A long 7" diameter duct is going to be limiting. I am sure @kaseki is going to chime in a minute and look up the exact numbers. But I can already tell you that things would vastly improve if you had the option of increasing duct diameter.

Can you access the vent/duct from the attic? An in-line blower might be an easier install & service than a blower on the actual roof. That is a very long run for the ducting, I think that warrants a solid HVAC consult. I found this table, https://www.engineeringtoolbox.com/equivalent-diameter-d_443.html Which implies that a 7" diameter duct can handle about 200cfm, but 200cfm isn't very much for a kitchen hood. But I could be reading that wrong ... definitely consult with a local place that does a lot of ducting work.

We cannot access the attic since we only have the first floor. The attic belongs to another unit. We have 4 units in the building. We all have our own ducts to the roof but mine is the longest since I have the entire first floor. The way I think about it, a 7x7" duct is 49"^2, whereas a 8" circular duct, which seems standard, is around 50"^2, so it's quite similar.

All you have to do is google: "1200 CFM external blower"

Here's one as an example: fisher/paykel

Actually I answered this, I think, in another open thread. Here is my comment:

Without addressing the actual flow profile, a 7 x 7 duct is 49 square inches, or .34 square feet in sectional area. At 1000 ft/min, the flow could be 340 CFM. At 2000 ft/min 680 CFM. I would consider that the upper upper limit. Also, the pressure loss will be higher for a square duct than a round duct of the same cross section, so blower selection should be based on total pressure loss calculated from the duct shape and length (or more conservatively, a 7-inch diameter round duct), the hood baffle pressure loss vs flow rate (may have to be estimated based on baffle manufacturer data), and pressure loss in the make-up air system (which needs to be defined during the process of dealing with the hood system).

Even so, the achieved flow rate might be marginal for wok cooking. You want around 90 CFM/sq. ft of hood entry area, and a hood that at a minimum overlaps the wok by enough to capture the rising and expanding plume, larger than the wok by about 1/6 the distance between wok and hood entry aperture around the hood boundary. Of course, this should be done for every burner to define the hood extent.

Do you have a practical way to go out a wall, or to expand the existing duct chase sectional area to accommodate a 8 to 10 inch duct?

What I would normally write is that we should start with requirements, which start with the cooktop size. Then we can use the ideas above to develop the hood size requirement, then the flow rate requirement, then the duct requirement.

Please provide the hood dimensions. Do I correctly assume that the hood itself has no internal blower? What does it use for a filter? Or do you intend to replace it?

Is this cooktop not on an exterior wall?

The hood is a copper 40”x20” over a 30” cooktop 57k btu max (all 5 burners at once). Unfortunately the hood is a custom piece and very old but quite beautiful. The cooktop is against the wall as is the range. The vent is in the wall inside the range, not the ceiling. Because of how the kitchen is built it’s hard to adjust this configuration and retain the range which we want to keep.

Here is a photo of the mushroom motor on the roof aged 30 years plus most likely. There are two other motors belonging to other units beside my motor.

The hood is also 34” above the cooktop. I know none of this is ideal!

The way I think about it, a 7x7" duct is 49"^2, whereas a 8" circular duct, which seems standard, is around 50"^2, so it's quite similar.

The actual math is a lot more complicated than that. It generally requires solving partial differential equations which is not something I am prepared to spend my time on. But assuming you actually have a full 7x7" and smooth wall surfaces, there is an approximation that should allow you to estimate the equivalent dimension of a round duct:

Deq = 1.3*(x*y)^0.625 / (x+y)^0.25 = 1.3*49^0.625 / 14^0.25 = 7.652

If we assume laminar flow (probably not quite true, but a fair approximation as long as the walls are smooth), then the Poiseuille equation applies.

It states that the flow rate goes down with the fourth power of the radius. For a 7x7" duct, that would be about 16% less than a round 8" duct or about 65% less than a round 10" duct.

Additionally, the flow rate goes down linear with the length of the duct. Let's say your 40' duct is about 5 times longer than typical residential ducts. Let's also assume that hood manufacturers would normally recommend a 10" round duct for the type of hood that you have. Then you only have about 7% of the capacity that the manufacturer recommends.

The biggest problem is obviously the length of the run. But the dimensions are also a very real factor. And the smoothness of the walls (if the duct isn't lined with metal but maybe just brick) shouldn't be neglected either.

In an ideal scenario, you'd either find a different location for where to vent to the outside resulting in a significantly shorter duct, or if that isn't an option, you'd increase the diameter of the duct dramatically. Something like 12x12" wouldn't be a bad idea. It's probably OK if you have to reduce back to 7x7" for the last two feet on the roof.

Chris Balster

Is there a filter in there anywhere?

34 inches high is fine. Mine is at 34.5 over an island. 20 inches deep is a tad shallow, but usable with keeping higher effluent loads on the back burners.

Keeping the brick clean must be a challenge.

The down-blast blower looks similar to the one I use for auxiliary venting. Mine was made by NuTone ca mid-70s, I think, and the zero static pressure flow rate is presently unknown because I can't find that one bit of historical data among my many cubic meters of stuff. Your motor may be different than mine. Examination of the fan blades may be useful in case they are encrusted.

Chimney likely has flue tiles, but they will be only quasi smooth.

A different blower could be acquired with parameters that will allow more flow at the cost of higher duct (flue) velocity. See Fantech 5DDD line as merely one example. Achieving the same chimney cap fit might be an exercise.

Higher velocity may not have much effect on duct noise with tile/brick/terracotta or whatever you have in the chimney. Grease impingement would motivate me to have filters made that fit where you can reach them a few inches above the bottom of the hood. Copper fittings could be fabricated. See http://www.handcraftedmetal.com/ for one source.

M

Pressure loss goes linear with duct length, but flow rate follows the fan curve, which is usually curved (non-linear). Because the curve is usually positive, initially the loss in flow rate with pressure loss will be less than linear, but at the higher pressure loss end, the loss in flow rate will be greater than linear. One wants to choose carefully if the pressure loss is anticipated to be great.

We haven't heard yet what the MUA solution is or will be for this vent system.

There was at one point a filter but I removed it as I took the previous control unit apart which had ceased working. Now I’ve connected a new controller to the motor and it works. I’m just going to take my time and carefully consider upgrade options.

here are pictures of the previous controller and the place it was located. In terms of the age of the fan and controller I can only deduce pre early 1990s based on discussions with my neighbors.

Wow, stunningly beautiful hood. You seem to be in a metro area so that means restaurants, which means there might be a specialty HVAC company in the area. IIRC, vent hoods in restaurants are much higher above the cooking surface than in a normal kitchen. Even if they aren't licensed for residential work, they may be able to spec out a solution (for a fee) that you could purchase elsewhere.

Or a blower installed at the top that really sucks. (This requires close blade to blower wall fitting, and the resulting acoustic noise might help keep birds away.)

On the removed filter, I would worry if wok cooking went on for a long time without a filter in place because if there is upward flow, a lot of grease will condense into the chimney, even when the chimney is warm. This will not only be a fire hazard, but some downward dripping might result if the grease deposition rate exceeded the grease rancidification rate.

Thanks. I’ll be putting the filter back in place over the weekend.

What are the dimensions of the current blower housing (eg, how large of a space do you have to mount something) and what do you have for power & wiring up there?

I am not a wiring expert but I installed a speed control switch by the hood, max 5amps 115v, which was matching with the previous non functioning Fan control from the Stone Age. the usual black and white wires which I flipped off from my normal electric panel. Not sure if that helps.

Chris: The available power will depend on the circuit breaker or fuse on the circuit you are using, although obviously one wouldn't want to draw more than 5A through the particular control you added.

I suspect opaone was also asking about the width x length of the sheet metal on which the existing blower on the chimney is mounted (appears to overlap the chimney). Or, perhaps, the diameter of the circular base assembly of the blower.