ATF: Bighorn - Whisper Quiet ducting design issue

You're missing how ducting works. Each vent is not supposed to "catch" the air going by. The AC unit pressurizes the duct and then the air goes out the vents. An A/C puts out "X" CFM of air. The vents are sized to put out "X" CFM of air. I don't know what the ratio is but there is one. Once the duct is pressurized the vents flow the amount of air they were designed for which should be less than the A/C is capable of to account for leaks. If the ductwork is leaky then pressure can't build up and force the air out the vents. That's why a sealed duct is so important. If putting noodles in just past the last vent helps the airflow it's because it blocked off a leak somewhere past it and it built up a little more pressure. There is supposed to be room past the last vent so it's not at a dead end, some design principle there but I don't remember what it is.

Take a balloon and put 6 pin holes in it. You can blow it up so far and equal air gets out each pinhole. Now cut a large hole at the far end and see what happens. No pressure buildup and no air out the pinholes (vents)

Look how much air a regular A/C puts out when the quick cool vent is open. Now where is all that air going in the ductwork?

I am watching this thread closely as we are having the same problem.

avvidclif said:

You're missing how ducting works. Each vent is not supposed to "catch" the air going by. The AC unit pressurizes the duct and then the air goes out the vents. An A/C puts out "X" CFM of air. The vents are sized to put out "X" CFM of air. I don't know what the ratio is but there is one. Once the duct is pressurized the vents flow the amount of air they were designed for which should be less than the A/C is capable of to account for leaks. If the ductwork is leaky then pressure can't build up and force the air out the vents. That's why a sealed duct is so important. If putting noodles in just past the last vent helps the airflow it's because it blocked off a leak somewhere past it and it built up a little more pressure. There is supposed to be room past the last vent so it's not at a dead end, some design principle there but I don't remember what it is.

Take a balloon and put 6 pin holes in it. You can blow it up so far and equal air gets out each pinhole. Now cut a large hole at the far end and see what happens. No pressure buildup and no air out the pinholes (vents)

Look how much air a regular A/C puts out when the quick cool vent is open. Now where is all that air going in the ductwork?

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Well, I need to "catch" some of it!

Another thought is with the new systems you have return on one side of the FW and outlets on the other side, cold air is suppose to fall and heat suppose to rise but does one think that with more then one return your sucking out too much air from more then one location? There has to be an answer to all these problems that everyone is having. I do know from personal experience it takes more then one AC to cool a 35 ft FW down in 90+ degree weather. But that's about all I know.

I believe avvidclif is on the right track. The system has to build enough pressure to force the air down through the vents. If not, where is that pressure leaking.

The units may have to come off the roof to make sure there is not a problem with the pressure bleeding into the return air keeping the system from reaching it's max pressure.

osims said:

I am watching this thread closely as we are having the same problem.

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Us too. I discovered the duct extends 36-38 inches beyond the last vent into the front and rear caps. I have blocked this dead space by making a 'dam' out of cardboard covered with aluminum AC foil tape. It has helped but what helps the most is removing the last louvered vent. There is plenty of air in the duct it just doesn't seem to want to move out the vents. I also believe part of the problem is the living room AC blows air toward the bedroom and the bedroom AC blows air toward the living room. They meet somewhere in the duct work and 'fight' each other on the 'trip' to the louvers, then little comes out. My thoughts.

Check the foam blocks at the front and back of the duct runs. They were an issue with mine. I have sealed them. I have also insulated much of the ceiling area inside the duct channel. It was a process but helped with condensation in kitchen and bath. My ductwork is also compromised by the framing of the bathroom vent and skylight and I can not get to them easy enough to fix it. I am getting good sir flow and can maintain 70 in 90 degree full FL sun... IF I can keep my front ac from freezing and leaking... Which is partly related as the ductwork end caps were allowing air mixing... Or at least that the operational theory for now.

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I've been following this post with great interest and I think Jim Gratz might be on to something worth pursuing. I have the older system not the "Whisper", but I could never understand why we couldn't get our FW cooler and I think Jim is onto something. I'm going to make some of those changes stopping air from going further then the last outlets and see if it makes a difference. My second AC (the turbo jet) is not ducted so it's only our old style main AC that I want to make a difference with.

Ok, because my phone is too big to fit in the vent/duct here's the best I can do to get some pics of where the AC air comes from and into the long duct. Looks like some issues but hard to tell.


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Erika, those pics indicate to me that the problem may very well stem from the install of the ac units from the factory. If that gap on the right is allowing the air to leak back into the return chamber of the ac it could very well cause the loss of pressure required for output and effect the overall cooling performance. If I understand correctly you don't have access to the unit from inside on the whisper quiet system to make sure that it is sealed off between the output and return. That could mean it's really hard to be assured of a good seal during the install of the unit on top.

Hope you get it figured out, it's gonna be getting hot soon here in Texas.

Here's a rough drawing of what I think I am seeing (and the airflow path for the output only. Not the intake.)

If that is the case, could you pull the AC and possibly clean up the area? But, do you think it would change the output airflow? I agree with the pressure theory. I think the ducts are loosing the pressure. Somewhere??? I have good airflow when I open the quick cool, close it and hardly anything out the ducts.

Some more info:

Based on my pic in post #69, i could see what looked like pieces of metal at the corners of the AC box and the ductwork that looked like they were not bent back around the corners properly. I squeezed my hand in there and did some work on that and tried to add some metal tape as well to cover any gaps. I need a smaller arm diameter!

Note, the following is with NO VENTS on the output side.

Before doing the above work, I used an app on my phone to test the wind speed. I was getting around 21mph out the vent closest to the AC, then 16, then 12 for the next two farthest away, toward the rear of the coach.
After the folding/taping I tested again, and got 24mph, 18, and 14.
I then blocked off just past the next duct opening to the forward in the coach, in the kitchen.
I tested again got 31mph, 21mph, and 17mph!
Plus, a significant improvement in the air coming out of the kitchen vent! Granted, this means no rear AC air is going to the bath or bedrooms. But I feel the improvement in the living area is worth it, since the bedroom AC does an excellent job to cool its areas.

I will have to wait and see when the outside temps get hotter to determine actual difference, but over all the interior feels much cooler.

That's what I've been thinking about, splitting the system, block off at bathroom and let bedroom unit cool bedroom and bath and living room unit cool LR

Travel Tiger what was the name of the app that you used? and when are expecting you new vents?

Jim Beletti,
As Heartland's Director of Owners Interests, would it be possible to have the Heartland factory air conditioning engineer/expert weigh in on this problem? Maybe explain how this system is designed to work. I'm sure many more owners than have posted here, are having cooling problems as well. Myself included. Thanks.

The IOS app was called ZephyrPro. I don't know how accurate it is, but I wanted something to quantify the difference I was making.


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Erika, I looked at the vents in our '14 BH, no whisper quiet, and they protrude into the boxed ductwork about 3/4". We experience good air flow so I don't believe the vent hardware is contributing to the lack of air you are experiencing.
It has to be a design or assembly flaw...


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travelin2 said:

Erika, I looked at the vents in our '14 BH, no whisper quiet, and they protrude into the boxed ductwork about 3/4". We experience good air flow so I don't believe the vent hardware is contributing to the lack of air you are experiencing.
It has to be a design or assembly flaw...


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Good to know! I have the new vents but since we are experimenting I haven't put the new ones on yet. They will require different holes to be installed.

TravelTiger said:

Good to know! I have the new vents but since we are experimenting I haven't put the new ones on yet. They will require different holes to be installed.

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Oh. BTW too there were foam blocks just past each end vent but no longer secure. It appears the ductwork continues on past that last vent to the end of the roof.
From an earlier post from way back when I had corrected that issue