super efficient duct work


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Old 09-17-05, 02:56 PM
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super efficient duct work

We are preparing to replace our entire heat pump system and duct work to move it from the crawl space into the attic. Many reasons for this, but my main question revolves around some very efficient duct work that a potential hvac contractor discussed with me. This duct work uses less registers and returns for the house. Some of the differences from traditional follow. Instead of having flexible duct work all the way to the boot, they use rigid elbows. Flexible duct work is only used in straight runs. Also instead of just using duct tape at joints, there is some sort of mastic tape. Also, there is a very rigid pressure test run on the system after the install. Supposedly, this means you can use say a 3 ton unit rather than a 4 ton unit on traditional duct work.

Some of the benefits include a much more efficient system overall and our local power company gives you a discount for the installation. Has anyone ever heard of something like this? How much does it cost vs. a more traditional system - 1.5 times, twice? Is it really worth it? Just trying to figure out if this is a rabbit hole I want to chase any further or not.

Thanks in advance!
 
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Old 09-17-05, 05:58 PM
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duct

if i understnd you right your saying there useing flex hose for the long straight runs then going to hard pipe to grills
 
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Old 09-17-05, 07:08 PM
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yes - plus they are adding mastic tape to all joints - not just duct tape. Don't know what else is being done if anything. And not sure just how much more efficient a system becomes with these revisions.
 
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Old 09-17-05, 08:03 PM
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Forgive me...but what you have described sounds like a normal duct install. No one uses "Duct Tape", and in most areas, Duct tape is not allowed by code.
 
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Old 09-18-05, 09:06 AM
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Steve-

hmmm...very interesting. This is exactly why I have come here looking for some assistance. The guy kept referring to it as "silver tape" - I'm just assuming duct tape. Maybe he meant the more rigid metallic tape?? I don't know. Just searching for some info.

Thanks!
 
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Old 09-19-05, 09:48 AM
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Ok, the silver tape is allowed by code in some places, and is a better tape for ductwork, but with that said, mastic tape applied with mastic is the best.

From what I have seen, and I am not a HVAC guy, the normal install usually has a main trunk from the unit, with flexible shoots off of it to the various output vents.
The trunk sometimes will taper and get smaller as it gets to the end of the run, thereby maintaining even pressure.
From what I understand, you want a riged box at the vent, and likewise a rigid connection on the trunk, and only be using flex pipe between the two.
When flex is bent too sharply it restricts airflow, and should only be used for gentle bends.
The rigid connections at either ends of the flex give a good surface for the flex to attach to, and makes sealing the joints much easier.

Like I said, I dont know much about the subject, but I am in the process of having a unit installed myself, and this is what I have gleaned from all the different sales pitches.

On a side note, you mentioned in your first post that you were moving the duct work from the crawl space to the attic, again I am not the expert, but as far as the duct joints themselves, the attic is the hardest on the adhesives used to seal the joints. The heat in the attic can approach 150 degrees F and is a known contributor to the failure of both mastic and foil backed tape.
 

Last edited by Stevetra; 09-19-05 at 11:35 AM.
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Old 09-20-05, 04:46 AM
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Get a few differant quotes and make sure they do load calculation "Manual J" and a duct calculation "Manual D" to make sure everthing is the proper size.

I hate flex duct and will only use it when rigid wont go. If it's used then rigid elbows would be best, but eventually flex duct fails and if you happen to get any critters in your attic they will burrow into it. If at all possible just go with rigid ductwork for the whole job.
 
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Old 09-21-05, 04:22 AM
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Thanks a lot for the thoughts everyone. As for the heat in the attic - this house is older and has soffit vents (2x12 spaced about every 8 feet), gable end vents and a thermostatically controlled power exhaust fan. As part of this upgrade, I am installing the 2" wide strip vent around the entire house in the soffit and a ridge vent for the length of the roof. This will give me much more ventilation area than what is there now. And I would certainly hope that my attic never reachs 150 with this amount of ventilation. My current fan has the thermostat set at 115. From what I understand, I need to remove the powered fan once the ridge is in place because it will mess up the chimney effect that is desired by installing the ridge vent and strip vent in the first place. I have many hot spots around the house and I hope that the new attic ventialtion and HVAC system will help this.

Thanks again to all!
 
 

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