ducts. A fan-powered on the supply duct has a bit more output than a bypass humidifier. I would not get the Model 500, that is too small. The 500 is meant for apartments, condos, and small houses. The 600 is the larger size of basically the same thing. The 700 adds a fan for additional airflow.

FYI the Lennox humidifiers are rebranded AprilAire humidifiers. The HCWB3-12A is like the model 500 and the HCWB3-17A is like the model 600. The 17 is the gallon per day output of humidity evaporated, not how much water it uses. They will both use 72 gallons of water per day if they ran for 24 hours nonstop. If you figure the humidifier runs half the time, the water usage would be around 1000 gallons per month. Try putting plastic on all the upstairs windows to reduce air leakage and humidity loss.

You might have to get creative with this one. Aprilaire 500 or 600 mounted on the return (you'll have to move your meat slicer), bypass duct up to the top supply duct, and to make this work better with variable speed, you might have add a 6" inline duct booster fan in the middle of the bypass duct. To control the booster fan you might need to add a 24VAC relay in parallel with the humidifier water solenoid. Pipe to hot water.

With the long runtimes I don't think a duct booster is needed. I actually reduced my 6" bypass to 4" for a small section to reduce short circuiting. Even with the restriction the humidity seems to keep up with outdoor temperature swings. The booster fan in series is an interesting approach to increase flow when there is a call for humidity. I've had great success with the temperature adaptive Honeywell humidistat or ecobee to control humidity based on outdoor temperature.

Yeah it's a 4 ton and Fujitsu with 3/4hp ECM, so it has a decent air handler. It's moving mostly around 800 CFM and up to 1600 ish at times. The humidifier is definitely meeting the humidistat at times and not running. Windows condensation seems to be managed nicely and comfort is about as good as I can do with the existing windows.

I have a very similar Fujitsu, I have an Aprilaire mounted on the return with the Honeywell humidistat mounted upstream. The humidistat is outdoor temperature responsive and wired in series with a 24v relay wired to a 12v output for heat call "on" from the board (this is needed for some heat pumps because they lack a 24v humidifier output). You can also use an ecobee as a temperature responsive humidistat using the window setting function the ecobee uses Internet weather instead of an outdoor temperature probe.

Used to use those, they sucked. If you dont fill with distilled water they get white powdery shit everywhere, and they have to be refilled often. Whole house is much easier. I change the Aprilaire filter once a year. Its only used in the winter. Never would I want to walk around filling 4-5 humidifiers when theres one that never needs to filled. Plus the Aprilaire bypass humidifier was only like $275. **EDIT** nevermind, OP is talking about something else apparently, he has no central air

I've got it and love it. I keep the humidity down low though and if I need a little extra for the bedrooms at night I can run something on the nightstand. Helps reduce those dry air bloody noses and teeth sensitivity. I forget which version of the Aprilaire automatic that we have, but I keep the dial set at four and I've never had an issue with mold. I just want to take the edge off the dryness, not turn the house into a sauna.

If you have central heating and air I HIGHLY recommend looking into a central humidifier especially if you are in a very dry area. Set and forget with auto water feeding and humidity setting with minimal (like 1x a year) maintenance. They are usually not super expensive to install either. I like the Aprilaire units.

If you own your own home, get the AprilAire that attaches to your furnace. Idk how much they cost but it would be cheapest in the long run.

If you want whole-house coverage, Aprilaire units connect to your HVAC and are the gold standard for set-it-and-forget-it. No daily maintenance, just annual checkups.