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Ok, I've got a couple minutes for a quick update guys. Sorry to be awol, had a busy few days of camping, boating and basically playing!
FACTUAL discoveries on this roof AC situation...
1- (Sorry in advance, this one's gonna sting a little.) MOVING THE PROBE WILL DO NOTHING. I took the time while I was trying to get on the road last Thursday and did the test I suggested here. Pulled the probe, put in ice and warm water. Unplugged the probe. Checked resistance of the probe (thermistor) while ranging the probe through various temps. Installed fixed resistor instead of thermistor.- Conclusions... Thermistor ONLY comes into play at 32 degrees or below. Result of thermistor detecting evaporation icing is it turns off the compressor. Amperage draw will go from ~15 amps to 5 amps (fan stays on) and inlet/outlet temps will equalize. If the probe detects icing OR the thermistor circuit is OPEN, your AC will be totally inop. It is not a variable system.
2- System design- As I've tried to say, it's OLD AND INEFFICIENT. (Fact, not opinion.) The system these RV roof AC's use isn't a variable displacement compressor, or cycling clutch orifice tube, or expansion valve system. It's the most basic system made, a CAPILLARY TUBE system. It's a system that basically has very little range, if operating conditions are out of it's optimum range, it's efficiency will drop dramatically. Meaning if it's super hot out or the heat load is excessive, it's out of it's range. (Again, FACT, not my opinion.)
edit- This is huge actually. There has been talk about BTU's. Some think "It's a 15k BTU unit, it's gonna put out the same as any other 15k BTU unit. And it's never going to put out much more or less than that 15k BTU rating." WRONG. It's tested at a set temperature to come up with that 15k BTU rating. What happens when the unit is used outside of that test range of temps? That's where one 15k BTU unit could possibly keep putting out 15k, but another poorly designed 15k unit might drop down to 5k when it's out of it's perfect efficiency range. This is a HUGE deal to me when I'm looking at solutions. AC certification testing (in my opinion) should be done at several different temp ranges. That would show which units work more efficiently when out of optimum conditions. Perfect example... I'd like to see the test results between a Coleman 15k and the Dometic Blizzard 15k when running on top of an RV in 110 degree heat. They could, and probably are VERY different. (Not saying I'm buying the Dometic yet. lol. It's still the same old design, just optimized a little better it looks like.)
3- The brand new (2018) Coleman 15k that is on the roof of my 2018 RV is LESS EFFICIENT WHEN MOVING as it is when stopped. Read my story about the rest stop in Utah. What I learned almost 3 decades ago still applies today 100%. I monitored amperage and inlet/outlet temps for 3 days in 100 degree temps. Amperage goes UP and outlet temps GO UP when you're driving at freeway speeds. Slow down to a stop and amperage drops, along with outlet temps.
The short explanation for this is the fact that the basic design has a harder time getting airflow in and out of the condenser and compressor area when there's a 60mph wind going over the entire unit. This to me is insane. Why on earth would you not exploit the benefits of airflow? That 60mph wind could totally eliminate the need for the condenser fan end of that 1/3hp electric motor. I monitor AC high side pressure on the engine AC system. Sitting still AC pressures (which track roughly the same as temperatures btw) will go as high as 300psi. (That means the high side lines and compressor are all around 300 degrees!) At 60mph, those pressures will drop to roughly half that, around 150psi. That's because the AC condenser is taking advantage of that free airflow. That's why your car AC works much better when moving. as it should. The fact these roof AC's don't take advantage of moving air is (in my opinion) insane.
4- These RV roof AC units are horribly ill-equipped to handle the heat loads of these coaches. My opinion, but if you fully understand the facts of the design, have the knowledge necessary to properly evaluate and assess, and have the personal experience with real-world testing, you will come up with the same conclusion.
The answer? I'll say this... I WILL FIND IT. lol. We were in 100 degree temps for 3 days. I started my generator and roof air at 9am the first day and it didn't turn off or cycle until 10pm that night. The lowest the inside coach temp went was 81 degrees. That's insane! And TOTALLY UNACCEPTABLE. The second day we left it all off while the RV sat at the boat ramp in the sun all day. The inside temp was 100 degrees with everything sealed and insulated up as well as possible. The awning was out all day on the sun side even. With generator/rear AC and the front AC on full while driving, it got down to ~87 degrees after a couple hours on the road. (The aftermarket AC's in the front of these cutaway E450's isn't that great either.) I burned about a 1/4 tank of gas in the generator alone over those 3 days! That's not acceptable to me.
I WILL figure this out guys, just hang in there and give me time to come up with a solution. I can tell you this, the things you all have come up with do help (with the exception of moving or removing the probe/thermistor). Make sure the cold and hot sides are separated. Make sure the vent routing is as efficient as possible and free from restrictions. Add vents like some have done, it will help. And insulate all the roof stuff too, it will help a little. But as far as a silver bullet to make these systems be able to keep up with the heat load of these RV's, nothing we've talked about here so far is it.
I'll be putting my thinking cap on to come up with the solution for MY coach. Hopefully I can come up with something that can be done in the hands of you guys too. In my mind, (and my amazing wife's mind also), labor time or money is no object to fixing this. We will not accept 87 degree inside temps even if it is 100 degrees outside. F that! I said something here when we first started talking about all of this... It was something like "These roof AC's haven't appreciably changed the basic design or gotten any more efficient than they were almost 4 decades ago." I still stand by that statement. The Coleman Rotary roof AC we put on our then new 1977 American Clipper was almost identical in design, operation and performance to the unit on our 2018 Thor. What a joke.
(BTW sorry to YELL and put stuff in bold. Just trying to make sure everyone gets the details in what I'm saying. I tend to type a lot and put a lot of important details in my posts. You might want to re-read it a few times to get it all!)
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