Newbie question about Duct Layout in Attic


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Old 01-04-07, 07:30 AM
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Smile Newbie question about Duct Layout in Attic

I have a new home that is just under two years old. The total sqft is 2680' with an unfinished basement. I read all the posts in the Duct System Forum and I'm still confused.

My situation is the normal...my second floor has heating/cooling problems. The front side of the house is hard to heat/cool and the backside of the house is always warm/cool. In fact, we have to close vents on the backside to keep it comfortable in the winter/summer.

I crawled up in the attic this weekend to address the problem and I have seen some things that concern me about the duct work. All of the duct work in the attic is flex duct with the exception of the plenum's and take-offs. The main line splits into a "Y" right off the main line plenum with two (2) 10" flex duct that runs to each side of the house. These 10" runs only go about 15' and then end into a triangle plenum with three 6" flex ducts coming off each side of the triangle plenum. The 6" flex ducts run anywhere between 6' to over 25'. They are twisted and have many turns threw rafters and such. The back side of the house has 6 shorts runs with the longest being about 12'. The front side of the house has long runs with the shortest being about 15' and the longest being over 25'. Now for my newbie question;

I always assumed that you ran one main line in the center and branch your supply lines off the main line and try to keep them all as equal in length as possible, is this not true?

Would it help if I ran one 12" main flex line down the center and then run the supply lines with dampers off the main line to adjust and equalize the airflow evenly for the second floor?

The sun rises on the front north east side and sets on the back of the house. There is 20" x 20" return vent in the main hallway on the ceiling at the top of the steps. There is also a 20" x 20" return grill on the main floor at the bottom of the stairs and a 14" x 14" on the return line at the base of the furnace in the basement. I cut all the doors on the 2nd floor at the bottom to leave a 1.5" gap hoping this would help the airflow problem, but that did not change a thing on the front rooms. It did stop the doors from banging when the system kicked on and off, but that was about it.

Any thoughts or comments would be most appreciated. Thanks in advance.

Jr
 
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Old 01-04-07, 09:37 AM
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Not there cant say for sure. You could try and run the supply flex duct so all the 6" runs are the same length. Try and work the many turns out of them. Most times a 10" 400cfm can go to two 8 "200cfm and a 8" can go to two 6 "100cfm
Or to run one duct down the line and take off it. You would have to make it out of ductboard and take all the 6" runs off it. You dont say how big the units are so I dont know the CFM you have to work with. Up in the attic you do want to use the silver flex it last longer than the black
 
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Old 01-04-07, 09:49 AM
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Ed, thanks for the quick response. I will post the HVAC specs tonight when I get home. The flex duct that is there now in the attic is brown in color.

I was planning on making the plenum's (6 total) for the main line out of 3/8" cabinet grade plywood (I'm an avid woodworker) and sealing all the take-off/dampers with mastic, let sit for 48hours before I hooked up the supply lines. I know it sounds like overkill, but here in Waldorf, MD we don't have much to choose from for HVAC supplies.
 
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Old 01-04-07, 11:16 AM
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Wood duct wont pass code on the supply side can on the return. In some citys. You want to use the insulate ductboard for the ducts up in the attic
 
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Old 01-04-07, 11:51 AM
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Did not know that, thanks for the heads up. Do you know of an online source that sells duct board and the cutter tool to the public? The reason I ask is because I just did a 30 minute Google search and came up with zilch.
 
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Old 01-04-07, 12:11 PM
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http://www.jm-builder.com/heavy-registration.php?section=Products&sub_section=Duct+Board&offer=ahs349


http://www.industrialinsulation.com/air_handling.htm

Try the bottom first ED
 
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Old 01-08-07, 08:16 AM
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UPDATE!

I crawled up in the attic and re-routed the flex pipe to remove all of the twists and sharp turns. I added 10 - 6" dampers to the supply lines, sealed everything up with mastic/zip ties and balanced the system.

100% improvement so far. Now I have to address the noisy return vent in my family room that has been a PITA since we purchased the home.

Thanks Ed, for your professional advice and stellar attitude. It truly helped me in so many ways.

Jr Lew
 
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Old 01-08-07, 08:28 AM
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Now I have to address the noisy return vent in my family room that has been a PITA since we purchased the home.
Might see if more open grill would cut down on the noise. Take the grill off and see. If the return box around it is big We some times line the duct there with scrap ductboard. That helps some times.
 
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Old 01-08-07, 08:52 AM
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The return vent in the family room is 20" x 20", it's up near the ceiling (9'). The return chase is 12" x 18" and goes down straight to the furnace. You can hear the fan motor running and it is very, very loud. We had to install a home theater stereo system there in order to watch TV, thats how loud it gets when the fan is on.

I was thinking about having some metal duct work made in the shape of the letter 'C' to run under the steps down the the floor and back over to the furnace. Maybe this will take that direct path to fan motor and quiet it up. The 'C' will be 8' x 4' x 8' with two 90 degree turns on each end.

The duct work will be hidden under the steps in the basement which is dead space anyways.
 
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Old 01-08-07, 11:13 AM
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Have also put some ductboard on the inside of the blower part of the furnace. This helps. Have you checked the blower when it runs? Is the blower wheel ok?? Might check it and see.
 
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Old 01-08-07, 12:06 PM
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the blower wheel is clean and oiled well. The fan itself is not noisy, it the air that travels threw the rreturn duct that is noisy. You can hear the fan creating the CFM for the system.

When you look into the fan housing, should the fan blades be exposed to the return vent? What I mean is that if I'm looking from the return vent at the base of the furance into the furnace fan housing, should I be able to see the fan blades?
 
 

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