Get with a Nate Certified Contractor, insist on the J-manual load and a D-Manual,[In contractor wants to talk U out of the D-manual...get ride of him...many contractors will talk J manual, J-manual...but could not even explain how the D-manual is filled out and forget about zoning its over rated in benefits. Install 2 small units on the long ranch layout. this is my opinion.

20 year old duct system is pretty old, if your ducts are made of flex duct or duct board there is a good chance a duct cleaning will not reach the cleanliness of a new duct system. With that said if you have a 2 story structure, it's unlikely you will have the access needed to replace every piece of the duct system. So there are trade offs going from replacement to cleaning, not to mention truth in cleaning. How are you going to know how clean the duct system is after the cleaning? The ads of duct cleaning typically start around $59 a system. At this price point it would be impossible for duct cleaner to stay in business, but that is the con game of this kind of service. If the duct system isn't thoroughly cleaned, it won't be long until you'll need to do it again. the duct above is a 35 year old duct. This type are impossible to clean well as the inner part of the duct has a cloth like material. This supply plenum was replaced recently. I don't know how recently, but you can clearly see the stuff that was growing in the ducts is making it's way toward the cool dark ac supply box. Here's another picture of same plenum. Nothing is cleaner, healthier than a new duct system. I service the Katy, Texas area.

Many answers related to this may be found on the Appliances forum where hood system issues are routinely discussed. There are code requirements for MUA, and a safety requirement. The safety requirement drives the code requirements, in a somewhat crude manner in most cases. The safety requirement is to not back-draft combustion appliances. This is moot if the appliances have their own make-up air and have sealed exhausts, or are in a space sealed from the kitchen or otherwise isolated. Code might apply anyway. The image below provides examples of the differential pressure between inside and outside that can cause back-drafting. Inches of water mean inches of water column, where atmospheric pressure can pull a water column about 407 inches high against gravity. Click to enlarge. A passive (no blower, large duct) solution should still have filtering, and in most cases heating. The filtering alone will cause some pressure loss. See table in the image below. Feet per minute (fpm) are per square foot of filter and thus equal CFM per square foot. So passive MUA is a potential solution where back-drafting risk is negligible. A duct diameter equal to that of the hood duct would probably pass code. In this case, it is better to ask (the AHJ) for permission rather than forgiveness. Where the pressure loss in the MUA path is too high, then an active (with blower) MUA system is needed. These can be controlled by various schemes to balance indoor pressure relative to outdoor pressure. It is worth noting that the actual flow rate of the hood system is likely only 2/3 of the blower's rated flow due to pressure loss in the hood system, particularly in the hood filter (mesh or baffles) vs. the blower's "fan curve." In order that the MUA not cause turbulence at the cooking zone that can cause failure of the hood intent -- capture and containment -- MUA needs to be introduced "quietly." That is, MUA flow should be expanded into a diffuser much larger than its duct size. If introduced near the hood, the MUA flow should be directed away from the hood (across the ceiling), and if introduced at the floor aimed so that doesn't interact with the cook moving about. Commercial kitchens can sometimes get away with dumping the MUA under the cooking zones, all of which are raised above the floor, but otherwise typically introduce the flow at the ceiling. Test kitchens use perforated walls without intervening equipment. Passive MUA is generally self auto balancing, but the balance is not too good as may be seen from the filter table above. Assume for convenience a 2' x 2' x 4" cassette pleated filter. For example, a MERV14 4-inch filter at 1200 CFM (300 fpm per square foot) would have a pressure loss of 0.29 inches, or 73 Pa, requiring a blower unless there were no combustion appliances. (Estimating 0.2 inches, w.c., at the 800 CFM a 1200-CFM rated hood blower would likely deliver against the hood pressure loss, or 200 fpm at the MUA filter, we are still at 50 Pa, and combustion appliances, if present, really need to be in their own room with its own MUA to get away with passive MUA. Active MUA where the only significant flow is via the kitchen hood can use hood system blower motor current sensing, duct flow sensing, or actual pressure sensing if the hood blower uses variable speed control. With multiple household exhaust blowers and with fireplaces (not sealed having their own MUA, and intended for use during cooking), pressure sensing based motor control may be the only practical scheme. For contrast, large buildings are kept pressurized to aid door opening against winds, passive bathroom exhaust flow, etc. They run constant rpm blowers that feed ducting that includes a bypass duct to the blower intake. This bypass is controlled by building internal pressure vs. external pressure. Passive pneumatic damper control is feasible for this.

@cat_ky Hmm thanks. We had a similar vent in the back of the house that I was able to remove the cover and insulate and it's done a great job keeping out the cold. Perhaps insulate it + a glass window? Or maybe I could stuff batt insulation in there, then some rigid foam insulation cut to size, behind the vent cover? Wouldn't that in theory do the same as a window? Just a way to block cold air from entering from the outside? Not trying to make this a big project and I worry the window would be complicated and also look odd out there.