I too have high static pressure
This is sort of a continuation of: http://hvac-talk.com/vbb/showthread.php?t=176863
My readings are (with a wet coil):
Stage 1 -- +.34", -.56" (set to 80% of 1200cfm)
Stage 2 -- +.44", -.65" (set to 1200 cfm)
So my return is a bigger issue right now, although I'm guessing increasing the return will lower negative but increase positive static? (I'm also planning on asking them to increase at least 3 of the 6" runs to 7" in the main living areas, so that should help. Enough?)
I've attached a picture of my return. It's an 8"x24" drop, connected to a curved ell, which is also 8"x24". The ell connects to a long trunk, which is 8"x16" on one side, and 8"x10" on the other. All returns feed into this trunk.
My question is...even if I increase the size of the ell and the drop, I'm only getting 8"x26" from the trunk, the way it's connected. I wondering how to increase the flow without having to rip out the trunk and replace it with a larger one. I've heard of using helper ducts on supply runs, can that be done on a return as well? I haven't spoken to my installer yet on the duct changes, I'm hoping to get some ideas of options before I hear from them.
What is the actual cfm you are getting?
8X24 should be close for 1200cfm.
What type of filter / filters are in system.
looks like a high static for 8X24 return duct for 1200 cfm
granted it could be a bit larger but doubt thats your main problem.
What is pressure drop across filter/or filters?
If this duct is lined you could have a loose piece of liner blocking the air flow. (wont hurt to look)
Last edited by obxtech; 07-23-2008 at 10:21 AM.
Not a good pic of teh whole return set up.
But, I would say its the way the drop is tapped into the horizontal run.
Hmmm, I'd be VERY happy if I have another problem, lol. It's a CleanEffects filter, according to the specs from the installer manual it should be creating a .19" PD at 1200 CFM. I don't have actual measurements for before/after the filter, but I can ask for those. Are CleanEffects known to cause higher PD than their stats suggest? (It should still be very clean, it's had less than 10 days of runtime since its installation.)
Actual CFM hasn't been measured, but according to the blower performance data, at .9" it would be pushing 1130CFM (when set to 1200.)
The return elbow does have turning vanes, I saw them in there before they installed.
Here are a few more that may give a better picture.
beenthere, how would you make that connection?
If the ell at teh air cleaner has vanes, then its about as low a pd as you will get there.
But, were you drop ties into the horizontal is very restrictive.
How would you make that transition? The installer did actually build that piece of it from scratch; it's the only part of the returns that they touched -- so I'm guessing I'm going to have to offer a suggestion as to how to change it. You can sort of see in the third pic where the metal changes. The horizontal curved ell is old, everything after it is new.
Originally Posted by beenthere
I know it's hard to say, but what would your ballpark guess be at how much the PD could be lowered by adjusting that transition?
A vert. ell with vanes and reverse transition would be a better connection.
or relocate the return "T" to line up with the return drop and use 1 vert ell with vanes.
Sort of like this...
Originally Posted by skunkiechris
Ah, that clarifies it. I was having trouble envisioning it. That makes sense, makes me wonder why it wasn't done like that to begin with....
That should cut the PD in half at that transition.
I think this is what obxtech had in mind...
Of course that looks like the best option -- that's exactly what my return trunk line looks like. However, my furnace is further down the line. If they were to tap into it and go straight down, it would actually be byeond where the trunk line narrows. I don't know why it's like that, or which happened first -- the original furnace was put there, or the return line was built, as I inherited both.
Originally Posted by joatmon123
Is going with a better transition like the first image likely to provide a significant enough decrease in PD? Or does it look like I really need to have that trunk re-engineered to allow the furnace to tap directly off of it?