By the way, it is a 74" inch long hood if that makes any difference....

What is the actual air intake aperture size at the bottom of the 72-inch hood? Let's compare the required flow rate to the zero static pressure flow rate of the blower.

Kaseki, thank you!!!

Hmmm...I'm not sure what the aperture size is as the hood has not been delivered yet. They are coming with the hood tomorrow to install. Here is the installation manual for the hood....but I custom ordered it to be 74" so not sure if everything is applicable. I looked through the manual and didn't see anything about bottom aperture size...

http://prizerhoods.com/wp-content/uploads/2012/11/INSTALLATION-INSTRUCTIONS-WALL-AND-ISLAND-1.pdf

OH, I forgot to add that the hood is a custom 27" depth if that makes any difference...and it will be installed 31 1/4" above rangetop grates.

OK, lets make some assumptions. Assume that the internal length is 70 inches and internal depth is 24 inches. Area is thus 11.7 sq. ft. Desirable actual flow rate is 90 times this or about 1000 CFM. A blower rated for 1200 CFM may be a bit underpowered for this given the actual duct, baffle, and MUA restrictions; 1500 CFM would be better. You can likely live with 1400. The difficulty is that their version of the fan curves (in duct feet) doesn't tell us what the baffle pressure loss is as a function of flow rate, which would have some duct equivalent length for each duct size and and hood size.

With respect to the cabinets, one could carry the facing/doors up to near the ceiling and build the cabinets without tops. Then an undeformed circular duct could be used with only a bit of protrusion into the cabinet space. But I suppose from your photo that it is a tad too late for that.

We can also do the original problem. Assume that the squish of the 10-inch circular duct is to an ellipse 7 inches in height (minor axis). It can be shown that the major axis will then be 12.3 inches (page 399, 1961 CRC Math Handbook). The sectional area is 67.6 sq. inches (c.f. 10-inch circular duct of 78.5 sq. inches). Using the equivalence approximation the equivalent round duct for the same pressure loss is 8.6 inches diameter.

Last, an 11 x 7 inch rectangular duct has a sectional area of 77 sq. in., almost the same as a 10 inch round duct. Without looking it up, this might be better than an 8.6 inch diameter round duct, although the formula for non-circular ducts would suggest only 7.5 inches equivalence. I think that formula breaks down when going fully rectangular. Looking up circular vs. rectangular duct loss is left as an exercise for the student.

Wow, Kaseki, thank you!! I will discuss with my contractor whether it is possible for him to switch to a rectangular duct if need be. But you're saying that even if the sectional area of the 11x7 rectangular is almost the same as the 10 inch round duct, it might only be the equivalent of a 7.5" round duct due to the inefficiency of a rectangular duct (vs circular duct)?

Let's say we are stuck with a round duct, not rectangular duct (install work being done tmrw, not sure if there is time to make changes as all supplies have been purchased and ready to go)...would you be ok with the equivalent of an 8.6" round duct or would you cut through the upper cabinets to allow for a 10" round duct? If it were your home?

If it were my own home, I would have followed systems engineering process and analyzed the Hades out of the relevant subjects to be sure that what I ordered was all compatible, and then would have collected all components on site so final measurements could be made before ordering cabinets and placing electrical outlets. Perhaps needless to say, this has (and did have) the effect of greatly dragging out the reno.

I would also have desperately tried to find a way to (a) use an exterior blower, and (b) find a place to put a silencer. And I would have to consider whether an 11 x 7 duct was too non-standard to contemplate.

I have to assume for your home, eliasgrace, that your duct path is pretty fixed and there is no point in discussing variants. I think I would go with the squished 10-inch duct and then do the following:

• Tape all duct connections with approved tape
• See how well it works with more challenging plume generating cooking.
• Listen for how noisy it is
• Most importantly, not initially close up the access so that any vibration can be suppressed with techniques we can address later, and in any case, horizontal ducts should have a way to get into them if grease build up is detected, in this case possibly in noxious ways thru the cabinets. (Most of the time, residential grease not collected by the baffles is too sparse for grease drips out of the ducting, but plan ahead.) Some thinking about that now may save a lot of pain later.* You may wish to use a wood cover that is magnetically attached so it can be pulled off.
• If the blower is roof mounted, allowance should be made for having to upgrade it if flow rate is too low for full containment of the challenging plumes. If the blower is internal to the hood, then allowance for adding a roof blower is worth considering, although that could have electrical consequences.

In spite of my comments above, I might be tempted to cut away the cabinets. However, this would change their structural strength without some additional structure that would have been much easier if added by the cabinet manufacturer. Replacing them would depend on their cost relative to the whole project. I'm not wearing your moccasins, so I fear that there is too little I know about all the details to assert that I know what's best.

Last, let me note that your hood aperture is larger than my hood's 10 sq. ft. I use a 1500 CFM @ zero static pressure roof blower and 10-inch duct. It is adequate even without completed pressurized MUA (although it won't be if other exhaust fans are running so I still need it). Your actual specific flow rate (with all windows wide open) may turn out to be less than 90 CFM/sq. ft., and the impact on capture and containment will need to be closely observed. Fluid dynamics (applies to hood operation) is not linear or independent of the overall configuration, and general rules of thumb are merely ways of avoiding solving nearly intractable problems. What is your MUA approach?

kas

*Fantech makes some clamp unions that could perhaps simplify taking out a duct section.

Kaseki, thank you so much for your response! We decided to keep the duct up behind the crown molding in the 7 1/2" tall space. My contractor thought it's the better option since putting it through the upper cabinet would require him to cut the duct into little sections, insert it into the cabinets and feed it through and then tape the sections together. Is there any other way to do it where you avoid cutting the duct work? So hopefully the duct in the crown space works out.

I was hoping to figure out a way for horizontal duct access...but short of taking apart the crown (which would be a big ordeal, I think, since it's a four piece crown), I don't think we can access the horizontal part? Hoping this will never be an issue! What do you mean by not closing up access initially? You think we should leave the crown off until the vent hood is up and running for a bit?

Do you suggest purchasing an anemometer to gage how many CFMs are being pulled up into the hood? To ascertain whether the squished duct is a problem or not? If so, is there a certain brand that you recommend? I'm thinking we should get a value of 1000-1100 CFMs since we have two 90 degree turns and I think every 90 is a loss of 100 CFMs (plus we have the inefficiency of the squished duct). If we can capture 1000 CFMs from a 1400 CFM blower, you think that's good enough?

Regarding makeup air, we have the Fantech MUAS 1600 installed and it looks like a space shuttle in our garage!!!

OK, with adequate (but not tested by me so I'm guessing here) MUA, your dominant pressure losses should be due to first the baffles, followed by the duct bends, the elliptical section, and then the straight circular sections. Try to get from elliptical to circular as soon as possible past the soffit.

Measuring actual hood flow rate is tricky. Someone specializing in this measurement can temporarily replace a exterior door and use a calibrated duct or unpowered fan rotation rate to measure it. An anemometer might be useful to see if the flow rate everywhere at the hood intake aperture is a desirable value, e.g., 90 ft./min.

A differential pressure gauge to compare inside the house vs. outside will let you know if the MUA is working correctly. These are not expensive. If you have any other combustion appliances with intakes connected via the house to the kitchen, your MUA should be trying to avoid more than 0.03 inches of water column lower house pressure.

And yes, leaving the access to the duct available for testing will reveal rattles that need to be damped.

Thanks Kas! So, would you recommend seeking someone how to measure the actual hood flow rate as opposed to purchasing an anemometer? What kind of technician would offer this kind of service?

I will definitely look into purchasing a differential pressure gauge! Any suggestions on what brand to get?

I think I was just commenting on your interest in the matter. If there is a back-draft safety issue, then I would want a differential pressure gauge to ease any concern, or at least carbon monoxide sensor alarms. If there is a plume spillage issue I would want to measure the flow velocity at various locations in the plane of the hood base. Otherwise, I wouldn't bother unless you have a technical interest in the subject.

There have been a lot of industrial mergers and collapses over recent years, along with changes in product lines. Any devices that I acquired a decade ago might not exist. I suggest searching on the Internet, including on eBay, for clues. "HVAC testing" might be a good search term if you want someone else to take measurements.

If I might interpret a bit, a small amount of 'squishing' is ok since the total area remains the same. Leaving it open gives you the option to go to a rectangular duct in that section if you need to later. A rectangular duct like this should be something any decent HVAC guy could make for you in less than an hour. The round to rectangular bits on the ends would require a bit of tin bending expertise.

And what kind of MUA do you have?

Weedmeister: as I noted in my March 6 message, the area of an easily fitted elliptical section having the same circumference as a circular section is moderately less, not equal. In that analyzed case, the 10-inch duct was "squished" to 7 inches. That is actually quite a bit of squishing, so a moderate sectional area change is not unexpected. Squishing a 10-inch duct into a 9-inch gap would, I think, qualify as "a small amount."

In any case, the reality is that achieved flow rate follows from the fan performance curve and the entirety of pressure losses (which increase with flow rate) of the system (cooktop to outside and back to cooktop).

The OP's March 8 message lists the MUA system.

K: yeah, I thought more about that after posting. 10" to 7" is a lot of 'squishing'. I was thinking less than that from the picture. And just glancing through all the text.

Too bad this wasn't measured before the cabinets went up. Dropping them another inch would have helped a bit.

A good idea, at least from the ventilation POV. I didn't think of that when commenting on alternatives. I'm not sure just what trade-offs are lurking in the history of cabinet height.