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Duct design, return locations.

Posted by mlo1 (My Page) on
Sun, Dec 30, 07 at 15:53

I have a new build 1-1/2 story craftsman style home. First level is 1400 sq.ft. with an open kitchen hearth room layout, mst BR in the rear. Second level is two kids room and small rec area, near 400 sq.ft.

Proposed heater location is under the stairwell which will allow supply and return ducting to be run vertical through a chase to above the second floor into the attic.

The system is to be an electric heat pump with 14 supply registers and two returns, one on each floor centrally located, up high. When I inquired about returns in many locations and specifically down low in stud cavaties, I was told that was "old school".

My climate is fairly mild in the northwest and I really do not see needing AC more than a few weeks a year. The company I am dealing with has a great reputation and is very expensive. I'm concerned about thier ducting philosophy though, as my little bit of research has indicated the opposite about return ducting if giving the option.

What is your take on this?


Follow-Up Postings:

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RE: Duct design, return locations.

Your "old school" questions are good ones, and even though te hnology has changed many fold over the years, the need for as much return as you can get is still true. As a matter of fact, other than bathrooms; kitchen and laundry rooms-- it would be best if you installed a return in every other room, including walk in closets. The big "kicker" with doing this expense. The more returns equals more cost, but it also equals more comfort.


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RE: Duct design, return locations.

does code allow you to locate your mechanicals under
the staircase?


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RE: Duct design, return locations.

I would expect code does, as the design and install is being handled by a HVAC contractor.

After inquiring further about adding more return air ducts into other rooms, it was reiterated that this would allow noise and conversation transmission through the duct work. They just really felt it was not needed for performance.

Is this a logical argument against more return ducting?

TIA


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duct work

Some extensive discussion during duct routing was interesting. Another point was brought up, that if you were to install more return ducts, the entire return duct system would have to be extensively designed. The trunk of the sytem would have to be capable of un-impeded system requirements or flow. The individual legs of the trunk, would have to be sized as to only be capeable of allowing a percentage of the systems needs. Forcing the other legs to perform and add up to full requirements. Only then would the system draw on all of the legs equally to provide any difference at each legs location. Other-wise you would have say 3 returns, with only two or even one of them, supplying all of the systems needs.

Not that this could not be done, it is. But you run into a point of diminishing return on the efficiency of the heating unit/fan having to operate under this design.

Problematic areas certainly may need this style design. He felt in my case it simply was not justified and would be a waste. But he said he would happily appease me an install as many return ducts as I felt I needed to be happy with his design :)


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RE: Duct design, return locations.

Having just 2 returns is a basic system, typical for a tract home. For my home i would have add at least a return in the master. Also a low return would be highly recommended in a colder climate.
Conversation transmission is nonsense. How you were explained the "legs" of the return system is correct but I don't understand why this would be a problem. It is normal design.


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RE: Duct design, return locations.

funnycide..Thank you for your input.

I'm on the fence here. The vertical supply duct was run up into the attic today with registers being fed into the ceiling of four seperate rooms on the second floor. Tomorrow the return ducting will be run vertical into attic of the second floor and will have one register through the cieling in the center of the hall. That return will draw from the 400 sq.ft. on the second floor that is comprised of two BR's a bath rm, and a rec area. Should I have this return split into 2 return's with one in the ceiling and one down low through a stud cavity, both in the hall?

TIA


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return ducts

Does it make sense to think of both stories as really one area? Atleast from the aspect of heat rising and filling the whole house from the top down?

If so, and specifically concerning the returns, would it not make sense to locate all returns for heat, down low on the 1st level? or do you need some return air being drawn from the second level to balance the supply legs between the two levels?

Maybe locate a damper controlled leg of the return to the attic and locate the register in the cieling of the upper storie hall, only to be used for AC. Sensable?


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Return duct II

Second floor has 6 supply vents, fed from the attic through the cieling.

Return air was suppose to be split into two off the 14" vertical supply from the air handler. One 14" low on the hall wall on the second floor, and a branch off of that feeding a 14" in the cieling of the first floor.

I was adamant that there be more returns on the first floor, especially down low and back in mst. BR. In order to do this, the 14" return duct being located low on the hall wall on the second floor would have to be branched off-of and fed down through a wall stud cavity and routed through a soffit back into the Mst. BR. There is no way to branch off of 14" return in the first floor cieling.

So to make sure the returns draw equall, should I downsize the second floor return from 14" to 12", as it will have a branch run near 20' and 45' with two wall drops into low return locations?

Should the single cieling return on the first floor be deleted or kept, if I am having them add two other returns? Does 8" duct to the two lower returns sound to big, maybe reduce to 6"?

My installers are basically doing as I say. Good, bad, or indifferent, I am just trying to garantee a good job.

TIA


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RE: Duct design, return locations.

For the second floor return you want it high. Either high in the wall or in the 2nd flr ceiling. What size is your unit? 3 tons?
Pictures would help.


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RE: Duct design, return locations.

Yes 3 tons. I'm driving between two locations to be able to use the internet. Pictures are difficult for me right know. I will try later tonight.

Ok, second floor return high on the hall wall instead of low. I need to get in the truck and drive...they are doing that now.

I ended up telling them to make the first story cieling return leg 10". And the return leg in the second floor hallway 12", which also is branched with a 10" leg to feed two stud cavity 8" drops low in the mstBR @ near 20'& 40'down the leg.

Thanks funnycide!


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RE: Duct design, return locations.

mlo 1,
Hey there, have you had a load calc done, Manual J and Manual D? Returns are critical especially if they are flex or rigid. The friction rate is different between the two.

Your second story has a load and so does your first story, but do you have a a large opening between the first and second story. 400 sq ft for the rec area and two rooms with a 14 " return air at nearly 600 cfms of air is quite a bit return. You don't want to have a negative pressure upstairs. When zoning your duct your returns should be dedicated to certain supply air vents to balance out the system. The air handler is under the stairs, will your return air grille on the first floor be close it? I have a link for you if you need more info on duct design. i hope i have helped raise questions now for you before it is too late. Are you in a rough-in stage of your mechanical right now?

Here is a link that might be useful: ACCA AND ASHRAE CERTIFIED HEATING AND AIR CONDITIONING LOAD CALC WITH DUCT DESIGNS


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RE: Duct design, return locations.

pearson21, thanks for the input!

I hired a well respected company months ago for my new construction HVAC work. Lets just say I have realized in the last couple days interacting with them, thier clean looks and polite demeaner have served them well. They were basically walking around designing ducts and such just "shooting from the hip". When I enquired about manual J or D calcs, the lead tech actually told me he does not really get involved with the scientific end of things he just knows what works!!!

I actually furnished this company a complete set-of building plans 6 months ago, and kept them aprised of the framing work for look-see's and rework if needed. This tech never saw them nor seem to care.

I have some understanding of pressures dynamics and its science from an unrelated field of expertise. Anyway, I quickly realized in the name of moving forward that I was going to become the lead. They are polite and making enough $$ off me, they don't care. So we are now co-designing this system "on-the-fly".Does not give me much time to really read up, though I'm trying.

The ducts are all metal ridgid. There is a stairwell open between the two floors. It is a slit landing design with two sets of steps. The return air on the cieling of the first story (10") is about 7' away from the air-handler.

I am in rough-in stage.


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RE: Duct design, return locations.

I don't know how cold it gets where you are, but around these parts, it is less than optimal design to be putting heating ducts in the attic, where they are exposed to the full brunt of the cold. To the extent that there is duct work up there, insulate it to the same degree that you insulate the ceiling, as the air inside the upper story is directly connected to the air in the ducts in the attic.

Also, hot air rises, and putting heating registers in the ceiling is the least optimal place for them in a low velocity system. Ceilings, however, are a good place to put air conditioning registers.


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RE: Duct design, return locations.

You are right, but I had no choice really. Fortunately our climate is fairly mild and the house is very energy efficient.

I spent the day doing looking at options in the structure for running ducts to be able to pull return air from down low on the 1st level.

I converted everything to sq.in. and started from the supply ducts back. My math shows that I need a 18" return trunk going vertical into the attic for about 20'. Then it Y's into a 14" and a 12". The 14" runs about 8' and y's into two 10"'s. One of those runs 4' into the cieling return for the 2nd floor (centrally located), the other runs 6' into a wall cavity down to the 1st story floor. The other 10" off the Y runs nearly 35' and drops into another wall cavity to the 1st story mst.BR floor. This area was previously landlocked fram any return action by a wall and short hallway.

The 1st story has a return ducted into the wall the furnace backs up-to under the stairs. It will be a custom plenum that basically yields a 12" x 12" volume and utilizes three stud bays for another low return.

Mathematically I end up with returns on the first story being oversized by about 15%. On the second story I am purposely undersized by about 20%. The only return (10")on the second story si in the cieling. All of the returns on the first story are just up off the floor in stud bay's. The 2nd story has 6-6" supplies and the 1st story has 9-6" supplies.

The furnace is a 3 ton Carrier infinity.

Am I on track or crazy?

TIA


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