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Old 08-08-2019, 06:36 AM   #1
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A Technical HVAC Question

With any ducted forced air system; residential, commercial, RV, etc., why is it that when you close registers to try to gain more CFM from other registers left open, you are never able to get all of that air that you just stopped moving to come out of the registers you left open? But, if you have a damper close to the air handler/fan and you close it to stop flow to those same registers, most of that air is directed to the open registers. I know there are scientific reasons because when I was a teenager, I worked as a helper for a relativeís air balancing company, but I didnít learn or retain much of this information. I know it has a little to do with air loss from joints, but I think it is mainly due to something related to static pressure. When we were balancing a buildingís system, the main things I remember about getting the right amount of air to the right places was adjusting the fan speed by adjusting belt pulleys, then adjusting dampers. Only fine-tuning was done at the registers once the correct CFM was obtained for each branch or VAV box.

If I was to close every register in my RV except for one or two, I notice very little difference in air flow at the registers I have left open. Why is this? Why is a closed damper (I wish RVs had these), close to the air source more effective at redirecting air than just closing registers?

Bruce
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Old 08-08-2019, 06:42 AM   #2
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My guess is the outlets you are leaving open are somewhat choked to begin with.
Far down the line, outlet not completely lined up with the trunk line that's molded into the styrofoam in the ceiling?
Also might have leakage elsewhere so the air is taking the path of least resistance.

I know my ceiling unit has mediocre flow in the bedroom ceiling but if I open the vents on the ceiling unit itself it will literally blow your hair around if you stand under it.
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Old 08-08-2019, 06:48 AM   #3
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My guess is the outlets you are leaving open are somewhat choked to begin with.
Far down the line, outlet not completely lined up with the trunk line that's molded into the styrofoam in the ceiling?
Also might have leakage elsewhere so the air is taking the path of least resistance.

I know my ceiling unit has mediocre flow in the bedroom ceiling but if I open the vents on the ceiling unit itself it will literally blow your hair around if you stand under it.
Iím certain everything you just said is very likely with an RV, but I know the same thing will happen with well-designed and well-sealed systems in all scenarios.

Iím just curious, and I know there are people here who are experts with this stuff.

Bruce
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Old 08-08-2019, 07:20 AM   #4
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If the air velocity out of a single outlet (after the others are shut) doesn't increase you have a blockage in the ducts which are rather small anyway. Our ducted system has a shutter on the bottom for an "air shower" when standing directly underneath. Massive amount of air comes out. If I close all but one ceiling outlets on mine the air comes out of the remaining out at a higher velocity but not as much as the air shower.

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Old 08-08-2019, 08:06 AM   #5
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You would be surprised how much more air flow you get from the ceiling vents if you block off the channel going away from the last vent opening. I blocked the 4 last vents to keep the air from going into dead space. Much more air flow.
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Old 08-08-2019, 08:53 AM   #6
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Your issue is your measuring device.

In the car world we refer to it as the butt Dyno! Not very accurate.

Your central fan produces so many cfm. If you measure every outlet and add them up you will come up with less than whatever the central unit supplies. That is duct imperfection losses.

The more you close the more loss thru leaks. Higher pressure in the system.

None of which your dyno can measure.

The better the design the less throttling thru diffusers the better.
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Old 08-08-2019, 02:55 PM   #7
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I just happen to be a licensed HVAC contractor that has been doing this since 1982. This is what is called sick duct work in the commercial field. With out turning vanes all you are doing is creating back pressure in the duct work especially if you are trying to push air to the farthest vents which could actually cause damage to the duct and the A/C unit. I have the same issue in my trailer as there are no standards or codes for mobile HVAC. I have attach a fan in our bedroom to the vent that draws air out of the duct and actually over cools the bedroom now but it keeps the wife happy. I don’t have a real fix for it due to the inaccessibility of the duct work but the small 12 volt fan is working for me.
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Old 08-08-2019, 04:00 PM   #8
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I just happen to be a licensed HVAC contractor that has been doing this since 1982. This is what is called sick duct work in the commercial field. With out turning vanes all you are doing is creating back pressure in the duct work especially if you are trying to push air to the farthest vents which could actually cause damage to the duct and the A/C unit. I have the same issue in my trailer as there are no standards or codes for mobile HVAC. I have attach a fan in our bedroom to the vent that draws air out of the duct and actually over cools the bedroom now but it keeps the wife happy. I donít have a real fix for it due to the inaccessibility of the duct work but the small 12 volt fan is working for me.
Sort of like the difference between pushing or pulling fish tape around sharp corners. Itís much easier to pull it than it is to push it.

Bruce
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Old 08-08-2019, 04:43 PM   #9
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Squirrel Cage blowers quickly get less efficient as back pressure goes up. They are designed for high volume/low pressure use.
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Old 08-08-2019, 04:55 PM   #10
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"rwhit's" explanation is a good one. I have past experience working with industrial HVAC applications and whit hot the nail on the head. I've seen large, 48" x48", duct work that we just could not get enough COM out of the end of the run although the engineer's calculations states we should have plenty. We've had to install booster fans within a duct run to get personnel happy.
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Old 08-08-2019, 05:26 PM   #11
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Some RV systems have a SAIL switch to shut down if the flow is too low and you could damage a furnace!!!
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Old 08-24-2019, 11:16 PM   #12
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Quote:
Originally Posted by rwhit View Post
I just happen to be a licensed HVAC contractor that has been doing this since 1982. This is what is called sick duct work in the commercial field. With out turning vanes all you are doing is creating back pressure in the duct work especially if you are trying to push air to the farthest vents which could actually cause damage to the duct and the A/C unit. I have the same issue in my trailer as there are no standards or codes for mobile HVAC. I have attach a fan in our bedroom to the vent that draws air out of the duct and actually over cools the bedroom now but it keeps the wife happy. I donít have a real fix for it due to the inaccessibility of the duct work but the small 12 volt fan is working for me.


We have the same problem with our trailer. Not to mention that the ac just sucks. Would you mind giving me details on the type of fan you use? Iíd like to put one in ours. Thanks so much!
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Old 08-27-2019, 07:35 AM   #13
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Because as you close registers you are trying to force more air through the other registers, which increases the air velocity and increases the static pressure in the duct system.

As the external static pressure of the system increases the design point on your fan curve moves to the left up the curve (known as the fan riding the curve in the HVAC industry) and the total airflow (CFM) reduces. If the static becomes great enough to move the operating point to the very left side of the curve the fan will spin without moving any air, this is referred to as fan "stall".
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Old 08-27-2019, 07:56 AM   #14
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Following... I too would like to install a fan to pull air into bedroom.
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Old 08-27-2019, 08:14 AM   #15
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This is the fan I bought and wire tied to the vent. I got power from the ceiling light next to the vent
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Old 08-27-2019, 12:22 PM   #16
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I totally agree with this but after being a trainer and customer service for years I try to make it where people outside of the trade can understand the explanation but you are right on point.
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