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Old 09-29-2010, 04:42 PM   #1
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New system, are my ducts adequate?


Hi,

I understand that no one can say whether a duct system is fully adequate without doing a Manual D, but I am trying to generalize if it is likely or unlikely that I would need to upgrade my duct sizing with an new system.

My current system is 20-years old and is using a 90% plus Carrier Weathermaker SX, with one speed blower and sealed combustion. It is 66K BTU with a 2.5 A/C. My house is 2100 square feet with an additional 800 sq ft. of conditioned space in the basement. I live in MN. I have a reducing trunk system running down the middle of my basement. The supply plenum is 19 X 17 which connects to an 8x20 duct running both ways. With a couple of exceptions each of the supplies is a 6" round or "equivalent" oval serving different rooms and spaces. The return drop is 8x24 dropping from a 8x16 running in both directions.

Main Floor - the main floor is approx. 1400 sf with 10-6" feeds and six returns using wall cavities.

Second Floor - the second floor is 3 BR and 2-baths, with 7 feeds and 4-wall cavity returns, one in each BR and one in the central hall.

The basement has three feeds and one larger return cut into the return drop.

The current system works fine, except that two BRs coming off the far end of the trunk, using oval wall stacks tend to be a bit colder in the winter and warmer in the summer. The entire system is also somewhat noisy when running the a/c with the high fan setting.

I plan on installing a new, high end, multil stage, ECM blower furnace and have read that they require bigger duct systems that may cause an increase in static pressure. I am concerned that my ducts may not be big enough or numerous enough. Would it be your opinion that I am either likely or not likely to need to change some portion of my duct system?

Thanks.

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Old 09-29-2010, 06:51 PM   #2
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New system, are my ducts adequate?


For the size of your existing equipment I can say you are safe with two 8x20 coming off each side of your supply air plenum.

VS or ECM motors do not require an automatic up size in duct work. Quit the Contrary, they can be used to (with in reason) to improve the air flow of problematic duct systems. It just depends on the individual system.
Of course there are some systems where only a total replacement would work.

I

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Old 09-29-2010, 08:08 PM   #3
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New system, are my ducts adequate?


Since you have 2 rooms that don't heat or cool right. Good chance that your supply reduces too much.
Noise is a good sign of an undersized duct system.

Your return drop is too small.

A VS ECM may very well increase your noise level from your duct system.

Which is louder, your returns or supplies.
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Old 09-29-2010, 08:31 PM   #4
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New system, are my ducts adequate?


I didn't catch the size of the return. But 8x24 on for 66k is is optimal. The rooms that are not warm/cool enough are out side walls. They are always cold no matter what the duct system. It's a regional problem relating more to
construction practice and under insulating. I have seen many new duct systems replaced using ACCA standards but the rooms with out side walls were still hard to heat and cool.

The return air trunk is a bit small but the cost vs benefits might not be worth replacing it for the amount of noise reduction the OP would get.


OP can you t
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Old 09-29-2010, 08:49 PM   #5
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New system, are my ducts adequate?


His rooms that are cold in the winter have the same size supplies as the rooms that are warm. They're undersized for those rooms, and so is his supply trunk supplying those runs.

Lots of those types of set ups around here.
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Old 09-29-2010, 09:04 PM   #6
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New system, are my ducts adequate?


I am gonna have to respectfully disagree Been.
This is a a typical mid-west problem unique to this area.
You got me trumped on duct design and air flow in most areas, but I am going to have to let my experience and "what I know works" in my region of the country speak for me here.
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Old 09-29-2010, 09:08 PM   #7
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New system, are my ducts adequate?


Those outside walls lose heat there the same as they do here.

So those rooms simply are not getting enough air.

I've worked in Rochester NY, that wind coming off the lake makes for cold rooms(2 outside walls) if you don't supply enough air to them.
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Old 09-29-2010, 09:17 PM   #8
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New system, are my ducts adequate?


Gonna have to stick to my guns on this one. Just seen it too many times.

Mi Mn...Great Lakes states with lakes the size of seas?

The so called Northern Express sweeps down from Northern Canada and whips across MN and Mi. We have lake St, Clair a stones throw away. The lake effect compounds the problem.
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Old 09-29-2010, 09:28 PM   #9
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New system, are my ducts adequate?


Extra air flow does not help if the wind is blowing cold temps against the walls faster than the airflow can keep up.
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Old 09-29-2010, 10:10 PM   #10
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New system, are my ducts adequate?


Thanks to both of you for your responses.

Just to clarify, the two BRs that are cold/hot each have two outside walls, but the supplies take off the trunk as round, transition to oval, up throught a common inside wall, transition back to round, run across each floor with a register in the floor under a window in each case. On the second floor is also the MBR, which is bigger and has three outside walls. However that room has two 6" oval supplies, but both actually come up through the outside 2x6 walls just inside of 2" of rigid insulation. This room tracks quite tightly with the thermostat in both winter and summer.

Given the general nature of my question, I guess i should not be surprised that one of you seems to think my 8x24 return drop is undersized, and one thinks it is optimal. Should the return drop have a cross sectional area similar to the supply plenum? 8x24 is considerably less than 19x17. Is it fair to say that the cooling load cfm is what will challenge the size of my duct system the most? Therefore, should my return drop be capable of handling 2.5 x 400 cfm/ton?

It appears that neither of you think my duct system will be woefully undersized for my new components. My thinking is that I will attempt to add some supply to the non-performing rooms and see what happens. The next step would be to possibly upsize the return trunk and drop?

Thanks again.
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Old 09-29-2010, 10:22 PM   #11
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New system, are my ducts adequate?


Return = 2 sq in x 66kbtu 132 sq in

Actual return 8x24=192
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Old 09-29-2010, 10:23 PM   #12
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New system, are my ducts adequate?


A 10X24 or larger drop would be more appropriate. The hard right angle from horizontal 16X8 to the 8X24 is very restrictive.
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Old 09-29-2010, 10:38 PM   #13
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New system, are my ducts adequate?


Quote:
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A 10X24 or larger drop would be more appropriate. The hard right angle from horizontal 16X8 to the 8X24 is very restrictive.

I don't like 16 x 8 either. But none of the people I have met ever wanted to change the RAT. Said they could put up with a little extra noise. And 8x24 I would on all new jobs go with a 10x25 wit an external filter rack and a slide filter cover.

I came across a 80kbtu 95% VS 2 stg drawing ten amps on 8x17 supply trunk and the RAT was 8x14. Ac was 31/2 ton. It was screaming!

Customer wanted it quiet. Was ready to pay for new duct work 'til he found out we had to tear down the drywall ceiling.
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Old 09-29-2010, 10:42 PM   #14
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New system, are my ducts adequate?


Lots of drops are undersized. And about 40% of the air flow problem.

Then squared corners/turns instead of radious ells to add more restriction.
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Old 09-29-2010, 10:42 PM   #15
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New system, are my ducts adequate?


Quote:
Originally Posted by civiltoatee View Post
Thanks to both of you for your responses.

Just to clarify, the two BRs that are cold/hot each have two outside walls, but the supplies take off the trunk as round, transition to oval, up throught a common inside wall, transition back to round, run across each floor with a register in the floor under a window in each case. On the second floor is also the MBR, which is bigger and has three outside walls. However that room has two 6" oval supplies, but both actually come up through the outside 2x6 walls just inside of 2" of rigid insulation. This room tracks quite tightly with the thermostat in both winter and summer.

Given the general nature of my question, I guess i should not be surprised that one of you seems to think my 8x24 return drop is undersized, and one thinks it is optimal. Should the return drop have a cross sectional area similar to the supply plenum? 8x24 is considerably less than 19x17. Is it fair to say that the cooling load cfm is what will challenge the size of my duct system the most? Therefore, should my return drop be capable of handling 2.5 x 400 cfm/ton?

It appears that neither of you think my duct system will be woefully undersized for my new components. My thinking is that I will attempt to add some supply to the non-performing rooms and see what happens. The next step would be to possibly upsize the return trunk and drop?

Thanks again.
Yeah you got a lot of travel there but the more take off you add the less volume for the rest of the house.

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