ReplyToo high of velocity, the air just blows on by. Add a scoop.
Oops, hope that aint DIY. If so a mod can remove and spank me.
Professional Member
Today i had a water source heat pump feeding a classroom and 3 offices. It has a main trunk line with 8" taps coming off of it. There are a total of 7 taps on this unit. 4 for the classroom and 1 for each office. Well the problem i am having is the one diffuser nearest to the unit isn't putting out hardly any air. I pulled the duct off and inspected inside the duct. At this point in the duct, it is like there is a dead spot. The air is just turbulating around the opening. This duct tap comes off right after the flex connector. It is the first fed. The diffusers that come off further down the main get plenty of air. Why is this? What can i do to the ducting to fix this problem? The office with the problem has the stat. So it is causing no heat or cool to go into the room. So it either burns everyone out or freezes everyone out depending.
Thanks for the help!
J. King
Professional Member*
Too high of velocity, the air just blows on by. Add a scoop.
Oops, hope that aint DIY. If so a mod can remove and spank me.
Politicians need to be changed like diapers, and for the same reason.
Mark Twain
Professional Member
How many people have messed with the downtream dampers to get more/less heat or cooling in their office?
A simple professional re-balance may help things. Either that or it may not have been properly balanced in the first place.
___________________________
-supertek65Chicago is an indian word for stinky!!!!!!
Professional Member
How far away is this duct from any changes in direction?
Is there a long section of straight duct before the tap or is it right after a change in direction?
Do you have shoe taps?
Professional Member
Too high of velocity, the air just blows on by. Add a scoop.
Oops, hope that aint DIY. If so a mod can remove and spank me.
Yea i was thinking that, but i just didn't know how much that would help. I haven't used them that much. I heard something from a sheet metal guy stating that the air is more even at the end of the trunk.
How many people have messed with the downtream dampers to get more/less heat or cooling in their office?
A simple professional re-balance may help things. Either that or it may not have been properly balanced in the first place.
Actually every damper is open all the way down the trunk line.
How far away is this duct from any changes in direction?
Is there a long section of straight duct before the tap or is it right after a change in direction?
Do you have shoe taps?
The main duct just comes straight off of the heat pump, goes into a flex connector, and is just a straight run all the way to the end cap. The duct doesn't change directions or switch sizes. The tap with the problem is right up next to the discharge of the unit. There is no long straight run before the taps.
Shoe taps? Are they taps with scoops?
J. King
Professional Member*
Might want to relocate the takeoff further down the duct where there is less turbulence and and the velocity has slowed, probably the easiest solution.
Politicians need to be changed like diapers, and for the same reason.
Mark Twain
Professional Member
How long is the plenum?Originally Posted by apprentice3
Usually with that many take offs and a long plenum, it's best to reduce it. A scoop would help, but a new properly sized plenum would be the best solution.
Professional Member
The rule of thumb I use is the first tap has to 3 times the diameter of the duct away from the discharge of the unit. So if the duct is 8" dia, then your first tap is set at 24" away from the unit. This also applies to change of directions.
Professional Member*
Is there a transition in duct size anywhere along the main trunk?There should be after a few taps are taken off.This will add pressure to your outllet.
Professional Member*
I'd be a little uncomfortable with a "scope" that might really disrupt the high velocity air stream coming off the unit as per your first thought
You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!
Professional Member*
+1
Professional Member*
I kinda missed the turbulence word first time around. That being said, I have seen it many times with high velocities that the air just rushes on by the first branch.
Politicians need to be changed like diapers, and for the same reason.
Mark Twain
Professional Member*
Sounds like you have no static pressure. Too many openings or too many air exits to allow for static pressure to build up and push the air out of the closest registors. Close down some of the dampers until you get static pressure in the trunk. I Should think you would need a 1/2 inch or so of pressure at 2/3 down the trunk run, on a unit that size.
Professional Member*
But don't you agree that close to the unit discharge we have all velocity pressure and won't have any static til the airstream slows down and we see the static regain we are looking for?
You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!
Professional Member*
A number of years ago we installed a make-up air unit for an industrial printing press. The mechanical insisted on a high velocity supply since the run was short and the we had plenty of fan on this extended plenum kind of design. well the air was moving around 1800 FPM and actually sucked air in on the first supply register. Just thought of that looking at your post. Funny huh?
You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!
Professional Member*
Static pressure should be at its higest at the fan exist and at its lowest at the end of the run, this is due to friction loss through the duct. The duct internal area and the number of exits need sized and balanced so as the air supplied to the duct is slightly higher than the air leaving the duct. This creates static pressure. Static pressure forces the air out of the registors. With out static pressure, the air will travel down the duct and the velocity pressure will force the air out at the end runs only. You need to have a pressurised duct to get even air flow to all supplies. How much static varies from type of system.
Professional Member*
Velocity pressure only exists with static pressure.
You can have static without velocity though.
Professional Member*
I agree with your quote but without motion what would be the point of a system that wasn't delivering anything?
Me and Mr Bernoulli Think of TOTAL Pressure the same way you think of WATTS You know Volts time Amps. Key being the watts stay the same and as the velocity slows the static builds. Now that doesn't mean there is all velocity pressure at the discharge just mostly velocity pressure.
That's my understanding of the Physics of the situation. If you see any flaws or have a simpler explanation, you know me, I am always open to being educated
You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!
New Guest
Is this a new unit? Has the unit and duct been sized correctly? Has it been balanced?
Posting Permissions