That is irresponsible of your builder and is a warning sign about their knowledge, quality and integrity. Reduction from 8" to 6" alone is a problem. It will reduce air flow and so the effectiveness of your hood and it will increase hood noise as well as the amount of electricity used. So increased cost, more noise, less odor, gas combustion by-products and other effluent removal. Flex duct creates two problems; It's a major fire hazard and it increases static pressure (reduces air flow). From a static pressure standpoint 6" flex is roughly the same as 4" rigid in most installations. At a MINIMUM you should have 8" rigid for your entire run if that is what your hood calls for (Many US hoods undersize this). If you have many bends then you should increase that to 10" to keep static pressure down.

I want to echo Weed's caution about being sure on code compliance when having your vent going out near that window. Last year, when I was doing a re-routed of the vent on my range hood, the local building inspector happened by. (We are on pretty good terms so there was no compliance issue.) I asked him about the rules and got a long dissertation as he handed vent tubing up to me. The gist of it was that the rules for range hood vents are different than for those that directly vent gas appliances (say a water heater power vent). There is some discretion when it comes to venting from a range hood over a gas stove. How far the vent has to be from an opening window also varies with whether the vent is above the opening window, at window level or below window level. I was told the vent that is above an opening window can be closer than one at or below window level. BUT, you want to be sure that, if you open the window while the hood is running, it won't be sucking in the vaporous stuff that was just blown out the vent.

clinresga: Thanks for all the info and good things to think about. I am glad to hear that you like the Fantech silencer. Our attic should easily be able to accomodate its size. Do you know if they reduce the effective CFM of the blower? (I hope to get off another project today and start calling various companies to figure out what we need). We will be using a custom surround - truly custom in the sense that our contractor/finisher will make it for us in any fashion we want. We haven't yet settled on a specific design, but it will be made of wood, painted the wall color and we'll probably try to make it "disappear" as much as possible. Ha! Given the nature of this project (restoring a 1920's remote fishing resort - off the grid and way in the middle of nowhere), we are working with a contractor who does beautiful work and can pretty much make anything out of any material. (You should see the cabinets he's made that look like they've always been there, the rock fireplace surround he built that is a deadringer for the 1924 one that came off in the 1970s, etc. He is a true artisan.) So for the range hood, we are constrained by the location of the refrigerator on the one side, and the size/depth of the range. I believe we will have a maximum of 43-44" for the surround (the stove is 40" wide and sits about 32" off the wall). It sounds like MA might be my first call if they can make a custom liner that is the right width and extra deep. We did hear back from the stove restorer, and the burners have a total of 72K BTUs (12K each) and the ovens have a total of 40K BTUs (20K each). I still think the FDK10 will work for us if we get to that level of pinching pennies, but we'll have to run the numbers to see! In another post, I saw you mention that you have a wall control for the blower and lights. Did that come with the MA? Or is it manufactured by another company? Are the blower and light controls on the same unit?

vinmarks No, in your case I was tempted to write that the VaH can make use of that size duct. I think VaH may be negligent if they don't point out that that duct size needs to be matched on the MUA side. As pointed out earlier, few in the kitchen hood sales chain want to raise the issue of make-up air and the horror it can be to retrofit into a renovation.* But lest others think that they need 12-inch duct for 1200 CFM with conventional blowers in lieu of using 10-inch for typical lengths, I wanted to point out that that would be only a modest improvement relative to getting the MUA in with low pressure loss. Pressure loss is counted from the cooktop to outside and then from outside to inside back to the cooktop. ------- *An example MERV7 high flow 4-inch deep pleated filter, 2 ft x 2 ft in area used to catch the big particles and bugs that the MUA air could pass into the house, used at 1200 CFM (300 ft/min) would have a pressure loss of 0.1 inches w.c. (See table from https://www.texairfilters.com/comparative-tests-on-pressure-drop-of-hvac-filters/ If one looks into VaH data, for example see below, 0.1 inches w.c. drops the flow rate of a dual blower (1200 CFM) hood to 1060 CFM. At 1060 CFM, the filter pressure loss is a bit less, so one really needs to plot filter loss vs flow rate against the fan curve implied by the data below to obtain the expected flow rates (excluding ducts, MUA register, and exhaust duct cap pressure losses which also need to be counted) . And for comparison, using an equation I found on the Internet at the HVAC engineering forum referencing the 2003 ASHRAE Handbook at Page 52.5 (confirmed in my copy) and adapting it with some trepidation to screening as was done at the forum, and assuming a screen of 70% openness across a 12-inch duct at 1200 CFM, the pressure loss is even higher than the pleated filter, reaching 0.7 inches, w.c. With a 15-inch screened duct, 0.1 inches w.c. at 1000 CFM could be achieved. The moral is, use a lot of screen area, or low loss filtering, or let the sparrows in.