Add on question.. the Modern Aire PSL-340 MAY be about the best bet for us ( albeit with some $$ ). It's been way too long since I have calculated duct losses so am appeakling to those here who are more "able"!

Modern Aire publishes their blower curves which are specified for 6 inch duct sizes. It seems they are ok with using 8 inch duct so am curious what their 1200 cfm blower will effectively do using 8 inch duct and a run of about 13 feet including two 90 degree turns. Thx!

Please provide a link to whichever blower curve is relevant to your intent. I will be very impressed if someone is making an affordable blower that achieves 1200 cfm through a 6-inch duct.

10-inch joists would normally be spaced 16 inches on center unless the loads above them called for tighter spacing and 12-inch lumber or trusses could not be fitted.

Hood overlap should be established by determining the largest pans likely to be used on each burner and projecting expanding cones above their rims with 23-ish degree half-angle sides. Where these intersect the height of the hood aperture above the pans you have the required overlap.

It is possible to capture effluent but not contain it due to insufficient flow rate, and also possible to have more than enough flow rate but marginal capture due to an insufficiently large hood aperture.

kas

Kas,

Thanks for the reply. First, after reading your reply it made me relaize I had lots of "stuff" in my head but I failed to explain things adequately. The Modern Aire blowers are rated at 600 cfm with an 8 inch duct but can be used with a 6 inch duct. The 1200 cfm versions are rated with a 10 inch duct but are specified as ok to be used with an 8 inch duct.

The curves supplied are for a single 600 cfm blower through a 6 inch duct.

The curves are here:

http://www.modernaire.com/Motor_Options_files/600%20Cfm%20Blower%20Specs%20PDF.pdf

Tonight I took a measurement of the floor joists using a coat hanger and I found a gap between a bathroom fan blower and the sheetrock for the coat hanger to fit between. It confirmed my suspicions that I have approximately 9 1/4 inch joists.

Now, I know I have an 8 inch duct size passageway between the joists though I don't yet know what size penetration I have going between the kitchen to the family room. Am guessing it will accomodate an 8 inch duct.

Option 1 is to buy the 600 cfm version and kow it will work with the 6 inch duc tin the event there is no suitable penetration. But, the rated cfm with losses will be less than 600cfm ( will have 13 feet of duct and two 90 degree turns ).

Option 2 is to assume I will have an 8 inch penetration and buy the 1200 cfm version, use it with 8 inch duct and loose a bit of flow BUT I can probably run at lower speeds most of the time and result in a quieter system for normal cooking.

I really liked the Prestige High Capacity hood design:

http://www.prestige-america.com/hcinsert.html

Seems to have a large capture surface area and little wasted space upfront. I can't find any information about blower specifications or if it can vent through an 8 inch duct.

Your point about having adequate capture and insufficient flow rate is well taken. I'm now leaning toward the Modern Aire PSL-340 and reducing the duct to 8 inches unless I can find more information regarding the Prestige hood. Thx again! John

The ModernAire fan has just the kind of behavior one needs for venting through ducts -- a low rate of flow rate drop with pressure loss. My guess is that the recurved blades and close fit to the wall provide this capability. If you conservatively assumed each 90 was worth 10 ft of duct (I don't recall the rule of thumb) you still get most of the rated flow. Also of note, the lower speed curves are also fairly vertical in this pressure loss region, so their flow rates will also be tolerant of duct length or restricted diameter.

I think you will like the 1200 solution better because, as you note, at lower flow rates the noise will be less. Internal fans put their noise in your face, so to speak, so lower speeds translate to lower motor noise and lower fan tip turbulence noise. Generated duct noise will be the same, but the fan assembly may block some of it.

Or, to put it another way, two fans each at a given flow rate generate twice the noise of one, but one fan running twice the flow rate may generate four times the noise, or at least four times the turbulence noise.

In any case, the anticipated flow rate for the fans selected and duct size used should be the input to calculating the duct air velocity to see if it falls in the usually suggested 1000 - 2000 ft/min range. For 1000 actual cfm, the duct area should be at least 72 square inches in area (0.5 sq. ft.). 8-inch duct is 50 square inches, so the velocities will be a bit high, but this is better than low for keeping grease from settling on the duct surfaces. This is another reason besides drag induced pressure losses for the recommended duct size to be 10 inches (79 square inches area). Rectangular duct is another option for inter-joist use as it will provide more sectional area in your case, but the 90s could be an issue depending on layout.

kas

Kas,

Thx.... I had not considered rectanglular ducting but it is an interesting option... I may be able to take a 10 inch 90 degree turn into a transition for the rectangular duct to head out to the external wall. I'll discuss it with my contractor but am feeling very much "safer" about at least fitting in an 8 inch round duct. Will probably buy the Modern Aire PSL-340 even though it is quite pricey. Supplying the fan curves was helpful. When I was considering the Imperial hoods, which use a single 7 inch duct for their 635 cfm liners, they stated clearly NOT to vent into a 6 inch duct as it would damage the blower. makes sense but the blower used by Modern Aire obviously can wisthstand much more backpressure and as you pointed out, does not lose much cfm due to the smaller duct. Just sounds like a higher quality better engineered blower fan. Course it does cost more :) Thx again!

We have a Prestige vent insert in a wood range hood above a 30inch induction cook top. Works well and is easy to clean.

⢠Vent hood insert for wood range hood - Prestige �" Pro-style Power Pack Hood Insert with Internal Blower System - Prestige Power Pack �" Model UIB28S1

Noise is a little loud when on high. Usually we turn it on high then turn the power down a bit.

Works great and is easy to remove the pro-style baffles and put them in the dishwasher to clean.