I got some numbers on temperatures. At 82F ambient, the AC can get the trailer to 75F at the countertop level. If you measure the exhaust temp it is around 120-124F. The cold output at the ACU is between 48-50F.
At this point the ACU seems to work fine. It blows cold. It vents hot. I decided to dig into the trailer some to figure out if something was causing it to fail to cool. I'm not the first to do it! Doing a search for "parkliner air conditioner" nets you a few valuable threads both here and elsewhere. Specifically see the one here by "Huck" and "homesteading" dot com. I cannot link them as it won't let me post here for some reason. I'll try to link one on this site
here.
I had several thoughts about what was going on. I'll list my theories below and what I learned.
Theory 1: Floor location prevents full cooling of trailer
Theory 2: Poor exhaust vent performance
Theory 3: Overly restrictive exhaust fresh air supply
Theory 4: Poor separation between hot/cold sides of ACU
Theory 5: Undersized ACU for application
Theory 1: floor location
This makes intuitive sense that it should not be great for performance. Even if a typical air-conditioning system doesn't blow from up high generally the return will be high on a wall or ceiling such that it is taking in warmer air and replacing it with colder air in a cycle. In the Parkliner ACU's design it takes air from down low and spits it out down low. And pointing laterally right at the door!
Using an infrared thermometer I verified that there is a distinct temperature gradient. See the graph below. It shows pretty clearly that 1) there a big difference between the floor and ceiling when the A/C is running alone and 2) when you add a fan to mix the air around.
Theory 2: Poor exhaust vent performance
The aluminum grill over the holes cut in the
fiberglass shell seemed to me to be quite restrictive. Then once I dug into how the "enclosure" was constructed I felt more and more like it was inoptimal. Consider this: if you took the exhaust pipe from a car and then stuck it into a box and then cut a hole in the box on the opposite side: would it work? Yeah, sort of. Hot exhaust gasses would surely
escape from the hole you cut but not before swirling, slowing down and heating the box to the same temperature as all the exhaust gasses. And that assumes the box has no leaks! I think the same thing is happening in the ACU enclosure. The enclosure warms up quite a bit and is not air tight. The PO did wrap the inside walls with reflectix which probably helps to some degree. You can still measure a distinct heat differences. Most surfaces in the trailer are mid 70s and the
fiberglass of the enclosure at the rear exhaust area are ~100F when it's running. That heat will radiate into the trailer. How much does it matter? I dunno. It's a relatively small area and typically the top is covered by a thick seat cushion which would be a good insulator. Hopefully some pictures make all this clearer.
I measured exhaust temperatures when the ACU was sitting on workbench in the open vs. the exhaust temperatures I read from the trailer grill. They were identical. Aside from the radiated heat from the enclosure I don't think the exhaust vent was restricting performance of the ACU.
Theory 3: Overly restrictive exhaust fresh air supply
See pictures. This is rather complex and I don't want to write it all up. The short version is the ACU needs fresh air to blow over the hot coils in the back to remove heat. The enclosure has very little space for this fresh air to flow. Pretty much no air can get to the left side, very little to the top and the right side has fresh air access via 6 holes cut into the enclosure. The PO did some rough cuts to the plywood to allow some air to the left. There ARE fresh air holes in the bulkhead below the ACU. You can see them in the pictures with the wires/plumbing in it.
In addition to perhaps not having enough fresh air to cool the condenser if fresh air intake is restricted it would mean the fan was creating negative pressure and sucking in outside/ambient air into the trailer. I think both of these ideas are reasonable but I couldn't think of any way to test them so I just moved on.
edit: I measured the fin area of the original ACU and the new one as well as the openings in the fiberglass. The numbers are: Orig ACU: 130 in^2; 8000BTU ACU: 150 in^2; 2 holes each 42 in^2
Theory 4: Poor separation between hot/cold sides of ACU
In a regular window-unit the "hot" and "cold" sides are separate by the window clamping down on the unit and little accordion separators. This installation doesn't have that. I think it simply relies on the tight squeeze of an installation to keep them separate. I didn't bother testing this and I regret that. I should have worked harder to create a sealed separation to see if it would have improved performance. As it is I don't know the answer. My gut tells me each side is "separated" as much as if you cracked a window in a room an inch. There is definitely a difference but there is also definitely a lot of waste.
I wish I had some pictures to share here but I don't.
Theory 5: Undersized ACU
Perhaps everything is designed/implemented acceptably well and a 5000BTU unit is simply not enough to handle the load. A poorly-insulated, sun-exposed, all-sides touching the outdoors little egg is very different from a room with a single outside wall in a well-insulated house.
Unexplored idea: Leaking door seal
I realized when writing all this up that one thing I had not considered was the door seal. The ACU points directly at the door. And if the seals aren't great there you could imagine cold air constantly leaking out through them into the less dense, hotter air outside. I'll see if I can figure out a way to test that in the future.
What do I think the issue is? All of them to some degree or another. I wish I had spent some time sealing things better before tearing it all apart but here we are. If anyone else has a Parkliner and wants to chat I'd be happy to give you the details of what I would have tried knowing what I know now.
Next up: A short aside about a roof A/C