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.

Devious and small duct paths have a lot of pressure loss. While it is possible to find a blower that can handle any combination of flow rate and pressure loss, you might not want to pay for it. If I wanted to go out the back of a hood and down the wall, I would peel the sheetrock and redesign the stud layout (as for a window, say) such that there was room for a reasonable duct cross section. This is where someone familiar with carpentry is needed, along with plumbing and electrical support. You might need wire splices along with the required accessible boxes for the splices. The plumbing scope and feasibility is not internettable (as Sophie puts it), although if you tear the wall open and provide photos we would surely be willing to comment. :) You would probably need to end up with wall access panels within your cabinets to pull off a lot of pipe, cable, and stud rerouting.

If it were my own home, I would have followed systems engineering process and analyzed the Hades out of the relevant subjects to be sure that what I ordered was all compatible, and then would have collected all components on site so final measurements could be made before ordering cabinets and placing electrical outlets. Perhaps needless to say, this has (and did have) the effect of greatly dragging out the reno. I would also have desperately tried to find a way to (a) use an exterior blower, and (b) find a place to put a silencer. And I would have to consider whether an 11 x 7 duct was too non-standard to contemplate. I have to assume for your home, eliasgrace, that your duct path is pretty fixed and there is no point in discussing variants. I think I would go with the squished 10-inch duct and then do the following: Tape all duct connections with approved tape See how well it works with more challenging plume generating cooking. Listen for how noisy it is Most importantly, not initially close up the access so that any vibration can be suppressed with techniques we can address later, and in any case, horizontal ducts should have a way to get into them if grease build up is detected, in this case possibly in noxious ways thru the cabinets. (Most of the time, residential grease not collected by the baffles is too sparse for grease drips out of the ducting, but plan ahead.) Some thinking about that now may save a lot of pain later.* You may wish to use a wood cover that is magnetically attached so it can be pulled off. If the blower is roof mounted, allowance should be made for having to upgrade it if flow rate is too low for full containment of the challenging plumes. If the blower is internal to the hood, then allowance for adding a roof blower is worth considering, although that could have electrical consequences. In spite of my comments above, I might be tempted to cut away the cabinets. However, this would change their structural strength without some additional structure that would have been much easier if added by the cabinet manufacturer. Replacing them would depend on their cost relative to the whole project. I'm not wearing your moccasins, so I fear that there is too little I know about all the details to assert that I know what's best. Last, let me note that your hood aperture is larger than my hood's 10 sq. ft. I use a 1500 CFM @ zero static pressure roof blower and 10-inch duct. It is adequate even without completed pressurized MUA (although it won't be if other exhaust fans are running so I still need it). Your actual specific flow rate (with all windows wide open) may turn out to be less than 90 CFM/sq. ft., and the impact on capture and containment will need to be closely observed. Fluid dynamics (applies to hood operation) is not linear or independent of the overall configuration, and general rules of thumb are merely ways of avoiding solving nearly intractable problems. What is your MUA approach? kas *Fantech makes some clamp unions that could perhaps simplify taking out a duct section.

Pumps and fans and measuring devices all like nice linear non-turbulent fluid flow for best behavior. It is possible that mounting the elbow at the hood might cause an interaction with the blower, but what I would expect is just noise and reduced performance. Any interaction would be due to the finite number of blades and the fan designer's expectation that there wouldn't be "feedback" from dynamic pressure pulses in the duct. This is an issue that cannot be easily predicted and that would be expensive to model, so your goal of obtaining reassurance is the best approach, so long as the conditions are the same; that is, the hood model and elbow location are the same. Worst case for no change in elbow or hood would be to have to add some flow straighteners in the duct. That is too far outside my experience to help with. I'm only guessing at the dimensions of the existing duct hole, but it looks quite modest in diameter. Going up in duct size would surely help if there is an issue. It would require a bit more carpentry, and implies testing with the existing intended configuration before closing up everything. Another bail-out option would be to remove the blower in the hood and put an external blower outside in lieu of the wall cap.