Duct sizing question for HVAC gurus

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Ozmosis

Ozmosis

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Hi

I have a ducting question for the HVAC gurus out there. I have a spider farmer 55 x 28 x 80 tent. I am ventilating with an AC Infinity 4" duct fan with carbon filter and using the tent vents for passive intake to the tent. The exit goes through 8 ft of 4" duct and out the window through a screened 4" vent. I know I am nearing the limit with the 4" fan in that setup but would like to squeeze a bit more performance out of it if I can. Right now with an AC controlling the room at 71 degrees I can hit 80 -82 deg F in the tent mid day with the fan controller set at 7 out of 10. You can really hear the restriction at the 4" exit screen so I am not sure if I am even getting any more airflow turning the speed up further due to the back pressure.

I know flex ducting has a lot of restriction and then the screened vent cuts the exit surface area. My question is if I used a 4" to 6" adapter coming off the infinity fan and then ran the exit though 6" flex ducting and a 6" screened vent would I get a reasonable increase in air flow? I know the CFM flow through the ducting would likely decrease but I assume total airflow would go up with less restriction in the exit ducting. Any thoughts on this?
 
freezeland2

freezeland2

2,540
263
Hi

I have a ducting question for the HVAC gurus out there. I have a spider farmer 55 x 28 x 80 tent. I am ventilating with an AC Infinity 4" duct fan with carbon filter and using the tent vents for passive intake to the tent. The exit goes through 8 ft of 4" duct and out the window through a screened 4" vent. I know I am nearing the limit with the 4" fan in that setup but would like to squeeze a bit more performance out of it if I can. Right now with an AC controlling the room at 71 degrees I can hit 80 -82 deg F in the tent mid day with the fan controller set at 7 out of 10. You can really hear the restriction at the 4" exit screen so I am not sure if I am even getting any more airflow turning the speed up further due to the back pressure.

I know flex ducting has a lot of restriction and then the screened vent cuts the exit surface area. My question is if I used a 4" to 6" adapter coming off the infinity fan and then ran the exit though 6" flex ducting and a 6" screened vent would I get a reasonable increase in air flow? I know the CFM flow through the ducting would likely decrease but I assume total airflow would go up with less restriction in the exit ducting. Any thoughts on this?
Think of it like a garden hose. The smaller the discharge the higher the velocity and less volume. The larger the discharge there is less velocity but higher volume. Personally I did the opposite of what you are suggesting. I upped the exhaust fan to 6” and discharge through a 6” to 4”’reducer. I took the 4” fan I replaced and used that for a supply. Both are plugged in to the same ac infinity 67 controller with fan speed set to minimum of 4 and max 10. Works great.
 
Ozmosis

Ozmosis

33
18
Yep I definitely get the garden hose analogy and volume of flow vs speed of flow. Just wondering about how much gain I would get going this route. Also curious why you would up your exhaust fan size but then restrict the volume of flow with smaller ducting. The difference between flow volume through a 4 inch duct vs a 6 inch duct is fairly significant.
 
freezeland2

freezeland2

2,540
263
Yep I definitely get the garden hose analogy and volume of flow vs speed of flow. Just wondering about how much gain I would get going this route. Also curious why you would up your exhaust fan size but then restrict the volume of flow with smaller ducting. The difference between flow volume through a 4 inch duct vs a 6 inch duct is fairly significant.
I did it to keep the heat down. The 4” running exhaust with passive intake just wasn’t enough. I went with reduction from 6 to 4 inch because I am utilizing an extra dryer duct I have to exhaust to the exterior of the house. Wasn’t going to demo my wall to up size that existing duct. It’s working fine like this. Plenty of air exiting the house. Zero odor in my basement.
 
Ozmosis

Ozmosis

33
18
OK I understand. Wanted to make sure I wasn't missing some trick I hadn't heard of lol. Glad to hear it's working good for you. What size tent are you in?
 
MacroLogos

MacroLogos

214
43
You can really hear the restriction at the 4" exit screen so I am not sure if I am even getting any more airflow turning the speed up further due to the back pressure.

Can you remove the screen, or change it to screen with larger holes?

Turning up the fan *will* increase CFM.. and pressure. Just not as much as removing that screen would.

I know flex ducting has a lot of restriction and then the screened vent cuts the exit surface area. My question is if I used a 4" to 6" adapter coming off the infinity fan and then ran the exit though 6" flex ducting and a 6" screened vent would I get a reasonable increase in air flow? I know the CFM flow through the ducting would likely decrease but I assume total airflow would go up with less restriction in the exit ducting. Any thoughts on this?

Going from 4" to 6" the whole run won't hurt CFM, but it won't help.

You started off with a bottleneck of 4", nothing you do with duct work after the point of that bottleneck will ever make it bigger, but you can eliminate making it smaller (since the bottleneck is the fan intake port itself, adding 6" duct work on the exhaust port won't change the size of the 4" intake port... see?).

You created a new bottleneck with the screen on the final exhaust.. so it's no longer even 4", its like 3.765" or something less than 4" (just an example, that number is fictitious).

Right at the end, you could go from a 4" to a 6" duct, and then the screen would be 6"... diminishing this new bottleneck. However, consider going 4" -> 8", due to the fact that screen can actually block 50%+ of the area depending on the screen.

Why screen the exhaust at all tho?
 
Last edited:
MacroLogos

MacroLogos

214
43
Going from 4" to 6" the whole run won't hurt CFM, but it won't help.
As I think on it a bit further, I am wrong here. It WOULD lower CFM to do this. Here is why:

The internal surface area of 6" flex duct is larger than 4". The surface is not smooth... and due to this... there will be more 'drag' and turbulence created in the 6" duct run.
 
Ozmosis

Ozmosis

33
18
Hey Macro. Thanx for helping with the brainstorming. I know the screening could be cut out or removed but then if I lose power, which happens from time to time, then the system is open to things wandering in. I do think at the least I am going to put a 4 in to 6 in adapter at the end of the run so I can put a 6 inch screened output port on the end instead of the 4 inch. Interesting point about the larger surface area causing more turbulence, but the fact remains that a 6 inch flex duct can pass more air than a 4 in. Even though my initial bottleneck is at the 4 inch exit from the fan the flow then enters the 4 inch flex duct which incurs a larger friction loss than the smooth 4 inch exit of the fan. The flow loss through a flex duct is more than the flow loss through a smooth duct. This is why I was thinking it may be helpful going up to 6 inch as it least it should reduce the further restriction the 4 inch flex is incurring over the fans smooth 4 inch exit port.
 
MacroLogos

MacroLogos

214
43
Hey Macro. Thanx for helping with the brainstorming. I know the screening could be cut out or removed but then if I lose power, which happens from time to time, then the system is open to things wandering in. I do think at the least I am going to put a 4 in to 6 in adapter at the end of the run so I can put a 6 inch screened output port on the end instead of the 4 inch. Interesting point about the larger surface area causing more turbulence, but the fact remains that a 6 inch flex duct can pass more air than a 4 in. Even though my initial bottleneck is at the 4 inch exit from the fan the flow then enters the 4 inch flex duct which incurs a larger friction loss than the smooth 4 inch exit of the fan. The flow loss through a flex duct is more than the flow loss through a smooth duct. This is why I was thinking it may be helpful going up to 6 inch as it least it should reduce the further restriction the 4 inch flex is incurring over the fans smooth 4 inch exit port.
It's not just about how much air it can pass... DRAG, bends, blocks (friction) are major factors in CFM. You're right about the larger size, but the amount of additional drag needs to be considered as well.

It's a really short run tho, so if you can keep the 6" pulled a bit tight so it's smooth, it could work... but the big issue is the screen. We're stepping on dollars to pick up pennies with this.
 
Ozmosis

Ozmosis

33
18
It's not just about how much air it can pass... DRAG, bends, blocks (friction) are major factors in CFM. You're right about the larger size, but the amount of additional drag needs to be considered as well.

It's a really short run tho, so if you can keep the 6" pulled a bit tight so it's smooth, it could work... but the big issue is the screen. We're stepping on dollars to pick up pennies with this.
I hear ya. Got a 6 inch exit port on order and will at least change out that restriction to a lighter one for starters.
 
MacroLogos

MacroLogos

214
43
Failing that, you might need a second 4" fan...

However, 82° is not horrible, and if you can adjust humidity, and if the duct change doesn't help drop the temperature... you can start managing humidity to keep the VPD in the green zone:
 
VPD Chart page 001
Ozmosis

Ozmosis

33
18
Good thinking on manipulating other variables, but I am about 2 weeks into flower and want to control my humidity at or below 50%. I know 82 degrees isn't terrible, it is just sort of the line in the sand I drew as the max temp I wanted to hit. I bought the 4 inch fan for a 36 x 20 x 60 tent in which it was adequate but I could see early on that my Xtreme auto was going to get to big for the space, so upgraded the tent to 55 x 28 x 80 which is pushing the limits on the 4 inch fan. Dutch Passion says Xtreme auto can go 4 to 5 feet and she is going to I think. I've done some LST to bring the height down which the larger tent has given me the room to lower and spread the tops out.
Really if the duct change doesn't get me where I want to be I may move to a 6 inch exhaust fan. I'm just trying to save the hassle of swapping a fan mid grow.
 
Blastfact

Blastfact

533
93
Was in the HVAC world for well over 30 years and a manufactures rep for the last 20 years specializing in pumps, boilers, chillers, blower, duct, pipe design, build. Your absolute biggest issue is the carbon filter on the suction side of the fan. Fans/blowers are pumps nothing more and nothing less. Everything you attach to the fan has a pressure drop across it. With the largest pressure drop being the carbon/pre filter and putting it on the suction side kills the fan curve. Basic fan/blower curves are created with nothing attached to the fan/blower. And none of the manufactures will supply you with a real fan curve. You never restrict a fan or pump on the suction side. With you wanting to exhaust outside your screwed. Going to 6" flex duct on either side of the fan will only help if you have a tight radius bend in the flex duct that is cutting the inside dia. of the flex duct down. By putting the carbon filter on the intake side of the fan the cfm/discharge pressure is pretty much cut in half. There is no magic bullet or engineering creative math that's going to correct the issue over all. With the carbon filter on the suction side a low NPSH situation has been created meaning cavitation and other ugliness is taking place in the fan housing and fan blades. I've attached a copy of a basic POS fan curve I got from AC Infinity concerning my two 6" units. Your 4" fans curve will be very much like this one except it will be a max cfm of 200 or so with a max of maybe 300 mm-H2O if your lucky. Remember this curve is created on just the fan assembly and nothing attached to it. It is spec'd to do 400 cfm max @ 0 mm-H2O and the point of no air flow is lets say 520 mm-H2O @ 0 cfm. If I put AC Infinity's 6" carbon filter with prefilter on the fan unit on the suction side this curve mathematically is then destroyed and near meaning less. So using a manometer I found the fan curve to change drastically to 200 cfm @ 200 mm-H2O. Lost cfm and head pressure,,, just gutted it. I then sewed one end closed on the prefilter and put it inside the carbon filter and slipped the filter on the discharge side of the fan. It then produced 275 cfm @ 225 mm-H2O and the fan is back on the curve and not in a low NPSH situation. And thats how I'm setup. I run a short 6" flex duct inside the tent put the carbon filter on the discharge of the fan with the prefilter inside the carbon and toss it on top of the tent. This quiets the fan down because all cavitation is removed because the fan blades are allowed to load properly to make head pressure which makes it better for cfm against the filter pressure drop. As the prefilter gets dirty the head pressure goes up because the fan is loaded correctly but cfm does go down and it then stays close to this over simplified curve. The whole point of a pump/fan is to create differential pressure. When you create that differential pressure, water, air, vapor what ever your pumping moves. I've also attached a Bell & Gossett pump curve to see what should be on that simpleton AC Infinity curve. If your fan as a way to set speed settings you can look at the real pump curve and get a feeling of how the fan curve is changing at different speeds by looking at the horse power and impeller trims. RPM is constant speed on the pump curve. If your eyes gloss over and you begin to feel like a deer in headlights. Your not alone. I've seen many mechanical engineering grads not understand one bit of this stuff as they size pumps and blowers. Good Luck!

AC curve

IMG 7902

Room2
 
Ozmosis

Ozmosis

33
18
Thanx for all the info Blastfact. Good stuff. The charts do look chaotic. They remind me of the impedance curve charts for vacuum tubes in vacuum tube amps. I do pretty much get the gist of what they are saying. I did not know, and am surprised to hear of the carbon filter being more restrictive on the input side. This goes against all the info that most of the fan companies are putting out in their literature as they mostly say the preferred way to use the filter is on the intake side. If the fans don't like the load on the intake, you would think the manufacturers would point that out so their customers would see better performance. I think AC Infinity does recommend calculating a 50 or 60% load penalty for their carbon filter, but they don't mention that penalty would be less on the exhaust side of the fan.
I do not have to vent out the window. I started the 1st couple weeks venting in to the room before I put the filter on as it was not needed for odor the 1st few weeks. Even with a dehumidifier running I noticed the room air was stale and a bit thick in the room. I really didn't want to recycle this air back to the plants. Based on some reading I did I decided to vent the output to the outside once I put the filter on somewhere around week 3/4. I also upsized my tent at that point. So I simultaneously increased my restrictions with the extra duct and filter while increasing my tent size. I probably should have upgraded the fan as well but was hoping to be done with the setup costs at that point. I thought I could squeeze a bit more performance out of the fan, but I may end up just needing the 6"
 
Blastfact

Blastfact

533
93
Thanx for all the info Blastfact. Good stuff. The charts do look chaotic. They remind me of the impedance curve charts for vacuum tubes in vacuum tube amps. I do pretty much get the gist of what they are saying. I did not know, and am surprised to hear of the carbon filter being more restrictive on the input side. This goes against all the info that most of the fan companies are putting out in their literature as they mostly say the preferred way to use the filter is on the intake side. If the fans don't like the load on the intake, you would think the manufacturers would point that out so their customers would see better performance. I think AC Infinity does recommend calculating a 50 or 60% load penalty for their carbon filter, but they don't mention that penalty would be less on the exhaust side of the fan.
I do not have to vent out the window. I started the 1st couple weeks venting in to the room before I put the filter on as it was not needed for odor the 1st few weeks. Even with a dehumidifier running I noticed the room air was stale and a bit thick in the room. I really didn't want to recycle this air back to the plants. Based on some reading I did I decided to vent the output to the outside once I put the filter on somewhere around week 3/4. I also upsized my tent at that point. So I simultaneously increased my restrictions with the extra duct and filter while increasing my tent size. I probably should have upgraded the fan as well but was hoping to be done with the setup costs at that point. I thought I could squeeze a bit more performance out of the fan, but I may end up just needing the 6"
The filters level of pressure drop is the same be it on the suction side or the discharge side of the fan. That number does not change, a pressure drop is a pressure drop. How the fan reacts to the pressure drop is what's important. If on the suction side the fans performance is gutted because the fan and housing can not be loaded properly. If the filter is on the discharge side of the fan and the suction side is open with no more restriction than the tent vent panels present then the fan blade and housing can load up properly and make cfm and discharge pressure. With the fan loaded properly the fan can make better discharge pressure to combat the pressure drop restriction of the filter. Most of the company's suggest the filter on the suction side for ease of use,,, plain and simple. Some will show discharge into the filter as a secondary way of setup. But never truly touch on the sizing, pressure drop. Remember your garden is not producing stale air. Your plants are feeding on CO2 in your air and making O2. They are scrubbing and filtering the air. Is it dramatic? No, but that is what they are doing. In my setup the return air filter and duct is a foot away from the door of my office. So any humidity that is exhausted from the grow space is going right into the central A/C and improving the A/C units wet bulb performance a small amount. So outside the A/C unit is pissing like a race horse on a flat rock and maintaining a perfect RH for the wife and I in the home at around 40% and have a very good temp differential of 20 to 21F degrees. That means I have 70F air hitting the return air filter and the A/C is putting out 50F air at a central located register. With the evap coil pissing the condensate out of the unit,,, ie: dehumidification the condensate is carrying away impurities in the air as the water catches them and they flow out of the unit into one of the wifes flower gardens.
 
Ozmosis

Ozmosis

33
18
OK. I think I get the whole fan thing. Basically the blades are designed to push into a loaded air space to the front of the fan. The fan naturally pulls air through it's back in this process, but the blades of the fan are not designed to pull a load. They will do it, but incur an additional hit in performance as they are engineered and oriented to be more efficient at pushing a load than pulling it. Is that about it?

That is a really nice synergy you have going in your HVAC setup, but of course one should expect no less from a lifetime HVAC guy. :-) I do not have central AC as I am in the PNW and it is uncommon here as we don't spend many days a year over 85. I am using a 6500BTU window AC to cool a smaller bedroom that the tent is in. I did find a quick fix to knock the temp down a couple degrees. I have an Infinity 6" Raxial booster fan. I had tested it blowing in a bottom port on the tent a while back and found little to no change in temp so I use it on a window vent to pull in night time air which has been around 60 degrees F lately. I got some MERV 8 filter fabric this week and strapped it over the duct and can now run the filtered night air into the bottom vent. Not tightly sealed but just with the flex duct set up loosely to the port to keep back pressure off the booster fan. During the day I had just been relying on the AC and passive vent, but found if I set the booster fan on a stool near the AC discharge it knocked a couple more degrees off my day temp. The downside is I am venting a good portion of my AC through the tent and out the window. I wonder if it would be more efficient on my main fan to set up to push though the carbon filter externally and put the filter near the window AC so the AC clips the humid air on the front end. What do you think?
 
MacroLogos

MacroLogos

214
43
Was in the HVAC world for well over 30 years and a manufactures rep for the last 20 years specializing in pumps, boilers, chillers, blower, duct, pipe design, build. Your absolute biggest issue is the carbon filter on the suction side of the fan. Fans/blowers are pumps nothing more and nothing less. Everything you attach to the fan has a pressure drop across it. With the largest pressure drop being the carbon/pre filter and putting it on the suction side kills the fan curve. Basic fan/blower curves are created with nothing attached to the fan/blower. And none of the manufactures will supply you with a real fan curve. You never restrict a fan or pump on the suction side. With you wanting to exhaust outside your screwed. Going to 6" flex duct on either side of the fan will only help if you have a tight radius bend in the flex duct that is cutting the inside dia. of the flex duct down. By putting the carbon filter on the intake side of the fan the cfm/discharge pressure is pretty much cut in half. There is no magic bullet or engineering creative math that's going to correct the issue over all. With the carbon filter on the suction side a low NPSH situation has been created meaning cavitation and other ugliness is taking place in the fan housing and fan blades. I've attached a copy of a basic POS fan curve I got from AC Infinity concerning my two 6" units. Your 4" fans curve will be very much like this one except it will be a max cfm of 200 or so with a max of maybe 300 mm-H2O if your lucky. Remember this curve is created on just the fan assembly and nothing attached to it. It is spec'd to do 400 cfm max @ 0 mm-H2O and the point of no air flow is lets say 520 mm-H2O @ 0 cfm. If I put AC Infinity's 6" carbon filter with prefilter on the fan unit on the suction side this curve mathematically is then destroyed and near meaning less. So using a manometer I found the fan curve to change drastically to 200 cfm @ 200 mm-H2O. Lost cfm and head pressure,,, just gutted it. I then sewed one end closed on the prefilter and put it inside the carbon filter and slipped the filter on the discharge side of the fan. It then produced 275 cfm @ 225 mm-H2O and the fan is back on the curve and not in a low NPSH situation. And thats how I'm setup. I run a short 6" flex duct inside the tent put the carbon filter on the discharge of the fan with the prefilter inside the carbon and toss it on top of the tent. This quiets the fan down because all cavitation is removed because the fan blades are allowed to load properly to make head pressure which makes it better for cfm against the filter pressure drop. As the prefilter gets dirty the head pressure goes up because the fan is loaded correctly but cfm does go down and it then stays close to this over simplified curve. The whole point of a pump/fan is to create differential pressure. When you create that differential pressure, water, air, vapor what ever your pumping moves. I've also attached a Bell & Gossett pump curve to see what should be on that simpleton AC Infinity curve. If your fan as a way to set speed settings you can look at the real pump curve and get a feeling of how the fan curve is changing at different speeds by looking at the horse power and impeller trims. RPM is constant speed on the pump curve. If your eyes gloss over and you begin to feel like a deer in headlights. Your not alone. I've seen many mechanical engineering grads not understand one bit of this stuff as they size pumps and blowers. Good Luck!

View attachment 1261677
View attachment 1261682
View attachment 1261683
Great post. I had ignored the carbon filter because it is not an optional component... but I had no clue there would be such a drastic hit to the cfm due to the fan blades being optimized to push.

We cool our shop using "5 gal bucket" type swamp coolers... and the radiator fans we use must be rigged to pull air, not push. They designed the fan blades & polarity on the circuit to simply be flipped to achieve this.

It would be nice if AC infinity would evolve their fans to provide this option. It would be so cheap to change fan blade design and optimize a fan to pull rather than push and sell both in a package with that dual fan controller.

We use a dual 6" fan setup with that AC 76 controller, and I just presumed the fans would be setup to push or pull... but it makes sense to optimize to one job or the other.
 
Blastfact

Blastfact

533
93
Great post. I had ignored the carbon filter because it is not an optional component... but I had no clue there would be such a drastic hit to the cfm due to the fan blades being optimized to push.

We cool our shop using "5 gal bucket" type swamp coolers... and the radiator fans we use must be rigged to pull air, not push. They designed the fan blades & polarity on the circuit to simply be flipped to achieve this.

It would be nice if AC infinity would evolve their fans to provide this option. It would be so cheap to change fan blade design and optimize a fan to pull rather than push and sell both in a package with that dual fan controller.

We use a dual 6" fan setup with that AC 76 controller, and I just presumed the fans would be setup to push or pull... but it makes sense to optimize to one job or the other.
There is no true optimization of a fan blade, blower wheel, open or closed impeller that will make a pump if you will increase it's ability to go into a vacuum and maintain good cfm/gpm and discharge head pressure. Which is what we are doing when we put a carbon filter on the suction side of a fan. We are now demanding it be a vacuum pump and they are very poor at doing that. This is all iron laws of physics. The only way this can be made better is with wet ring tech which would require much more hp and a way to control either a oil or water wet ring so more vacuum could be pulled. And then make the fan or pump a min. of two stages. You don't want that monster in your grow space. What they are doing at your work for cooling is old school evaporative cooling. No different than say a cooling tower where there pulling heat out of the condenser side of a chiller system. Or as you said a swamp type cooler. If those types of systems are engineered/sized correctly and ran properly they do develop a type of wet ring seal between the fan tips and fan housing. It's known as the vapor ring. But it's hard to maintain the perfect vapor ring. To little vapor ring and eff. goes down. A little to much and you have just enough water at the blade tips the the water starts cutting a groove in the fan housing. And way to much blows water out of the cooling tower cell straight up in the air causing increased water and chemical usage. You can set a common blower like we use in our exhaust fans and do a simple test with your hands or a couple pieces of cardboard. Put the fan on a table or in your lap. If in your lap you can feel cavitation and harmonics better. On a table you can hear them better. Fire the blower up with no filters on it, open suction and discharge with the speed controller set at full speed. Now just feel the air moving through the blower. Now block half of the suction side with your hand or cardboard and feel what you lost in discharge volume/cfm and pressure. Now block half of the discharge side of the fan and feel how much pressure and air you feel. Thats what a carbon filter with prefilter is doing to your airflow in simple and realistic terms. If you have a monometer and can measure the difference you can plot it on a simple curve.
 
MacroLogos

MacroLogos

214
43
There is no true optimization of a fan blade, blower wheel, open or closed impeller that will make a pump if you will increase it's ability to go into a vacuum and maintain good cfm/gpm and discharge head pressure. Which is what we are doing when we put a carbon filter on the suction side of a fan. We are now demanding it be a vacuum pump and they are very poor at doing that. This is all iron laws of physics.
So you don't find a fan blade with a "foil" design (similar to how modern sail boats use air flowing over a sail's surfface to "pull" the boat, rather than using it to "push" it, and air plains use this same principle in wing design to create lift), to be creating a vacuum?
 

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