New ducts to lower static pressure

[key1cc]

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.
http://hvac-talk.com/vbb/showthread.php?t=175955
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.

Key1

[beenthere]

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.

[key1cc]

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.

Key1

[key1cc]

Here are the pics, The duct pro has a run open so he can't give me static for another hour or so.

[key1cc]

More pics

[captube]

To link your other thread

find it and open it up, right click on the address bar and copy

edit your original post, choose the little blue ball with paper clip (insert link) and paste the address.

[key1cc]

To link your other thread

find it and open it up, right click on the address bar and copy

edit your original post, choose the little blue ball with paper clip (insert link) and paste the address.

Thanks.

Done.

[key1cc]

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.

Key1

[beenthere]

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.

[key1cc]

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.

Blower set at 350 CFM/Ton

Coil = TE50660C2 10B2722AP 410a TXV installed max pressure = 450 PSI

[beenthere]

Is it a 4 ton condenser, or 5 ton.

[udarrell]

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 believe the list of static pressure results he left at the house.

He said the measurements 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" reduced to 5" before wall stack."

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 responsible 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. Key1

You don't reduce static much by going from 8" back to 5" in the wall stacks.
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

[key1cc]

Is it a 4 ton condenser, or 5 ton.

4 ton xl15i/xv95 100k btu blower

[beenthere]

Thats a high PD for 1400CFM through a 5 ton coil.

[key1cc]

Thats a high PD for 1400CFM through a 5 ton coil.

Did not know I had a 5-ton coil.....thought it was matched to the condenser at 4-ton.

If that PD is high does than mean something is not installed correctly? Or is it that there is a range and I am the unlucky one that got the high end of the range?

Does Trane have specs. (Equipment specifications)...and if so would this PD fall outside of those specs?
Key1

Thanks.

[beenthere]

If I'm reading that mod number right, its a 5 ton coil.
Which isn't unusual on 4 ton systems.

A 4 ton running 1400 usually isn't that high.

Could be just that model coil is restrictive.

I would expect around .2 to .25 for a 4 or 5 ton coil at 1400 CFM.

[key1cc]

You don't reduce static much by going from 8" back to 5" in the wall stacks.
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

Well.....there you have it......
With a coil sitting at 0.37 SP...... leaving the available SP for the entire supply, filter, and return duct system at only 0.13......It is safe to say that with a 2 story house and 4 ton condenser....It is not realistic to think I can get down to the name plate max SP of 0.5. I checked the specs of the media filters including the Honeywell F100, F200, and aprilaire 2200 and all have PD of close to 0.2 at 1400-1600CFM.....which would imediately take me over the max SP even if all supply and return ducts were disconnected.....

However, I am not one to walk away from a challenge .....(my wife calls this new HVAC system my obsession and wishes we stuck with the old one) I tell her the old one may have had high static also we just did not know about it.

Anyway, I will have installed a very large return drop (from current 24x10 to new 24x20). I have been looking inside a lot of the new model homes lately and I have never seen one larger than 24x14.

After that, I will have only the large 8" run to the master bedroom enlarged all the way to the register as this should increase comfort in that room.

At that point, I'll live with what I have and go ahead and purchase the 10 year labor warranty (I was given 6 months to make up my mind so I did not rush it).

Key1

[pstu]

Key1, I am a homeowner with Trane equipment. I wonder how much was saved by not going with the variable speed furnace. I replaced old Consolidated furnaces with Trane's 2-stage and variable speed, the deciding factor was no fan tables available (airflow vs. static pressure) and that annoying label on the equipment saying never operate above 0.5 ESP. My ESP had already been measured at 0.7, but the new air handler is rated up to 0.9. You show a degree of fanaticism which I admire.

My original return was a bit restrictive and measured 0.45 inch vacuum using some medium quality pleated filters. Essentially added a whole lot of capacity via 2 additional returns and got that down to about 0.2. Still using pleated filters but there are two 20*30*1 ones for a 3-1/2 ton system, that makes for low airspeed at the filter face and therefore pretty low pressure drop.

It is my understanding that Trane specs the same exact coil for 3, 4, and 5-ton systems. Maybe not all the time but sometimes. So your system would be pretty normal to use a 5-ton coil but I marvel that the pressure drop over your coil is supposed to be 0.37. Do I understand correctly this is calculated and not measured? My system has transitions before and after the coil and that makes room for pressure takeoff points so I could easily measure the pressure drop over the coil itself. The hot attic dissuades me from being too curious about that exact measurement though, that and the fact my total supply side static is about 0.35 or less (it varies greatly with airflow).


Best of luck -- Pstu

[beenthere]

pstu

Take a static reading with your blower at 1400 CFM, and 1600 CFM.

[pstu]

Beenthere, is the message to say ESP varies with airflow? I totally agree if that is what you are saying. My system is set up for 350 cfm/ton, plus about 20% additional slowdown when the humidistat is above its setpoint. So I have watched ESP move between about 0.40 and 0.55 under those two conditions.

There is a simple equation that fairly accurately relates static pressure as a function of airflow, is there not?

Best wishes -- Pstu