# Thread: New ducts to lower static pressure

1. My furnace is an AUD080R9V4K, it clearly gives performance data with ESP at 0.9, 0.7, 0.5, 0.3 and 0.1 inches water column. And the performance is approximately linear with respect to ESP, I did not even look for a thing like a label stating a different max ESP.

I will have to look around in my notes for manometer data at 1400 CFM. When this furnace was installed in 2004 we had an old Intertherm AC for which everybody was seriously confused as to the size (Outdoor label had been erased by weather). Perhaps we at one time had it set for 400 cfm/ton and 3-1/2 tons. At some later point my AC pro scrutinized the compressor motor and gave the professional opinion that its size indicated a rather "light" 3.0 ton AC. Then in 2007 it was replaced by a 3.5 ton Trane XE1200 and by that time I wanted 350 cfm/ton (unless there would be a good reason to do otherwise).

If not mistaken I have seen more knowledgeable pros refer to a simple equation which would predict this ESP at 1400 cfm with fair accuracy. Wondering at present if Beenthere directly wants to know or has in mind teaching me something. If the latter what might be the lesson?

Best wishes -- Pstu

2. I could predict it.
But as with all predictions. Until tested. The answer is just a prediction.
And actual reading erases all doubts.

For the record.
If your blower is moving 1200 CFM at .55" static. If you increased CFM to 1400, your new static would calculate out to .75", at 1600 CFM it would come out to .98"

When the formula is applied to TESP, it doesn't always come out right. As some fittings become more turbulant. And add additional resistance that the formula can't allow for.

3. I could predict it.
But as with all predictions. Until tested. The answer is just a prediction.
And actual reading erases all doubts.

For the record.
If your blower is moving 1200 CFM at .55" static. If you increased CFM to 1400, your new static would calculate out to .75", at 1600 CFM it would come out to .98"

When the formula is applied to TESP, it doesn't always come out right. As some fittings become more turbulant. And add additional resistance that the formula can't allow for.

4. Originally Posted by key1cc
.....My duct pro is taking a little long to get my new 24x20 radius return drop (RD) so I called a sheetmetal shop closest to my house just to get an estimate on availabilty. It turns out the closest place that can fabricate the RD is the same place that installed my system. I put in a request with my measurments to the sheetmetal guy at that place through the phone....he said no problem I can make this in an hour or two...I or my boss will call you back with a price....15 minutes later the owner called me and asked are my measurements correct.....he said he had never heard of such a thing as a gigantic 24 x20 return drop...He said that thing will move 3500 CFM of air...he said what size system do you have...I told him XV95-100k btu/XL15i-4-ton....he said we usually put in a 24x10 for that....that is all you need because it will move 1600 CFM.....I replied I know, your guys installed it.... and I have high static pressure sitting at ~1....It got very quiet for a moment, then he asked what is your name again....I told him and he pulled my file and said he will call me back tomorow because he wants to talk to his guys first......the plot thickens...
Key1
Well....I spoke to the owner today.

He indicated that if I convert my 24x10 return drop to 24x20....I will hurt the return pressure upstairs.....and...it may invalidate my warranty...

Is he correct?

He also asked what does Trane have to save about the proposed Return Drop change. I told him I had not spoken with Trane because I would have had to tell them your guys installed my unit and the SP is close to 1.... almost twice the reccomended max ....and I did not want to cause them problems as they seemed like great guys...not knowledgeble but good customer skills...and that I can work with.

The ownwer was thrilled at my response and indicated he would prepare the new RD and would give me a good price.....

More importantly, Is he correct about the upstairs return pressure .....and the warranty invalidation?

Key1

5. No.
Increasing the return drop size has no more bearing on the Trane warranty, then if it was a free return.

It should increase the amount of return from upstairs.

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You may want consider enlisting this shop owners help in remedying the problem. If you only use him to get the parts you want, you may be missing out on some free possibly valuable information and free help.

This is an interesting topic. Hope you work goes the correct way. I may have to work on my system with a high SP.

7. Originally Posted by dash
I'm sure you are joking... I read your linked thread and I don't have a beemer in the garage...Also, unless Beenthere is willing to come across from PA to NJ I suspect I would have a difficult time finding that type of talent in my neck of the woods. That is one incredible install. 0.35 total ESP for a 5 ton? I would not thought it was possible.

We rechecked my pressure drop across the coil today in a different location in the plenum and got a slightly different number.

Positive pressure before coil = 0.59 Neg Pressure = 0.32 (all vents open, system unbalanced)
Plenum pressure location 1 = 0.21 location 2 = 0.24.

Which means I'm looking at a measured pressure drop across the coil between 0.35 and 0.38. Either way if that is the coil pressure drop alone..... I could never hope to achieve the total ESP of 0.35 that you acheived even with no ducts and a simple filter.

By the way what was the PD across the coil on your install from the link?

Key1

You may want consider enlisting this shop owners help in remedying the problem. If you only use him to get the parts you want, you may be missing out on some free possibly valuable information and free help.

This is an interesting topic. Hope you work goes the correct way. I may have to work on my system with a high SP.

Not sure how "valuable" the info would be. It seems to be much more valuable info here on this site. Read Beenthere's previous post right before your post. The 2 pieces of valuable info I was given today by the owner were inccorrect and could lead me down the wrong path.

This whole HVAC business is quite a mystery to most but as the saying goes....In the land of the blind, the man with one eye is King.........

Key1

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I'd have to be at the shop to look it up.

I do know that it's a 5 ton zoned system,designed for .5 or .6 static at 125% of 2000 cfms and it opertes at 1750 cms,hence the low static,plus it has two 5 ton Air Puriifiers,running 875 cfms each.

This home was for a vaery good friend that's in the industry,and money was no object.So since the zoned system "can" run at 2000 cfm,we used the max., plus 25% for zoning.

This was also new construction,horizontial was the best choice for the given space.

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Originally Posted by dash
I'd have to be at the shop to look it up.

I do know that it's a 5 ton zoned system,designed for .5 or .6 static at 125&#37; of 2000 cfms and it opertes at 1750 cms,hence the low static,plus it has two 5 ton Air Puriifiers,running 875 cfms each.

This home was for a vaery good friend that's in the industry,and money was no object.So since the zoned system "can" run at 2000 cfm,we used the max., plus 25% for zoning.

This was also new construction,horizontial was the best choice for the given space.
I think you posted in the wrong thread.

http://www.hvac-talk.com/vbb/showthread.php?p=1937063 (Once again, the install looks great.)

11. Originally Posted by RyanHughes
I think you posted in the wrong thread.

http://www.hvac-talk.com/vbb/showthread.php?p=1937063 (Once again, the install looks great.)

Key1

12. Even on a 5 ton coil at 1700 CFM. I would have thought you'd have a .2" to .25" PD across a wet coil.

13. Originally Posted by dash
I'd have to be at the shop to look it up.

I do know that it's a 5 ton zoned system,designed for .5 or .6 static at 125% of 2000 cfms and it opertes at 1750 cms,hence the low static,plus it has two 5 ton Air Puriifiers,running 875 cfms each.

This home was for a vaery good friend that's in the industry,and money was no object.So since the zoned system "can" run at 2000 cfm,we used the max., plus 25% for zoning.

This was also new construction,horizontial was the best choice for the given space.
By the way Dash,

I know how strongly you feel about turning vanes....When I asked the owner to give me a quote on the 24x20 return drop to replace my 24x10 I asked him to quote it with Turning vanes. He indicated that it was not necessary because he is making the el curved on the front and the back for smoother air-flow and that the vanes add a lot of cost.....

I don't feel to bad about it because another point that you have made is that it is better to not have vanes than to have them installed incorrectly....So far my installing company has not given me the confidence that the vanes would be correctly installed.

Key1

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