1. ## Curious Solution.

I had a T&B some time back in a three story building where the third floor was being rehabbed for a brokerage firm.
An AH was installed and when on line and confirming the fan and motor were maxed out the air flow at the last few terminals was not even close.

I called the engineers (another State) and told them what I had. Their solution was one I hadn't seen.
They sent drawings to the mech contractor telling him to extend the discharge plenum and make a tap, send the new run around the space, (think doughnut) and connect the run to the end of the original run. In other words, feed air from both ends.
I remember this was sometimes done in houses way back (not often) in plans I've seen in books. Not sure why because of extra duct needed but it should lessen pressure looses.

It did work in fact fairly well. I never knew why the original duct design fell short but has anyone seen this solution done after the fact not being part of the original design. Sort of a static regain of sorts.

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That's how a lot of city water loops are done in the streets it helps balance the pressure from one side of the loop to other.
It really should be done that way on duct work as well but nobody wants to pay or give up to space for the extra duct.
It really screws up a neighborhood if a pipe breaks in the street no one in the city water authority remembers were all the valves are, so the water flows till someone finds the last magical hidden valve.

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It seems to me that each terminal now has 2 parallel paths for air flow.
Assuming that resistance to air flow has similar characteristics as resistance to electrical current flow, then the total resistance is less than the lesser of either of the two paths.
I don't know how to write an equation here, but the total is equal to the product of the two (multiply the two) divided by the sum of the two (add the two).

4. Makes me wonder if the original ducting was larger, if there would have been a problem on those taps at the end of the run.

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Originally Posted by BBeerme
Makes me wonder if the original ducting was larger, if there would have been a problem on those taps at the end of the run.
That would have been the correct thing to do originally, but once the mistake was made, another small run was probably faster, and cheaper.

6. Originally Posted by BBeerme
Makes me wonder if the original ducting was larger, if there would have been a problem on those taps at the end of the run.
If the duct was sized properly, all probably would have been fine.
Most know if when questioning an engineer, their answer might be any number of things.
Usually it's better to just lay the results in their lap.

7. Originally Posted by Nuclrchiller
It seems to me that each terminal now has 2 parallel paths for air flow.
Assuming that resistance to air flow has similar characteristics as resistance to electrical current flow, then the total resistance is less than the lesser of either of the two paths.
I don't know how to write an equation here, but the total is equal to the product of the two (multiply the two) divided by the sum of the two (add the two).

Because the two ducts are unequal total resistance might be the reciprocal
1/ ((1/r1)+(1/r2)) If I remember that right.
Probably the TEL total equivalent length of both ducts gives the total friction would equal total resistance then the static pressure represents voltage and velocity = current.
Never thought of it that way but seems valid. Might be the way the Ductalator was created.
Often it seems "Laws" like Fan Laws and Ohms law cross over in a similar way.
I don't know what went South with the original duct sizing or how they arrived at the new duct size. By some of the other things that went wrong it would be a guess at best.
I might have looked at the terminal totals, see where the air went short, then size the new duct to offset those values. Or just use the SWAG (Scientific Wild A\$\$ Guess) method.

The space was originally served by one large unit 100 ton. As with many buildings like this, renters wanted their own units/controls so the original unit was drastically oversized and noisy. When the engineers called me asking about the noise complaint in the space and how loud it was my answer was "Do you remember the Engine Room on the Starship Enterprise?" Not wanting to be a smart a\$\$, just wanted to drive a point home and what happens when a 100 ton unit morphs into a 12 ton.

8. Originally Posted by Nuclrchiller
That would have been the correct thing to do originally, but once the mistake was made, another small run was probably faster, and cheaper.
Yes. I thought, given the problem, it was a good solution for both the reasons you mentioned.

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This has been an interesting discussion, and an interesting thought exercise. Thanks.

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When I run compressed air lines in a shop I hook the ends together. It helps with the volume tremendously.

11. I totally get it.

I guess you could say that I'm mildly impressed that the engineer came up with a reasonable solution.

I was actually going to run the duct that way in my house years ago. Then figured any benefit out weighed the extra work and design. One duct goes this way, and the other goes that way. Each with its own branches or take offs.

In my case with the ducting, I went with an alternative, I chose to go one size over to reduce velocity and noise. Small house, so any extra heat gain through ducting not that big of a deal. And it is quiet!

I can't stand noisy air conditioning. Air conditioning should be felt, not heard.

Originally Posted by joeysmith
When I run compressed air lines in a shop I hook the ends together. It helps with the volume tremendously.

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Have had to run a parallel control air lines down thru multi- story to boost air pressure at the bottom, starting off with 20 psi in the penthouse and only have 15 psi ten stories lower was causing a lot of control issues. The straight shot of air got all the floors running and cooling.

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13. Hmmm, sounds like pneumatics. But the concepts are all the same.

Originally Posted by Chev5372
Have had to run a parallel control air lines down thru multi- story to boost air pressure at the bottom, starting off with 20 psi in the penthouse and only have 15 psi ten stories lower was causing a lot of control issues. The straight shot of air got all the floors running and cooling.

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