After not being able to cool my coach I installed the third mid-ship 15K air conditioner, plus the second awning in an attempt to cool it down. These efforts (and expenses) really had no effect on cooling down the coach. Regardless of what I did, in 90 degrees or better heat, I could never get below 80 degrees; and this is just not comfortable.
The first idea I wanted to try was making a custom plenum box. The existing box is what appears to be typically used in the industry. It turns out this is highly ineffective as it is a horrible way to direct air. This is what I did:
First, I pulled the rear-most air unit. I was cursing the entire time as both the air conditioner and the roof were glued to the gasket/seal. From what I have now learned, this is not the proper way to do this. Only one is supposed to be attached to the gasket and the other is just a compression fit. I spent over 6 hours removing the air conditioner and gasket. This was really an unnecessary job.
The plenum box looks like this:
In the above pictures, you can see that the air actually has to fight to get into the small opening of the vent. You have a vertical wall above and below the vent that does not assist this effort at all.
Pretty standard from what I have seen during my research into this issue. But, this is not an efficient way to move air at all it turns out. The air conditioner sits atop the plenum box and slams air into the box. The air moves around until it finds the opening (vent) and finally moves down the line.
I decided to make a custom plenum box that fits into the existing box. My box would funnel the air directly into the vent eliminating the chaos of air slamming around the plenum.
I started with a piece of foam between hard paper that I purchased at my local craft store. I cut it to the size of the floor of the plenum box. Prior to placing it, as it was too thin and would have left me with part of the vertical wall, I placed two shims to bring the bottom of the floor to the exact height of the vent.
Then the floor
As you can see, the floor Is now on the same plane as the vent. A little silver foil tape and on we go.
Next was this back wall. I scored the back of the piece so there would be a slope to it; again, guiding the air to the vent.
Some more foil tape and on we go
I then cut a piece to fit atop the plane of the top of the vent, setting it at an angle to meet the air conditioner once it is reinstalled.
Once the air conditioner was reinstalled, everything was silver taped to provide a clean, smooth surface to guide and direct the air directly into the vent (I had to remove the fan & motor assembly to tape everything).
So, let’s talk number because all of this is moot if I can’t back it up (and I can).
I purchased a digital anemometer and took lots of readings both prior to any modification and after. Here are my results:
Rear Zone Air Conditioner – No modifications:
Rear roof vent: 700 Feet per Minute
Mid roof vent: 900 Feet per Minute
Front roof vent: 787 Feet per Minute
Total Feet per Minute: 2431
With the new custom plenum box:
Rear roof vent: 706 Feet per minute
Mid roof vent: 1201 Feet per Minute
Front roof vent: 1142 Feet per minute
Total feet per Minute 3049
Difference: An increase of 25% airflow
I did the same upgrade to the mid-coach air conditioner. Here are those results:
Mid Zone Air Conditioner – No modifications:
Rear roof vent: 334 Feet per Minute
Mid roof vent: 137 Feet per Minute
Front roof vent: 433 Feet per Minute
Total Feet per Minute: 904 Feet per Minute
With the new custom plenum box:
Rear roof vent: 590 Feet per Minute
Mid roof vent: 216 Feet per Minute
Front roof vent: 1004 Feet per minute
Total feet per Minute: 1810 Feet per Minute
Difference: An increase of 100% airflow*
*This number may be a little off as when I pulled the middle air conditioner I noticed the “technician” that put it up failed to place the foam barrier above the metal separating wall between the return air and cooled air.
I did notice an anomaly that I didn’t expect and was actually quite shocking to me. Air is being sucked back into the vent while using the air conditioner. When using the rearmost unit, the cooled air comes out of the vent at the front edge of the roof vent. I can place my hand and feel in the middle of the vent there is no air coming out at all. And, the digital anemometer showed the air is actually being sucked back into the vent. This should not be happening and it happens at all three vents. I took a video of this which you can see here:
This is really bizarre to me and someone much smarter than I needs to figure out how and why this is occurring. But, if the unit is delivering cooled air and then sucking it out, then this cannot be very efficient.
But, I wasn’t done yet. I was curious to see if I could even do better. I was not sure why when using the rear air conditioner there wasn’t what I considered a lot of air coming out if the rear roof vent. So, I tried a little experiment and wow, the results are amazing. I placed a block, actually some silver tape in the middle of the air vent at the front of each vent. Holy crap, did this change the air flow:
Man, this was crazy how much more efficient this was. Here are the numbers:
Rear roof vent: 1575 Feet per Minute
Mid roof vent: 960 Feet per Minute
Front roof vent: 964 Feet per minute
Total 3499 Feet per Minute
This is an increase of 450 Feet per Minute over and above the plenum box modification. But, I still wasn’t done. I had seen where some folks were putting short pieces of PVC pipe inside the vent to provide more space. My vents seem compressed to me and are just 13/16ths at the smallest measurement. So, I cut 6 2&1/2” PVC pieces in 2” lengths and shoved them inside my vent, raising the top of each vent. The numbers speak for themselves:
Rear vent with tape and pipe: 1732 Feet per Minute
Mid vent with tape and pipe: 1122 Feet per Minute
Front vent with tape and pipe: 846 Feet per Minute
Total 3700 Feet per Minute
I started with 2431 Feet per Minute and ended up with 3700 Feet per minute a 52% total increase in airflow on the rear unit alone! Imagine the rear and mid-air units on at the same time?
The total cost was around $20.00 in silver tape and foam board. If the air conditioner isn't glued to the roof (via the gasket) then it should take around two hours per air conditioner. I hope this helps folks.