Author Topic: Roof Air Install - straddle the center rib?  (Read 5438 times)

Offline rv_safetyman

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Roof Air Install - straddle the center rib?
« on: July 01, 2006, 09:55:59 AM »
I am going  to redo the roof airs on the Eagle.  The front AC is installed in the escape hatch.  I am going to restore the hatch and move the front air forward to cool the driver going down the road.  I am going to install a third AC about mid-roof.
 
The Eagle has a frame rib going right down the middle.  I have heard of folks installing the AC so that it straddles the rib.  You would loose some air flow, but not much.  The installation would be very quick if I take that route (wire has already been run)

I certainly have the skills to cut and weld an opening, but my time will be limited between the current trip I am on and the next trip.  With all the computers I have (engine ECM, two transmission ECMs and the Trace inverter) it will take me a half a day to unhook all of the connectors before I start up the weldor  :(.
Jim Shepherd
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Offline Nick Badame Refrig/ACC

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Re: Roof Air Install - straddle the center rib?
« Reply #1 on: July 01, 2006, 11:11:02 AM »
Jim,

You will have no problem offsetting the roof unit. They are pretty versitile. Of course the side of the rib you choose to strattle will
be the side that the condensate water will run off to. Under normal circumstances the condensate water will pool up in the
condencer fan area so the ring around the fan blade will pick up the water and sloosh it through the condencer coil to help cool
it down.

Hope this Helps
Nick Badame-
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Offline H3Jim

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Re: Roof Air Install - straddle the center rib?
« Reply #2 on: July 01, 2006, 11:15:16 AM »
Jim,

I just cut an opening with my sawzall.  Since I was so close to the ribs and struts, I merely put treated wood in between the inner and outer roof structure for support.  Even without the clamping action of the installed air conditioner, I was able to stand on the edge of the open hole with no discernable deflection.  What do you need to weld again?

JIm

Nick,

I think he is going to center the unit, and leave the strut, which will run down the center of the opeinng.  I did offset mine, and I am happy with the looks inside and out, the strength etc.
Jim Stewart
El Cajon, Ca.  (San Diego area)

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Offline Beatenbo

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Re: Roof Air Install - straddle the center rib?
« Reply #3 on: July 01, 2006, 04:40:37 PM »
Unless you have Carrieres. Offset or unlevel and they will drownd you. I saw a GM a while back with 2 Colemans on the front side by side. I bet 27,000BTU kept that Buffalo cool. ;D
« Last Edit: July 01, 2006, 04:45:24 PM by Beatenbo »

Offline Ross

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Re: Roof Air Install - straddle the center rib?
« Reply #4 on: July 01, 2006, 06:12:13 PM »
I welded in a frame for the center AC.  An off center AC would absolutely drive me nuts.  I like things square and symetrical.

Offline jjrbus

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Re: Roof Air Install - straddle the center rib?
« Reply #5 on: July 02, 2006, 12:07:17 AM »
Going where no man has gone befor!   The total area of the roof rib, assuming 1.5" wide by 14" equals 21 sq in. 14x14" is 196" so you have less than 10% blockage.   Looking at most vents on roof AC's there is not a total of 196 sq in of opening. On the surface it does not seem to be a problem. An inverted v of some type on top of the rib might cut down on any turbulance?
 As I think back on putting in my ducted AC's a 10% restriction in the system did not seem to be a problem. The only thing I can think of is condensation, it might be a good idea to drill a few holes and put spray foam in the center rib. I'm surprised this has not been brought up befor.
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Offline TomC

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Re: Roof Air Install - straddle the center rib?
« Reply #6 on: July 02, 2006, 06:22:28 AM »
I also have the rib going down the center that I did not want to cut since I have three roof airs.  My bathroom is on the right side and is four feet wide (enclosed type-don't like and never have liked walk through-but that should be another post) with a two foot hallway and two foot cabinet against the left wall for the reefer, washer/dryer and closet.  So my design is already off center.  The center and rear A/C's are offset just to the left of the center roof support with a 1x2 wood frame around the opening.  The front A/C is close to the front and offset to the right so to have better aim at the drivers seat.  Since my bus has a flatter roof than others, it isn't very noticeable.  Works well and would do it again.  My theory is the designers of the bus put that center support in for a reason.  I just don't like to cut and try to redesign things if I can help it.  Good Luck, TomC
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Burgermeister

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Re: Roof Air Install - straddle the center rib?
« Reply #7 on: July 02, 2006, 08:30:05 AM »
You can remove the center rib as long as you properly "box" the new opening with/using two parallel stringers.   

If you were really worried about it, diagonals from each corner (front and back) meeting in the center, co-incident with the original stringer as it continues for and aft would take care of the loads nicely!


Offline gus

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Re: Roof Air Install - straddle the center rib?
« Reply #8 on: July 02, 2006, 12:36:11 PM »
I'm no structural engineer but I don't see any way a box could ever pick up the load of that center stringer without using diagonals somewhere.

 It appears to me to be almost the equivalent of a boat keel.
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Offline jjrbus

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Re: Roof Air Install - straddle the center rib?
« Reply #9 on: July 02, 2006, 01:10:16 PM »
 I spoke a little quickly on this, I have ducted Dometic AC's, I don't see straddleing the center rib with this type of unit a problem. The Carrier is another story. They have a flexible, insulated duct about 4" by 6" dropping right down the center of of the AC unit right where the rib would be. You could probly work around this, but it would be eaiser to box in the opening. You would have to look at the type of unit you are installing. The non ducted Dometic may not work in this situation either.
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Offline JackConrad

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Re: Roof Air Install - straddle the center rib?
« Reply #10 on: July 02, 2006, 02:19:18 PM »
Jim,
  Is your hallway centered in the bus or slightly offset?  If it is offset, you would probably be better off offseting the AC to the side of the center rib, so the AC could blow the air further (instsead of the air partially hitting a wall).
  On the Carriers and Colemans i have installed, there is a duct from the evaporator in the Ac unit on the roof that comes through the 14 X 14 opening into the distributuon housing that is inside the bus. This duct is right in the center, where your rib is. You might be able to seal the duct around your rib. Check this duct location on your AC before cutting your hole.  Hope this helps, Jack
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Offline rv_safetyman

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Re: Roof Air Install - straddle the center rib?
« Reply #11 on: July 03, 2006, 05:51:39 AM »
Thanks for all of the input. 

I really want to locate the AC in the center of the bus.  I saw one Eagle with the ACs offset and it did not look very good.

They are Carrier units.  I don't recall a tube being a part of the installation, but it has been a couple of years since I installed the first two units.

One post made the observation that the rib was a fairly small percent of the 14X14 opening.  However, the actual input opening for the air imput to the bus is less than half of the area.  Thus the rib becomes a bigger factor.

As I noted in the first post, I am just trying to avoid having to disconnect all of the ECMs in the bus.  Was also trying to avoid having to do all of the draping of the interior.  The more I think about it, the less of an issue it becomes. 

I think I will break down and fabricate the boxes and do it correctly.

Again, thanks for all the help
Jim Shepherd
Evergreen, CO
’85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
Somewhere between a tin tent and a finished product
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Offline gumpy

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Re: Roof Air Install - straddle the center rib?
« Reply #12 on: July 03, 2006, 05:58:35 AM »
Jim,

Do it correctly the first time and you won't regret it the rest of the time.

craig
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Re: Roof Air Install - straddle the center rib?
« Reply #13 on: July 03, 2006, 06:23:30 AM »
Gus, 

Without going into a lot of calculations, it's hard to say, exactly, if the skin or the stringer is the major load carrying member.

I'm of the opinion, (but guessing at this point) that the stringer isn't the same type of load bearing member as a boat's keel.  I think the stringer contributes to stiffening the skin to resist compression buckling,  already addressed by the spanwise curvature of the skin.

Offline Melbo

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Re: Roof Air Install - straddle the center rib?
« Reply #14 on: July 03, 2006, 07:09:44 PM »
I am not familiar with carrier units but on colemans the output duct is only 4 by 4 and I think it it offset to  the drivers side.

So if you mounted the unit centered on the roof the vent may miss the center rib completely.

Just a thought but it would save cutting the rib.

Melbo
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