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
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
