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Old 07-19-2018, 04:41 PM   #72
gmtech16450yz
Senior Member
 
Brand: Thor Motor Coach
Model: Vegas 27.7
State: California
Posts: 289
THOR #10907
This is the discovery I made about 25 years ago on improving the efficiency of these RV roof air conditioners...

We had a 19' Tioga Class C. We absolutely LOVED that thing, it was amazing. We logged over 150 separate trips in it, the Dodge powertrain had over 200k miles on it when we sold it. It didn't come with a generator or roof air. It was built in '73, most RV's didn't have either of those things stock. It didn't have room for anything like an Onan, I actually had to take the propane bottles out of a compartment and install a single propane tank underneath to have room for even a tiny portable generator.

I took a portable Yamaha generator, a 600 watt max unit, and built it into the compartment. Everyone that saw or heard what I was doing said I was crazy. 1- You can't permanently mount a portable generator in an enclosed cabinet. 2- It's WAY too small to run the Coleman roof air I had just put in it. 3- You'll never be able to run a portable generator while driving down the road. 4- The roof air is going to burn up from improper voltage/ amperage. 5- The generator is going to burn up from lack of airflow. 6- The motorhome is going to burn up from having a gas tank inside a closed compartment.

So yeah, I ignored all that and did it! Ducted air into and out of the compartment, ran fuel line from the motorhome gas tank, built a super quiet but more efficient exhaust system for the generator that exited outside the compartment, and wired up a transfer circuit into the coach. It all worked awesome but as I expected, the amperage output of the generator was right on the cusp of tripping it's circuit breaker when the roof AC was used.

So I worked on increasing the power of the generator and reducing the power draw of the roof air. My mods made the whole system work great, except for in extreme conditions at times. Fast forward a few years and we're on a trip through the Utah salt flats in the middle of a super hot summer. I had noticed that the AC and generator would work fine when stopped, but when we started driving it would trip the breaker because the amp draw of the AC was too high. I had my wife drive while I watched my ammeter I had wired into the generator output. The amperage was fine when stopped, but kept rising once we started moving. We pulled over at a rest stop in the middle of the Bonneville salt flats and I literally climbed up on the roof and took the cover off the AC!

I looked at the way the system was designed and was a little shocked. It's backwards!!! The condenser is at the back of the unit, the fan pulls air from outside the rear of the unit, through the condenser and then out the sides. So while at a rest stop in the middle of Utah, I pulled the roof air off, rotated the entire unit 180 degrees, and bolted it back down. (Yeah, I'm just a little crazy.) My wife gets behind the wheel and starts driving while I'm in back watching my ammeter. OMG my theory was right, the amperage started DROPPING as the RV got going faster!

From that day, our AC unit on that old Tioga was mounted backwards. (Backwards according to how Coleman said to mount it and how 99.9% of roof air conditioners to this day are mounted.) It was always more efficient when driving, and the efficiency when stopped didn't change a bit. We used the cr@p out of that generator and AC system until we sold that RV a couple decades later.

So sorry for the rambling post, but it's VERY relevant to this thread and it's good info for anyone that has issues with AC performance while driving. To this day, RV roof AC units are still built in this inefficient way. Why? Like I said, they're selling plenty of them, why would they bother spending money re-designing something that sells? Lots of people don't even believe you're supposed to use the roof air and generator while driving! It's just a perfect example of thinking outside the box. Sometimes something that sounds crazy ends up being not so crazy.

Here's a pic of our old Tioga. If you notice the roof AC, it has extra vent holes cut into the front of the cover. I originally just turned the entire unit and cover around. Later I modified the cover so it looked like it was mounted properly, but the inside unit was still mounted backwards, condenser in front so it could suck in all that fresh air. Actual, factual data from before and after amp readings and years of verification proved that the crazy idea I had at a rest stop in Utah was valid. Don't EVER let anyone discourage you from pursuing crazy ideas!
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