OK, since my name was called, I'll try not to make it in vain. :) Yes, more than one fan in series (but with enough separation to ensure blade to blade transient effects aren't present) will improve flow because the overall pressure drop is shared between fans (but not equally unless they are identical fans given identical power). If for example, the duct (and hood and hood filter and MUA) will cause a pressure loss of one-inch at 300 CFM, then two identical fans capable of 300 CFM at one-half inch will be required to flow 300 CFM at one inch. To predict the performance of two different fans, the overall pressure loss vs. flow rate is needed as well as the fan curves of both fans. Without knowing what the MUA pressure loss is, and the hood filter pressure loss is, estimating from just duct losses will not be very close to reality. I would add, however, that adding a fan may be a good idea when the goal is to not change anything else. Note that the roof cap in this case is not very amenable to adding a fan to one duct.

There are (at least) two options -- (a) blindly following the hood manufacturer's recommendations and (b) deriving from basic principles. The latter approach follows: Flow velocity at the hood entry aperture should be around 90 ft/min for good containment with baffles. I have no data for mesh filters. Ninety ft/min is equivalent to 90 CFM/sq. ft. of aperture area. Ninety times the aperture area is the total desired actual full power CFM. The desired duct velocity is 1000 ft/min or more (but not over 2000 ft/min). A six-inch round duct has an area of 0.2 sq. ft. and an eight-inch round duct has an area of 0.35 sq. ft. So divide the result of paragraph 2 by each of these values and see which is closest to your goal. Note that the blower rating has to be significantly higher than the result of paragraph 2 because it has to work against the pressure losses of (mainly) the hood filter, ducting, and make-up air. A factor of 1.5 may be adequate. Given an already selected hood with a specified blower, I would take the 600 CFM specification and divide by 1.5 to get 400 CFM hoped-for actual flow rate, then divide by the two duct sectional areas to compare 8-inch with 6-inch duct diameters. The results suggest the 8-inch duct would be better.

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.

OK, wok cooking: This requires the highest level of residential ventilation if you want all of the grease and water to be expelled instead of condensed on the interior. Start with capture. The plumes from pans expand as they rise. We believe that a good approximation for hood capture width is the cooktop width plus six inches. For a wall hood, the front-to-back opening should be wall to 3-inches past the cooktop (toward the cook). Lights and switch panels add to this, depending on how the hood is configured. Continue with containment. Given these aperture dimensions, modified by the reality of whatever products are actually available/affordable/aesthetic, and how the cooktop is installed in the counter, one computes the area of the hood entry area in square feet. Multiply by 90 CFM/sq. ft. (equivalent to an intake air velocity of 90 ft/min). This is the required or actual CFM. Multiply by 1.5 (if you don't want to do a complete analysis) to get blower-rated CFM, which is only valid when the blower is hanging in free air. Next is ducting: This is usually specified by the hood manufacturer, but the goal in cold climates is operation near 1000 ft/min, and otherwise in the range of 1000 to 2000 ft/min. This is given by duct sectional area divided by actual CFM. Don't forget noise: Best blower noise reduction is achieved by using a roof blower or in-line duct blower, with a silencer mounted between the blower and the hood. Silencers are about 4 inches larger in diameter than the duct, so silencer installation needs to be considered ab initio. Last, but really contemporaneous, is make-up air. MUA must be supplied such that the house internal pressure does not fall so far that combustion appliances, if any, are back-drafted. Some such devices, if not supplied with their own MUA, can be very sensitive. Even without such devices, one wants MUA pressure loss to be a small fraction of the cut-off pressure of the blower (top left of the fan curve). Review MUA related threads on this forum for more information.