Rereading this thread in its entirety -- a trip down memory lane -- reminded me of some web sites with filters on their search function such that when one clicks on enough constraints there are no candidates left to purchase. feisty68's constraints have a similar effect. Thinking that maybe there is some way out of the box, two concepts come to mind to bring up: (a) One can choose a hood with internal blowers (as seemingly required) that has more specified flow than the external duct can handle. In this case, at full power the flow will be low, (or possibly zero, depending on the fan curve shape), but when run at part power to yield an appropriate flow rate for the duct, there should be reasonable flow. Whether this is adequate flow would require some analysis. At partial power the blade tip speed will be less and lower noise should result. For example, a VaH hood with two motor squirrel cage fan assemblies run at part power would likely still expel grease onto their housings as intended. Choked flow, however, may be sooner (CFM-wise) than a similar over-sized conventional blower / baffle system due to the nature of a squirrel-cage blower (as implied by VaH CFM vs. pressure loss data). I wouldn't want to guess which approach was louder at the power setting that was at the point of diminishing returns, CFM-wise. Related to this category is using an inline axial blower designed for higher pressure loss at the desired actual flow rate. Fantech might help here. Such blowers may be noisier, so an in-line silencer on the hood side of the blower might be essential. There are screaming-mimi aerospace fans that can push 1600 CFM through a six-inch duct. (b) The other concept is a hybrid exterior/recirculating hood scheme. I think both Best and VaH have examples of recirculating hoods. In this concept, if the actual systems are so configurable, the path to the filter pack would be split with one branch going outside. When the pressure losses of the exterior duct path rose high enough as blower power increased during heavy cooking, additional flow would be through the filter pack and back into the room. Some recirculating hood users here have commented on effectiveness and noise and their threads might be discovered and checked for relevance. Obviously, the simpler approach here is recirculating without engaging the external ducting. In that case, a separate soffit blower, soffit face register scheme could be used for further kitchen overall ventilation, helpful (slowly) when the oven is opened. kas

To bastardize a phrase, measure first, cut second. Get the flow measured with the house configured in whatever closed up condition is typical. The original fan system may be completely adequate, although adding a control for variable flow might be helpful if it is a bit loud at full power. The interior of the ducting needs to be inspected. You could have a fire hazard waiting to happen. How much grease has accumulated will depend on the duct velocity and 110 years of cooking history. Grease precipitation will increase as the cooking effluent cools, so the condition near the roof may be worse than over the stove. One purpose of filter screens and filter baffles is to be a fire block. I think some of the custom hood makers could construct an insert that provided filtering without having to include a fan assembly if the original fan provides good flow with some pressure drop simulating baffles. kas

If you like reading, you have come to the right place, because we have a large number of hood related threads that can be read for as much theory and practice as you can stand. There are at least two suppliers of recirculating hoods with sufficient filtering that they might quality as at least marginally effective (from my purist point of view). One is Vent-a-Hood and the other is (as I recall) Best by Broan. There may be others and I hope those here who have or know of them will pipe up. Searching the manufacturers' web sites should provide specific information. Good quality recirculating hoods will have a succession of filters to remove grease and odor. CFM can be determined from the hood aperture area, and we don't yet know what size hood you may choose from among the limited number of recirculating hoods. For example, for a 2-ft front-to-back by 3-ft side-to-side aperture (6 sq. ft.) to achieve a goal of 90 ft/min across the hood aperture would require 540 CFM. Actual blower rating has to be higher due to pressure drops, but these may be accounted for in how a given recirculating hood is specified, so the usual factor of 1.5X that I suggest to make up for this may not be needed. Also, recirculating provides its own make-up air, so that factor is not applicable. I'm assuming that there is a good reason for not attempting to duct the cooking effluent outside. Duct installation may range from easy to extremely difficult, and without knowing more I won't attempt to provide encouragement for adopting the superior approach of external exhaust. kas

We are in need of ventilation for our 31.5" French range. It's not a flame thrower like some newer ranges but we cook most of our meals and often at high heat settings for meat and vegetables. We only have 19" between the bottom of the hood and the ceiling. We have a structural beam running right above the center line of the future hood. This leaves us two options: 1) we can do a double 90 immediately out of the insert--from there it is a 3' straight run to the roof, or 2) we can have the insert or hood custom made to have the duct exit offset 10" or so to clear the beam. This would leave us with a straight shot to the roof. Modernaire charges $240 for the custom work. This leads to my first question: In your estimation, would you get more performance from a 400cfm motor with a straight duct run or a 600cfm motor with a double 90? The cost difference between the 400 and 600 is about the same as the custom charge. The hood will be 36" wide. Second question: If we buy an insert now, can it be installed while we decide what we're going to do about the canopy/surround? Thank you.