Return Air Duct

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  #1  
Old 10-24-20, 12:23 PM
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Return Air Duct

Getting ready to replace furnace and get quotes. The manual D design says I need two 14" air return supplies. It asks for one to be 14" x 14" and the farthest one to be 14" x 20". Both connect to a 14" round duct that connects to the furnace with a 16" round duct. Not sure why both return supplies can not be 14" square. Not sure I have room for the 14" x 20" in the front hallway. Heating total is 1005 CFM. Does the larger 14" x 20" pull more air even though both are connected to a 14" duct. Don't know if the distance between returns makes a difference, but the 14" x 20" will be about 28 feet from the 14" x 14".

Originally they had a 12" round and a 14" round and I asked if the larger return size would help run the system quieter. They suggested reversing the 14x20 and the 14x14 and increasing the 12" duct to 14".
 
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Old 10-26-20, 09:08 AM
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"Supply and "Supply Duct" refer to conditioned air entering a room. "Return" and "Return Duct" refer to room air going back to the unit.

The Manual D calculation has some questionable numbers:
1. The first (closest to unit) return branch is 14" for 494 CFM (480 FPM velocity) and the second is 12" for 506 CFM (670 FPM).
2. The first (closest to unit) filter is 14x20 (1.94 SF @ 255 FPM) and the second is 14x14 (1.36 SF @ 372 FPM).
3. The 16" return main handles 1,000 CFM (750 FPM).

Why is the longer duct smaller? Why is the closest filter larger? The changes you described will correct those, leaving the larger issue: Why does the 16" exceed the 700 FPM Manual D velocity limit? I'd think you'd want 18" there.

Equivalent sizes are fine, use a "Ductulator" instead of calculating areas. Filter sizes should both be the same, 14x20 or equivalent. That's if you're willing to accept that any inleakage to the return ductwork will be unfiltered. You might instead consider a media air cleaner at the unit, similar to Honeywell F100 MERV11, size 20x20 or 16x25.

The diagram does not show the branch supply and return volume dampers required by Manual D (It's a book by the way - Not a software program).

I did not check anything on the supply side.


 
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Old 10-26-20, 09:12 AM
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The diagram shows round metal duct throughout. Rectangular equivalents are fine as mentioned. There is no flex duct shown, a Good Thing, since there'll be no flex duct workmanship issues to worry about.
 
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Old 10-27-20, 01:34 AM
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You'll also need to establish a Manual D "Low Resistance Return Path" at every large room separated from the return grilles by a door, too. AND why two returns? A single central return will be fine with the Return Path.


 
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Old 10-31-20, 10:51 AM
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Thanks for the information. Thought I would receive responses by email automatically, but am not. I just had to replace the circuit board on my 25 year old Payne furnace to keep it going.

Preparing for the new furnace I replaced all the supply duct work with 26 gauge ducts.

You mention, "Why does the 16" exceed the 700 FPM Manual D velocity limit? I'd think you'd want 18" there." Not sure what this statement means, but will inquire about with the HVAC installer. Present return system is rather noisy, so what to try and keep as quiet as possible. I want a two stage furnace with an ECM and maybe it will run quieter on low speed.

As for the filter, I was considering the Honeywell F200F1625 or the Aprilaire 3410. My daughter has the electronic Carrier PGAPAXX625 with germicidal, but the cost is five times more. Which type would eliminate the most dust?

The Manual J and D report is 22 pages and you are correct the diagram does not show the branch supply and return volume dampers required by Manual D. Forgot to mention all supply and returns are in the floor.

Lastly, we only have two doors, one for a bath, and one for a bedroom. The bedroom door is rarely shut. Would either of these rooms need a jumper duct?


 
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Old 11-01-20, 01:13 AM
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The 16" is too small, you need 18" to comply with the velocity requirement. Any competent Manual D practitioner should know about velocity limits.
As for filters, you need to first ask the designer what he allowed for filter PD (pressure drop). The F200 MERV 13 will have a higher PD than the F100 MERV 11 I spec: At 1,000 CFM 16x25 MERV 13 0.17" and MERV 11 0.10". The 0.07" difference is HUGE, considering a good design only allows about 0.30" for the ducts and fittings. If you're intent on MERV 13, you'll need a 20x25 filter and 6" long transition at the furnace inlet (0.11" PD, almost the MERV 11 16x25).
"Rarely shut" is no different than "always shut" when it comes to designing duct systems. Do provide the required return path at the BR. Bath will be fine with the normal door undercut.
With the floor supplies and returns "Throw" (the distance conditioned air is projected above the floor) can be a problem with two stage blowers: Manual T requires 6 foot throws to avoid "hot head, cold body" complaints, and throws change with two speed operation. That's really troublesome with oversized AC, because the system rarely operates at high speed; couple that with "one outlet size fits all" thinking and you can forget about comfort.
 
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Old 11-03-20, 03:21 PM
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Savoy gave the following response concerning the filter box, "We design for a MERV 8-11 rating (mid-efficiency) vs a high efficiency such as MERV 13. A MERV 13 will add more resistance on the return duct side.

Manual D requires a friction rate of 0.60-0.180. Your designs are 0.104 and 0.116 which are in the middle of that range and should be able to handle a MERV 13 filter. Most people are quite happy with the mid-efficiency filters too.".

Our home has no air conditioning and ceilings are 10' 6". Savoy recommends Day and Night G96CTN0601714A furnace.
 
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Old 11-03-20, 10:18 PM
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"Manual D requires a friction rate of 0.60-0.180." That's total BS. And they probably meant 0.06, not 0.60.

My Manual D (2014) is a 276 page book, and there's a more recent version (2016). Ask them which edition they're using, what page the friction rate requirement is on, and why they're not adhering to the velocity limits. OH...AND think about another contractor.

https://www.acca.org/store#/productD...-fc15b428dd54/

Another thing: You don't need much heat, and a few percentage points of efficiency won't make a noticeable difference in your bills. Ask for a less efficient 40,000 BTU/Hour furnace (nice to see they actually recommended the smallest furnace in the "96" series, but the less efficient series are available in 40,000). It'll require less airflow (quieter) and be far less expensive to fix when it breaks. And ask who's responsible for registering the furnace for the extended warranty if you use this company.
 
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Old 11-04-20, 10:40 AM
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Thanks again. Savoy just did the calculations. Will be using a local HVAC company for new installation and I think they use Rudd HVAC. The load calculations were done in 2012 and I just kept repairing my existing furnace. Don't know why, because it is a 25 year old Payne 110,000 BTU unit. Duct work and furnace are under house in crawlspace. Last year I replaced all the duct work and registers. Being as I waited so long, should I have the load calculations redone to current 2016 standards?
 
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Old 11-04-20, 11:57 PM
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I see no need to recalculate, please post the Manual J cover sheet, we could use that as a basis for choosing the furnace capacity. I'm still thinking 40,000, but we'll see. It'll be interesting to see the outdoor design temperature and what he assumed for duct losses and infiltration.
 
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Old 11-05-20, 10:34 AM
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Here is a screenshot of the page. I do have an email saying the following, "We recommend a 60-65k output furnace with 2 stage if 2nd floor is not to be conditioned for some time.". I have not done anything to the 2nd floor as yet.


 
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Old 11-05-20, 11:07 AM
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Maybe you need this page also.



 
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Old 11-05-20, 02:45 PM
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In the "Bad Old Days" we took window U factor as 1.13 for then current single pane double hung windows: Heat loss would have been U x A x TD = 1.13 x 1SF x 31F = 35.03 BTUH/SF in your climate using their 31F TD (It's actually 32F because they used 37F outdoor instead of 38F and 68F indoor instead of 70F). Today's window U factors are routinely 0.35 or thereabouts. So you gotta wonder about the "glazing" at 28.2 BTUH/SF, meaning they assumed 0.91 U factor!

Infiltration is calculated at 8,611 BTUH divided by (1.085 x 31F) = 256 CFM for the 1st floor sounds high.

I'm feeling much better about the 40!

And do your window rough openings total 6,729/28.2 = 239 SF? Strange but true: I ran across one practitioner who took every window rough opening as 15SF regardless of actual size!
 

Last edited by ferd42; 11-05-20 at 03:34 PM.
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Old 11-05-20, 06:39 PM
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Just measured all the windows for a total of 233 square feet which includes 12 square feet of glass in 2 doors. As a mention the humidity in our house runs about 70%. House is post and pier and ranges from 2 to 5 feet above ground.
 
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Old 11-06-20, 09:58 PM
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Glass in doors is included in "Doors", so they overestimated your window area too. 40,000 is looking better and better.
 
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Old 11-10-20, 11:35 AM
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These are the changes I should make to the installer.

Stay with a Merv 11 filter.
Increase return air duct to furnace from 16" to 18".
Not sure about single stage or two stage furnace or 80% or 96%.
Still not clear on what size return air plenum boxes to use.
 
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Old 11-10-20, 03:14 PM
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Stay with a Merv 11 filter. YES - 16x25 MERV 11.
Increase return air duct to furnace from 16" to 18". YES.
Not sure about single stage or two stage furnace or 80% or 96%. SINGLE STAGE, 40,000 and 80% IS FINE FOR THE LITTLE HEAT YOU'LL NEED - BUT DO CHECK TO SEE IF 80% IS ALLOWED IN CA. IT'D BE GOOD TO HAVE A "CONSTANT TORQUE ECM".
Still not clear on what size return air plenum boxes to use. DON'T UNDERSTAND, PLEASE CLARIFY.
 
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Old 11-10-20, 06:04 PM
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Need to have larger return air plenum boxes made for the 14" return ducts. Should they be 14" x 14", 14" x 20" or some other size?

Present returns are 10" with a 10" x 12" plenum box in the front foyer and 14" with a 12" x 14" plenum box in the rear hall area.
 
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Old 11-10-20, 10:33 PM
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The grilles will have to be 20x14 or 14x20 for 500 CFM. That's with H&C #672 or equal, with 1/2" fin spacing. 0.03" pressure drop is usually used, rather than a velocity.
 
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Old 11-11-20, 06:36 PM
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If the return ducts are 14", why do the grilles need to be 14" x 20"? Also, can I add a third return in our Mud Room? If so, does that change all the return sizes?
 
  #21  
Old 11-12-20, 01:16 AM
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14" fits between ceiling joists or wood "I" beams spaced on 16" centers, if your joists are on larger centers, use wider grilles. If you have trusses on 16" centers, 14" wide grilles are no good, of course. If you do end up with 14" wide, you might want to make the boxes tall enough to tap the sides for the 14" ducts.

Why add another return (you've already got more than you need)? I've always believed return ductwork should be minimized, so the blower's "OOMPH" can be used to overcome supply side losses, and allow for extensive balancing where required. If you do decide to install a second unnecessary return, leave the other two grilles and ducts as they are.
 
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Old 11-13-20, 01:31 PM
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Will skip the third return air.

Which way is better for mounting the floor plenum box. It seems the side return air mount and bottom return air mount both need about the same amount of depth. The side mount would need a much larger box though, but if something is dropped in the grille, it would be easier to retrieve.

Instead of adding the Merv 11 filter box to the furnace, would a filter in each return air work better?
 
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Old 11-13-20, 03:33 PM
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By "Floor Plenum Box", I assume the return grilles are in the floor and you're asking whether to connect to the side or bottom of the "Pan" or "Box" under the grille. If that's the case, you must connect to the side to provide the NFPA 90B-required "Receptacle" to collect embers, etc. if there's a fire - a Code requirement that may have originated more than a century ago. Make the box 2" deeper than the bottom of the duct connection. In an earlier home, I put the floor grille under an armoire (it had "feet", with about 6" between the bottom and the floor) and provided a duct access door in the box so I could clean it out from the basement.

If you use filter grilles, any inleakage to the return ducts will be unfiltered. Do put the 16x25 MERV 11 MAC at the furnace inlet.
 
 

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