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07-21-2008, 11:11 AM #1
New ducts to lower static pressure
Supply duct installation is almost complete. The system is a Trane XV95-100k btu/XL15i -4 ton system.
The history can be found at an earlier thread called “Is 0.5 Max static Pressure a realistic target” by Key1cc. Here is the link.
In summary I had 3 trunk-ducts coming off the plenum one 12x9 out the top, one 8x9.5 out the back (that turned out to be ~ 8 x 7 because the installer used a flexible rectangular extender that folded into the air-stream), and one 12 x 8 trunk coming out the front (that I kept the damper closed because it led to the garage and laundry area where I do not require AC). My system had 16 total round runs off the 2 trunks that were open. All 16 lead to room registers. 13 were 5 inch and three were 6 inch. All static pressure (SP) was checked with factory filter after filter in blower bay and after blower before coil and with CFMs set to 350/ton. The SP with all dampers open (including the garage area front trunk) was 0.6 pos. and 0.32 neg which is a total of 0.92. This setup caused the upstairs to be 10-12 degrees hotter than the down stairs so the system was balanced with three 5 inch dampers closed and the front plenum damper to the garage closed. As a result the upstairs was now only 3-4 degrees different than down stairs but the static pressure jumped to 1.1 (0.8 pos and 0.3 negative).
Now for the current situation.
The 12x9 duct trunk coming out the top was not changed (at the plenum).
The 8x 9.5 partially blocked trunk coming out the back of the plenum was converted to a 16x 8 for the 1st 5 feet then transitioned back into the 8 x 9.5 to continue supplying three 5” runs and one 6” run.
The 12x8 that went to the garage was disconnected and the plenum opening was converted to a 14x14 four ft trunk off the plenum that made a 90 degree angle turn using 16x12 trunk for 5 ft then transitioned to a 16x 8 that took over supplying 7 of the 11 runs that were originally coming off the 12x9 trunk….thus leaving the 12x9 with only four 5 inch runs to supply.
The supply work is not done but so far 6 of the 5” round runs have been converted into 6” runs in the basement only. Before going up the wall stacks the 6” runs are transitioned down to 5 inches with a reducer to that they can connect to the existing 5” pipe in the wall stack.
I almost forgot…..Additionally, a new 8” hole was cut in the back side of the plenum for a new 8” round run that goes about 10 ft total with (three 90 degree elbows) before transitioning with a reducer into the 5” just before going up the wall stack. This run is for one of the registers in the master bedroom.
My duct pro is at the house now so I will have new static numbers after lunch and possibly pictures.
Anyone care to speculate what the static may be now?
Changing the Return drop will start this week….going from a 24x10 to a 24x20.
07-21-2008, 11:36 AM #2
I thought you were going to increase all the supplies in the basement to 6"
Just a guess.
About .2" less after all work is completed.
07-21-2008, 11:50 AM #3
After the work started we decided to increase 3 supplies to 7" and the rest to 6"....It just has not been done yet.
I'm home for lunch now I'll write back shortly with new static info and pics.
07-21-2008, 01:07 PM #4
Here are the pics, The duct pro has a run open so he can't give me static for another hour or so.
07-21-2008, 01:09 PM #5
07-21-2008, 01:23 PM #6Professional Member
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07-21-2008, 01:53 PM #7
07-21-2008, 06:20 PM #8
I have new static pressures...
I have new static pressures...and now I am more confused than ever. I just got off the phone with my duct pro because I could not beleive the list of static pressure results he left at the house.
He said the measurments were made with all dampers and registers wide open.
Keep in mind the old numbers under these conditions were 0.6 pos and 0.32 neg for a total of 0.92.
Here is his list of new numbers (before all runs are completely taped but no obvious leaks)
Pos. pressure = 0.57
Neg pressure = 0.34
Total = 0.91
Plenum Pressure = 0.20
Next, with filter door wide open (for example of what impact more return may have if I increase the return)
Pos pressure = 0.65
Neg pressure =.04
Total pressure = 0.69
My first question was why didn't the total pressure change much. He said a couple of possibilities..... It could be return air starved...... or the fact that I did not want him to rip out walls yet so he tied in all the runs to the existing 5 inch runs in the wall stack and that is the limiting factor (see picture in this thread on "8 in reduced to 5 before wall stack" for example.
My next question that I just thought about while sitting here is.....does this data say that the pressure drop across the A coil is resposible for 0.37 of the positive static pressure? (Pos pressure-Plenum pressure?) If so is that high? or normal? If that is what the data is indicating, How could the nameplate justify 0.5 Max If the new factory filter is 0.08? That leaves 0nly 0.05 for the entire duct system in an entire whole house.
I eagerly await the perspective of the pro's here.
07-21-2008, 07:25 PM #9
Your supply was restrictive to the point that your system wasn't moving set air flow.
So after incresing supply and return. The return is now getting more air, so the supply static remained the same because of increased air volume.
If you had increased your return only. Your supply static would be higher then before.
A .37 PD might be a tad high for the coil, but not by much. 4 ton coils range from .25 to .4" PD.
What CFM is your blower set for again, and what size coil do you have.
07-21-2008, 10:08 PM #10
07-21-2008, 10:23 PM #11
Is it a 4 ton condenser, or 5 ton.
07-21-2008, 10:33 PM #12
That is a no, no, it produces too much back pressure.
The boots, the supply registers & everything must flow the velocity FPM & CFM volume with minimum static pressures rises.
The Return Air filter Racks must be, way oversized, & all Return ducting with a low Fiction Rate. - Darrell
07-21-2008, 10:40 PM #13