My advice has to start with the observation that pulling greasy cooking effluent into your attic is certainly unwise, and, I expect, a violation of code most places. I am reminded of the meat packing plant in Cambridge MA in the 1960s that caught on fire -- an impressive sight from a few miles away. Every part of the path for greasy cooking effluent should be metal ducting until exhausted outside.

I'm in the Hoods-R-Us camp, so if you revise your plan, a search here using hoods and ventilation as topics will provide a lot of information that we can expand on if necessary.

kas

I think you misunderstood my question, I would never vent into an attic, only THROUGH the attic to the outside. I have had a nonfunctional downdraft system in place for years, so I have had no venting at all. My plan would simply be to have a working kitchen ventilation system using large ducting and a remote blower to take kitchen exhaust outside. I believe it would certainly work better than the downdraft system. Any thoughts?

I must have been misled by the "whole house fan" reference.

Yes a hood is superior to a down-draft system because the cooking plume rises to the hood where it is (when correctly designed) captured and contained and transported to the outside. One can have a blower in the hood pushing to the outside, or a roof blower pulling from the inside (either way the pressure loss is the same), or an in-line blower. A roof blower provides room in the attic for a silencer (muffler) which will significantly remove blower blade turbulence noise. Turbulence noise from the baffles will still be present, but this is usually modest.

Ideally, the hood should overlap the hot pan surfaces by enough to capture the rising and expanding effluent. A minimum of three inches all around is typically recommended, but this should increase if the hood is materially farther above the countertop than three feet.

Flow rate (CFM) should provide enough velocity at the baffles that all of the uprising cooking plume, upon hitting the baffles, is pulled through. This should be about 90 cfm/square foot of hood interior aperture if you do hot cooking such as grilling in a pan or on a grill surface, or wokking with typical rising hot peanut oil vapor.

The rated cfm of the blower is for conditions of hanging in space, so the pressure loss of the entire path (including getting air back into the house -- make-up air) has to be accounted for and the reduced blower output compared to the necessary actual flow rate. I would take the area of the hood aperture, multiply by 90, mulitply by a 1.5 fudge factor for pressure loss, and find a blower in that ballpark. Exactitude is not needed here because there are too many unknowns to provide a refined analysis.

kas

Your mistake was calling it a "whole house fan". That description is reserved for a specific type of fan in the International Residential Code, and refers to one of THESE.

What it appears you are asking about is more directly explained in the British Columbia Building Code. Most specifically, 9.32.3.9(6) & 9.32.3.12(2) , where if the edge of the vent intake is within 4' horizontally of the the edge of the cooktop, it needs a grease filter, or the entire inside of the duct needs to be easily cleanable. The IRC doesn't have a specific measurement for this.

There are different types of ventilation. If it is just intended to ventilate the kitchen, or if it is designed as mechanical ventilation for the whole house, can make a difference in the methods. If the fan is over 400 CFM, you will also need makeup air.

Is that what you meant? Or did you mean one of those supposed "hoods" that are flush with the ceiling? Those don't work much better than downdrafts, unless you have really serious pulling power, because the plume expands so much as it rises. Yes, in the old days they just had plain old rotary fans, but they didn't do much except stir around the air to make it smell better, and were more of a clever idea than an effective appliance. The real question is where are the steam and smoke and lighter than air particulates and cooking gasses and all going to go? No situation is perfect. They don't make extractors for ovens any more, nor vent ovens to the outside like they used to. My oven has a gonzo air filter but cooking smells still get through (not nearly so much as without, though), plus when you open the door all bets are off. Given that, if you don't really mind if much of the gunk gets into your new kitchen, because you're used to it from your old kitchen, the ceiling vent can only help. If you want it to do the same good job as a proper island hood, forget it. If you get a hood the same color as your ceiling, it'll be surprisingly unobtrusive.

Thanks for all the practical suggestions from kas, Fred s and pllog.

Since I will have a 36x20 inch island induction cooktop, it sounds like I should put as large of vent opening as possible on the ceiling on order to capture as much exhaust as possible. If painted ceiling color, I think a 3-4 sq ft opening is as large as I would be able to go without it looking overwhelming. I was planning on using a decorative heater vent opening and install that in the ceiling and connect to all metal ducting and a remote blower of 400-600 cfm. (If I use calculations suggested by Kaseki.) Making sure to add a backdraft/ baffle/ muffler in the system. This is what I had originally envisioned, thanks for your support. I know it would not be a perfect system, but it should be better than no ventilation, or a downdraft system. Would I need makeup air? What about a grease screen? I could hide that behind a removable vent cover perhaps. There would be no baffles, as I was planning on using a decorative heating vent cover. What size ducting would I need?

Anything flush mounted to the ceiling is going to be next to useless. As bad or worse than a downdraft.

Get a hood. You can't have your cake and eat it too.

The primary purpose of residential kitchen cooktop ventilation is not removal of waste BTUs. It is capture and containment of greasy and smokey effluent plumes from hot pan surfaces. As long as a down-draft can capture and contain an effluent stream rising past it at 3 ft/s you won't need a hood. Velocities won't be this high if frying is done at a minimum temperature, (e.g., quiet bacon), perhaps aided by a grease screen over the pan. However wokking and grilling and searing can reach this level.

kas