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Old 12-27-2016, 04:48 PM   #1
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Downdraft vent ducting - Qs about duct size and transitions


Hi! I'm new to the site - thanks for hosting this fantastic resource! I found the DIY Chatroom because I have some questions that I'm trying to answer about replacing my downdraft and fan with a much more powerful version. Yes, I realize that an overhead hood would be much, much better but it's not an option in this house.

Here is the background (the questions are at the end if you want to start there): I’m replacing a 450 CFM blower fan downdraft unit (which uses 6” round duct) with a larger downdraft unit matched with a 1100 CFM blower fan that has a 10” round input and output duct connections. The goals are 1) much more air volume since the new range has a griddle + much higher BTU burners and 2) quieting the whole thing down (the old fan was in the cabinet, the new one is inline under the house).

Here is the full description of the run:

It is relatively short - max 15’ total but with three 90°ish turns and 1 rectangular to round conversion at the beginning and (possibly) one round to rectangular conversion at the end.

From the downdraft, it starts out as 3 1/4” x 10” going straight down through the floor from the bottom of the downdraft unit. It converts to 7” round in the crawlspace (this is the largest transition that I could find - didn’t see any 3 1/4” x 10” x 8” or larger diameter transitions). The 7" round then takes a 90° turn, expands from 7" to 8” and then from 8" to 10” and attaches to the inline blower fan. The output from the blower is also 10”. This is where it gets interesting…

My problem is that the largest exit hole option that I’ve got in the outside wall is max 14.5” x 7” rectangular OR I could fit up to two 7” round vents. The current hole is a single 6” round. So, my current plan is to run 10" round from the blower output and convert it to 14" x 6” rectangular, make a rectangular "S" curve and then exit the house using 14" x 6". The crawlspace is below grade and so that is why the run requires an “S” shaped rectangular duct on the way out - it has to go 90° up and then 90° horizontal to exit.

The largest rectangular wall exit vent that I have found online is 6” x 10”. That equals 60 sq. in. of area where a 10” round pipe is providing 78.5 sq. in. of area, about a 25% reduction. So, I think that I need to have someone custom make me a 6” x 14” (84 sq. in. area) exit vent. I could do 6” x 13” but with the transition I figure a little bigger is better and I’ve got the extra inch to work with.

Another option would be to use a pair of 7” round vents - that would give me 77 sq. in. of area exiting the house (almost enough). I still need a custom vent cover because they'd be almost touching each other at the exit point. Also not sure how the 10" round to 2x 7" wye impacts the calculation - maybe I'd actually need a 10" x 8" x 8" wye which would be too big for the exit hole.

Now, the questions:

1. Is it always a bad idea to reduce the size of the duct area after the inline blower (i.e., between the blower and the exit point from the house). For example, transitioning from 10" round duct at the fan to 7" round duct at the exit point would be really easy! This seems to be highly discouraged - everything that I've been able to find says that it would increase static pressure, work the fan motor too hard, cause premature failure of the fan, probably would make excessive noise, etc.

The reason that this is even a question in my mind is that the output from the downdraft is only 3 1/4" x 10" which equals 32.5 sq. in. and this is matched by the manufacturer to a fan which has 10" input/outputs. Seems weird that I need 78.5 sq. in. of area (10" round) exiting when I only have 32.5 sq. inches of area at the starting point...

2. I understand that I should have 10" round ducting for at least 2' prior to the blower unit (i.e., don't go from 7" to 10" right at the unit). Should I also have some 7" duct before the 7" x 8" expander and some 8" duct before the 8" x 10" expander or can I go straight from 7" to 10" right after the 7" 90° turn?

3. As per above, I have limited exit options for the duct exiting the house. The largest possible hole would be either 14" x 7" rectangular or 2x 7" round ducts. Converting from 10", which would be better? Either would require a custom made wall vent as far as I can tell. Round seems to be the most efficient based upon the length conversions for 90° turns (as an example) that I've seen online. If round is better, would the 7" round need to be 8" or larger due losses at the wye (i.e., 10" by 8" x 8" vs. 10" x 7" x 7")?

Thanks in advance for your thoughts.

Best,
Ruprik
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Old 12-27-2016, 05:05 PM   #2
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Re: Downdraft vent ducting - Qs about duct size and transitions


Just my usual concern. If you have any naturally drafted appliances, furnace/boiler, water heater, or fireplace (or other), then you will need a matching powered supply of make up air to prevent backdrafting. If those appliances are already sealed combustion or otherwise immune to house pressures then forget I mentioned anything.

Not an air flow guy, but the size changes and bends will definitely reduce your total air flow.

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Old 12-27-2016, 05:18 PM   #3
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Re: Downdraft vent ducting - Qs about duct size and transitions


Thanks, Bud. There is a window right by the stove - the thought was to open that up whenever running the downdraft (as we do now, but the old downdraft has a much lower CFM). No water heaters or other gas consuming devices inside the house but a gas log fireplace with a relatively tightly sealing chimney damper exists. Would you expect an open window (42" x 12" when open fully probably) to be sufficient for the make-up air source?
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Old 12-27-2016, 07:30 PM   #4
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Re: Downdraft vent ducting - Qs about duct size and transitions


Relying upon humans to open a window is a poor option. Someone say, it is cold out and the window gets closed. But, with your only backdraft concern being the gas fireplace it isn't as bad as a naturally drafted water heater, but no the dampers do not seal well and if the FP is running (damper open) all of the exhaust will likely be pulled into the house.

Building codes (or best practices) specify any range hood in excess of 400 cfm requires a dedicated make up air system. Whether that has made it to code I'm not certain but it should be mentioned in the literature related to kitchen exhaust fans.

The other down side is, that size fan will replace all of the air inside a typical home in 20 to 30 minutes. Think of it as cooking outside.

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Old 12-27-2016, 08:03 PM   #5
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Re: Downdraft vent ducting - Qs about duct size and transitions


You could get a Y fitting and split the single 10" duct to two 7" ducts and go from there.

1100 cfm is a heck of a lot and as the others have said, you will have issues with back drafting.

hopefully it has a lower speed if only one burner is on. otherwise you'll have to open a window every time you cook or provide some form of make-up air.
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Old 12-28-2016, 02:25 AM   #6
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Re: Downdraft vent ducting - Qs about duct size and transitions


Thanks, Bud and 12345a.

The downdraft has an "infinitely adjustable" potentiometer so generally 400 CFM or so is probably all that is needed for the single burner or maybe two that are in operation - we aren't running a commercial kitchen. But when that griddle is on, cooking sausage, etc., we'd probably need to crank it up. The 1100 CFM rating is for all 6 18k BTU burners and the griddle running simultaneously - not likely to have that happen often if ever.

I'll look at make up air options - I don't want to suck all of the door and window seals off! I already have a 2nd 3 1/4" x 10" hole in the floor that I could use under the range to feed air back in (we also moved the island about 10 inches).
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Old 12-28-2016, 02:52 AM   #7
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Re: Downdraft vent ducting - Qs about duct size and transitions


18k btu burners? six of them? normal gas stoves have 5-8k btu burners.

what do you cook that needs 18k btu burners?

With even half the burners on it's like running a residential furnace in the house with the vent pipe disconnected. Then trying to remove all the exhaust from the furnace with a remote exhaust fan.

You'll definitely want to make sure this fan is ducted so it moves full rated cfm and find a way to get makeup air.
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Old 12-28-2016, 03:27 AM   #8
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Re: Downdraft vent ducting - Qs about duct size and transitions


"What do you cook that needs 18k but burners?" - nothing but it can really bring that pot of spaghetti water to a boil fast! It's handy for the wok too - stir frying works best with a lot of heat.

18k are what what all of the "pro-sumer" ranges seem to have now-a-days. There are a even few that are 20k each. But generally not using more than 15-25k total btus at any one time I'd expect.
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Old 12-28-2016, 05:12 AM   #9
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Re: Downdraft vent ducting - Qs about duct size and transitions


Code requires any exhaust fan rated at 600CFM or larger to have an interlocked make up air supply.

Its has a small opening at the stove, so that it has a lot of ability to pull the smoke, smells, heat, and grease into it.

The fan is rated at 1100 CFM at what static/ESP pressure?
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Old 12-28-2016, 03:18 PM   #10
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Re: Downdraft vent ducting - Qs about duct size and transitions


I attached the chart that came with the fan specs - but here are a few representative values that I estimated from the chart:

~1120cfm @ 0.0" H20
~790 cfm @ 0.5" H20
~550 cfm @ 1.0" H20
~420 cfm @ 1.5" H20
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Downdraft vent ducting - Qs about duct size and transitions-static_pressure_1100cfm.png  
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Old 12-28-2016, 03:42 PM   #11
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Re: Downdraft vent ducting - Qs about duct size and transitions


This looks like a good solution for the make-up air. http://www.fantech.net/globalassets/...air-system.pdf. Certainly appreciate the input on this.

The code referenced per the above link is that MUASs are required for range vents > 400 CFM as of 2009.

With that out of the way, any feedback on Qs 2 and 3?

On #2 - is it a better design to have a gradual expansion taper in the duct (i.e., 2' for each transition) or is it no different to just go from 7" to 10" within 20" or so (i.e., just put all of the transitions back to back)?

On #3 - I saw 12345a's feedback that the wye w/ 2 7" round ducts exiting the house would work but its not clear to me which would be the better solution. Rectangular seems to be significantly less efficient for airflow but it's not clear to me how to evaluate the two options (i.e., the 10" x 7" x 7" wye and less area of that solution is better or worse than the 10" round transition to 14" x 6" rectangular solution). Either would work fine with the space.
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Old 12-28-2016, 03:50 PM   #12
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Re: Downdraft vent ducting - Qs about duct size and transitions


Quote:
Originally Posted by ruprik View Post
This looks like a good solution for the make-up air. http://www.fantech.net/globalassets/...air-system.pdf. Certainly appreciate the input on this.

Over kill, as your exhaust be between 500 and 550CFM.

The code referenced per the above link is that MUASs are required for range vents > 400 CFM as of 2009.

With that out of the way, any feedback on Qs 2 and 3?

On #2 - is it a better design to have a gradual expansion taper in the duct (i.e., 2' for each transition) or is it no different to just go from 7" to 10" within 20" or so (i.e., just put all of the transitions back to back)?

Gradual is better.

On #3 - I saw 12345a's feedback that the wye w/ 2 7" round ducts exiting the house would work but its not clear to me which would be the better solution. Rectangular seems to be significantly less efficient for airflow but it's not clear to me how to evaluate the two options (i.e., the 10" x 7" x 7" wye and less area of that solution is better or worse than the 10" round transition to 14" x 6" rectangular solution). Either would work fine with the space.
The wye will be more restrictive to air flow.
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Old 12-28-2016, 04:46 PM   #13
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Re: Downdraft vent ducting - Qs about duct size and transitions


Quote:
Originally Posted by ruprik View Post
I attached the chart that came with the fan specs - but here are a few representative values that I estimated from the chart:

~1120cfm @ 0.0" H20
~790 cfm @ 0.5" H20
~550 cfm @ 1.0" H20
~420 cfm @ 1.5" H20
You'll have to size the duct so the pressure drop total is 0.2" total or less.

I suggested the Y fitting because i don't think you want a 10" hole in the side of your house.

Determine the equivalent length of all fittings including the hood, decide which fittings you need to use.

google has everything you need. elbows have a equivalent length much greater than the actual length.

Add up the lengths, add the length of straight duct. this is your total equivalent length.

Determine the standard friction rate needed by multiplying the desired pressure drop by the quotient of 100 ft divided by the total equivalent length.

From there, take the cfm on the chart you posted at the total pressure drop you're using (maybe 0.1" or 0.2") and plug it into a duct calculator online, along with the friction rate.

the calculator will tell you which diameter you have to use.

This is the proper way to size ducts if you need a specific flow rate.

It's rarely done properly because people like rules of thumb.

Every application is different and the flow rate will vary.

You will not get 1100 cfm from a fan rated at that. If you get 900 to 1000 you're doing very well, and 800 wouldn't be bad by any means.

There are contractors slapping in 200 to 300+ rated cfm range hoods and using 4" flex pipe. that rating gets cut to 100 cfm or less in the real world.
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Old 12-28-2016, 05:21 PM   #14
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Re: Downdraft vent ducting - Qs about duct size and transitions


Quote:
Originally Posted by user_12345a View Post
You'll have to size the duct so the pressure drop total is 0.2" total or less.

I suggested the Y fitting because i don't think you want a 10" hole in the side of your house.

Determine the equivalent length of all fittings including the hood, decide which fittings you need to use.

google has everything you need. elbows have a equivalent length much greater than the actual length.

Add up the lengths, add the length of straight duct. this is your total equivalent length.

Determine the standard friction rate needed by multiplying the desired pressure drop by the quotient of 100 ft divided by the total equivalent length.

From there, take the cfm on the chart you posted at the total pressure drop you're using (maybe 0.1" or 0.2") and plug it into a duct calculator online, along with the friction rate.

the calculator will tell you which diameter you have to use.

This is the proper way to size ducts if you need a specific flow rate.

Range hood/stove exhaust duct work is not sized that way. As it can cause grease fires.

It's rarely done properly because people like rules of thumb.

Every application is different and the flow rate will vary.

You will not get 1100 cfm from a fan rated at that. If you get 900 to 1000 you're doing very well, and 800 wouldn't be bad by any means.

There are contractors slapping in 200 to 300+ rated cfm range hoods and using 4" flex pipe. that rating gets cut to 100 cfm or less in the real world.
On a good fan, they run about 180 CFM, which would be proper for a 4" exhaust duct.
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Old 12-28-2016, 05:32 PM   #15
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Re: Downdraft vent ducting - Qs about duct size and transitions


if the op wants a specific flow rate would have to go through that procedure regardless.

the grease factor i didn't think about but it makes sense in the same way that if dryer ducts are sized too large, the velocity isn't high enough to vent the lint.

if there's a risk of having grease buildup there would have to be a minimum velocity to size to, and if the fan is inadequate for the required cfm, get a more powerful fan.

180cfm through 4" is a lot especially if the pipe is long and/or or flex, most hood instructions spec 6".

main point was that the ratings are totally deceptive and calculations are needed to see how it will perform in the field.

they shouldn't be allowed to sell fans with cfm ratings at 0" static. imagine if they sold furnaces like that.
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Last edited by user_12345a; 12-28-2016 at 05:36 PM.
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