OMG! That's an awesome resource. Thanks for the link! There is an older area of homes that I've been working in a lot lately. Lots of 40, 50, 60 year old homes, many were custom and designer back in the day. What an odd collection of vents and returns! Creative HVAC is an understatement. :D Unfortunately, they've been painted and painted and painted and look awful at this point. I know most of them have lead paint - which homeowners don't think about when they take a piece of sand paper to them for a repaint. Sanding and salvaging is often the chosen option 'cuz it can be hard to find replacements. Having an on-line resource to order new is like the highlight of my week!

It sounds like your home had floor furnaces at one point. The large return air grilles sound like old openings for floor furnaces, used a long time ago. Where to start? First, your system should have filters somewhere in the return duct or inside the air handler or furnace. I would look really good to make sure you haven't missed something. I've seen filters placed in really weird places. Sometimes filters aren't found for years. By then they're pretty stopped up. As far as the return grilles being large, the larger the better! One thing that kills your system and your energy efficiency is duct work that's just too small. Modern systems are made to run with very little static pressure in the ducts, around 1/2 inch. That's not much, so I wouldn't worry about the large ducts. The filters are another story. You really need a filter in your return duct somewhere. If there's not one, i would install filtration. Usually the cheapy filters work best as they cause the smallest pressure drop in the duct. You mentioned the refrigerant leaking out. If you have a refrigerant leak it should be repaired. That stuff ain't cheap. Besides it's illegal for a technician to keep adding without repairing the leak. If someone is having to add refrigerant, you definitely have a leak, no question about it. If the service provider you called did not look at your duct work, did not tell you your system had a leak, and did not offer sound recommendations, please ditch them and find someone qualified. As far as your system coming on and off every ten minutes or so, that's called "short cycling" and isn't good for your system, your energy costs, or your comfort. It sounds almost like your system is either too large for your home or you have massive air infiltration. Either way, this needs to be addressed. Really, I would recommend you find a good service provider in your area that can be trusted to offer technically correct advice. I usually find it best to ask friends, co-workers, or other family who they use and trust. In the meantime, you may want to check out a couple of articles I've written dealing with heat loads and your comfort. Here's one HERE if you would like to read it. Let me know if there are specific things I can help with. I'll be happy to help if possible.

The return is BEHIND the wall and RUNS to the UPSTAIRS. There is no vent cover for it in the kitchen (side note, an air return should NEVER be in a kitchen), but it's behind the wall that the cabinets are going on. I'm definitely not comfortable attaching to one stud, even more so since it doesn't land in the middle of the 24 x 36 cabinet

Venting Options Many people feel they cannot use a portable air conditioner because they do not have a window in the room they are trying to cool. This is not a problem! Though most portable air conditioners are designed to vent out of a window (most come with a handy, easy to install window kit), you can get creative when thinking of venting options. You can vent through the floor, ceiling, even a chimney. You can even vent into another room. The primary goal of venting is to get the hot air coming off the compressor out of the room you are trying to cool. As long as you can accomplish that, then you and your portable air conditioner will have a very happy life together. https://www.centralacwarehouse.com/ Venting From a Casement Window Many people think that they cannot use a portable air conditioner because they have a casement, or crank out, window. Don’t fret, this is easy to overcome. You simply need to get a piece of plexiglass cut to the size of your window, and attach it like a screen so that the window will still open (usually can be done with strong Velcro). In that piece of plexiglass, have a 5 inch hole cut near the bottom. Then you simply put the vent hose in the hole, and you are good to go. Plus, you do not lose the view from the window when you use the plexiglass!