Try this first before you buy 2nd AC unit or bring your unit in for AC service.

[halfprice]

Moved probe up and could only get 61 output

I moved out and had it hanging by the vent. 61* again

Moved it to the original spot and im at 56* but inside temp is now 83* so im at 27* swing

Also pulled the end duct vents and found they keep on going to cool nothing

So ill stop at home depot to get some expanding foam spray to blick tge vents off. This way all the air will come done the vents.

The front vent channell went a good three feet. The rear about one foot. No need to cool these areas

Jerry

[Bob Denman]

Thanks for a most interesting, and VERY cool thread!
(I'm still trying to get my head wrapped around how to do this! As descriptive and complete as your posts: I'm a mechanical dummy! )

[halfprice]

Originally Posted by gmtech16450yz

Pull it out and put it in a glass of ice water.

lol. That's what she said.

Seriously. Try putting the probe in a glass of ice water, and then in a glass of hot water. I really think the first step is to understand how exactly the probe is working. If you can determine the exact water temp the probe reacts to as far as compressor cycling, then we're onto something. And compressor cycling might be (is) totally separate from thermostat cycling you know. If you don't have any way of monitoring amperage draw, you might be able to hear a change in compressor noise or vibration when it's running.

As far as moving it an inch and a half and getting a change from 59 to 61, I'd almost call that within sample variation and not really conclusive. (NOT being snotty, just trying to help.)

Im not going to do the water test. Its at 55* now and inside is dropping fairly quickly. So im pretty happy with it as the probe is concerned

I will buy the frost king insulation and climb up top tgat might help. I sure dont think it will hurt anything.

And a foiled taped all the seams in the ceiling area eventhough it looked pretty good. Guess that cant hurt either.

And gm you dont have to feel i get offended at your suggestions. I dont so suggest away

Jerry

[Long & Winding road]

Ok great. Let us know your results once your all done.

[gmtech16450yz]

Dammit you guys you're making me late for my boating weekend!

I'm trying to get things loaded up and ready to go but I've also been trying to get answers for you all too. I did the testing. I know how the system works. I'll explain later but don't worry about debating, theorizing or trying out stuff, I have the answers. Hang in there and let me at least get to the lake. lol.

[Bob Denman]

Have fun!

By boating; do you mean:

Fishing?
water-skiing?
tubing?
cruising?
Fixing?
or
drinking?

[vegasruv24.1]

I 'm not one to argue with the sign post and take the wrong way home, but if I tell you the best way to go and you know a better way, I'll listen,... and I have noticed this... the more heated the debate... the more people think before they post,... and that leaves the results... a half dozen or so get involved , while thousands reep the benefits,..I for one have for years done mods to ac's try'n to get the meat locker temps inside...it's a long hill....So, I read here lots of ideas, the most I've mess with,.. but gotta say, the insulating the roof duct caught my interest...so up on the roof I went again, temp gun in hand...lots of over 200 degree readings...as far as insulating the box....what did that really do... the box is already insulated ...but for sound, I had put phat mat on it... but I had the unit off..so this time, temp gun in hand, I took the cover off while it was running,... as soon as I removed the back half.... bamm… there's the problem...the reason that the fan suck in instead of blowing out is it does two things... one cools the condenser , and two,... cools the compressor... while the temp at the condenser was like 130...the compressor was at 220... and whats happening is that 200 degree air is forced up and around the air box... that was also at 200 degrees...so with the cover off, I found that the whole unit worked better.. and whats going on is the cover won't let the heat out, so,... with the cover off,.. I went and drank a beer...when I went back up there... the condenser was at 130.. the box was 110, the compressor was at 180,.. so my thought is what needs to happen is the front section of the cover needs to be sealed to not let the hot air in and around the air box...and I don't know if insulating the top and side does this.... but if it doesn't, I'll put a foam seal on the cover,... I also noted that there were only the four screws on the top,,, and not the usual ones on the bottom sides... mine was getting blown way out there from the fan..

so my conclusion is this...the insulation on the box wall by the compressor is a must...and next, I need to figure out a way to modify the rear section to let the heat out and at the same time keep from turning it into a bird house...lol

[Long & Winding road]

Good Ideas. Let us know how it works out.

[JamieGeek]

Had the RV home today and had a few minutes so I figured I'd pull apart the A/C shroud on the inside:

What you see here is that the divider between the intake and output is firmly in place, its pretty well sealed up through the roof, and the freeze probe is dead center of the condenser (this is what it says on our datasheet re the A/C: A/C, ROOF, 13.5M BTU, MACH 3, BLACK--I gather I have a Mach 3 13,500 BTU A/C). We have a very early Axis so my guess is that Thor was building them more carefully at first...

Sadly I only have the fancy laser temp gun so I can't measure the air temperature (I'll have to rectify that when I can).

Since I had a roll of duct tape handy I thoroughly taped around the edges of the divider since it only had some foam to seal there. I also removed the freeze probe and taped it to the A/C supply line (so its just hanging in the air stream):


Didn't have a glass of ice water handy to try that trick but I did hold the probe in my hand to warm it up but after a few minutes I really didn't notice any temperature difference (kind of hard without a thermometer and I likely didn't wait long enough).

We're camping this weekend, sort of, but I don't know if I'll have enough power to run the A/C (a family member is having a BBQ and invited those of us with RV's to stay at their house--they have a few acre's).

So that is my very very very unscientific experiment LOL (yeah I can do better...).

[Long & Winding road]

Originally Posted by JamieGeek

Didn't have a glass of ice water handy to try that trick but I did hold the probe in my hand to warm it up but after a few minutes I really didn't notice any temperature difference .

Oh....Didnt I tell you..... These are "Rectal Thermosters" .... not for "Hand" use. LOL Use it properly and let us know your results.

When you get a chance do some more testing and let us know the results. I will test mine a few diff ways this weekend if I get a chance.

[Chance]

Originally Posted by JamieGeek

....cut.....

What you see here is that the divider between the intake and output is firmly in place, its pretty well sealed up through the roof, and the freeze probe is dead center of the condenser (this is what it says on our datasheet re the A/C: A/C, ROOF, 13.5M BTU, MACH 3, BLACK--I gather I have a Mach 3 13,500 BTU A/C). We have a very early Axis so my guess is that Thor was building them more carefully at first...

.....cut......

Did you mean to write evaporator in lieu of condenser?


Your results are not unexpected. I designed and installed a few 300-Ton blast freezers with 10 X 30-Ton evaporators, and if you could stand the -40 F air moving at high speeds (wind chill close to -100 F), you could actually see the frost form on the coils (evaporators) over time. During start-up I made a lot of quick trips in and out in a freezer suit to collect data. Anyway, we fed liquid from bottom, which resulted in initial frosting at bottom, but for the most part the coils would frost throughout their entire surface before we’d put them through a hot-gas defrost cycle — one coil at a time so blast freezer could remain in operation at all times.

Ice would indeed form a little thicker at bottom near liquid feed, but the difference in temperature across the entire face of coil isn’t as large as some here would think. As long as there is liquid to evaporate, temperature differences across evaporator isn’t huge. If it were, something else needs to be corrected. It’s all pretty simple science when you break it down, and equipment manufacturers had a lot of great data we could confirm in field.

The same principles apply to air conditioning, except that when operating well above freezing, it makes design and operation much simpler. Coolers at or below 40 F air temperature have similar freezing issues, but at A/C room temperatures of +/- 70 F, it’s easy to avoid freezing by controlling the suction pressure.

[blw2]

wow, the thread really exploded. I recon I'm probably the engineer who was meant to be on the receiving end of some snarky comments a couple pages ago.
I don't really want to get into the weeds on that, except I do want to point out that my comments were originally driven becasue I was trying to understand the worth of insulating the plenum vs moving the temp probe, etc.... but with the data presented it wasn't telling me...so I was hoping for some clarification.... and that evolved into a simple effort of trying to help folks to better understand a little something...

By no means am I or was I trying to say that the mods discussed weren't helpful...of course they were.... As pointed out empirical evidence proved that they were.The OP would of course know if it's cooling better now than before. I was just trying to better understand the mods...

Originally Posted by JamieGeek

.....

Sadly I only have the fancy laser temp gun so I can't measure the air temperature (I'll have to rectify that when I can).

......

I assume you mean one of those infrared temp 'guns' with a laser pointer....
I've found that you can get a pretty good sense of what's going on by measuring the surface temp of the diffusor/register after the air has passed over it for a few minutes. Not traceable to lab standards.... but a good approximation....

and yes, for what it's worth, I'm a mechanical engineer. Used to work in my dad's appliance repair business as a teen and summers during college, fixing window air conditioners mostly. That was many years ago & I'm not an HVAC expert by any means, not even close. I did take an HVAC engineering class as an elective in school though....not that that means much either.... it was interesting at the time though, learning about cooling and heating loads, properly sizing unit capacities, etc....

[gmtech16450yz]

blw2- I'm SO SORRY, I did not mean you. I'm not that great on remembering screen names, but I actually remember yours as being someone who contributes solid, quality posts. Sorry! Keep contributing!

Dammit, see, that's what happens when you come at any situation from a negative instead of a positive. My post about a member was negative. Like I said, I really try to live my life in the positive. I'll try to keep it that way here.

[gmtech16450yz]

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!)

[gmtech16450yz]

Just starting to do my research and getting the wheels in my head turning... lol.


I don't think I'm going to have much luck trying to redesign or heavily modify the existing Coleman 15k unit. There's simply too much wrong with it. haha. Dometic did try to optimize the basic RV roof air design, but it's still a pretty primitive unit that only really works well in a certain set of conditions. Poor airflow, weak/cheap compressors, small condensers and evaporators without enough tubing passes and finally the capillary tube limitations are just too much to overcome. I could replace the capillary tube with an expansion valve, but I'd still have the other problems. I could increase airflow through the condenser and compressor area, but the static restriction of the capillary tube would still hold back any increases in efficiency. I could increase the compressor output, modify the capillary tube and tweak the refrigerant volume for better cooling, but still have the undersized evaporator that would hold back any other improvements.

So basically Coleman did "OK" with the dirt cheap components and design they've used for 4 decades. After all, it probably costs them around $100 to build one of these units that retail for between $500-$700. Why bother changing it when they're probably selling all of them they make?

This is how far out of the box I'm willing to go... haha...

https://www.pioneerminisplit.com/col...ystem-full-set



So far in my trying to figure out a solution for our coach, this looks interesting. Unfortunately we're limited to 110 volts in our RV's, otherwise I'd be looking at 220 volt systems. But look at the design- little things like brushless motors. Wow! Hey Coleman- try jumping into the '90's and embrace brushless motor technology! Geez.


I should probably add a little basic physics lesson to all of this discussion. All we want in our RV's is cool or comfortable air when it's hot outside. To do that, we need to REMOVE heat. That means we're going to be creating heat to end up with cool. I know that's pretty basic, just thought I'd start simple.
That's why what I mentioned about stationary RV's vs. moving RV's is a big deal. Removing heat is way easier if you have access to basically "free" 60mph airflow. The design of these roof AC's doesn't take advantage of it in any way.

The next basic principle is the system has to control the pressures in the condenser and evaporator to efficiently remove the most amount of heat. You can either do that by cycling the compressor, changing an orifice/valve or simply making the system work without any feedback or control whatsoever. The final example is what we have on our roofs. The output is fixed. Which btw is why the thermistor/probe mod to move or remove it does nothing. IF it was an expansion valve or cycling clutch system, it WOULD have made a difference. It's not.

So now that you understand that, look at what these ductless split systems are doing- They have brushless compressor motors! Wow, like I said, welcome to the '90's. lol. So to control evap and condenser pressures, they simply speed up or slow down the compressor. That's huge.

Anyway, I hope some of you guys learned something so far. haha. I'll keep thinking and researching, then I'm gonna order something and start modding. Yikes! I'm excited because I know there's a TON of room for improvement on what's been the same old cr@p they've been putting in RV's for decades.

[gmtech16450yz]

$728 for the hardware is relatively cheap. Grafting it into the Vegas would be interesting though. I think it could be done pretty cleanly. Like I said, I'm willing to do whatever it takes. As much cr@p as I give Thor for their garbage assembly, we really do like this Vegas and plan on keeping it a long time.


https://www.amazon.com/WYS012-17-Air...QYE6GY7DTWHJ0G




.

[Beau388]

Any good electrician can wire your coach for 240 volts. You just need a separate entry panel with circuit breakers and your present entry panel becomes a sub panel. If you have a 50 amp coach, it is your panel that splits the legs to two 120 volts circuits.

[BradnKaren]

Our 2017 38ft Challenger has the exact same two Coleman 13,500k BTU units that our 2002 38ft Southwind had, but the inside temps between the two motorhomes in the exact same weather conditions is night and day. The Southwind would hold 72 degrees and lower all day long with no trouble. The Challenger struggles to hold at 78-79 degrees when the outside temps are 89 degrees, as they are today, as I sit here in my RV. I don't think the A/C units are the problem.

[gmtech16450yz]

Originally Posted by Beau388

Any good electrician can wire your coach for 240 volts. You just need a separate entry panel with circuit breakers and your present entry panel becomes a sub panel. If you have a 50 amp coach, it is your panel that splits the legs to two 120 volts circuits.

Good point but I should have said for OUR use, 90% of the time our AC is going to be used, we'd be on generator and not shore or campground power. So what I meant was "I'm limited to 110 volts if I want to use the AC on generator power".

As far as being able to make those kinds of mods, I replaced the weatherhead and main drop on our house last week. lol. The city inspector kept saying "You need to hire an electrician for that". I said, "Can you just please come out and look at what I've done first?" lol. Yeah, I got my building permit signed off after the inspector couldn't find any part of what I did faulty. Even down to torquing the main breaker box lugs properly to 250 inch/lbs. So yeah if I only used the AC on shore or campground power, I'd put in a 220 volt circuit so I could use a 220 volt residential AC unit.

[Long & Winding road]

Great info GM tech, Did you have your shades done in all your windows (including the cockpit) when your rig was in full sun?

Did you use any of the reflective bubbles in the RV at all?

I think the only "easy" way to make the AC more efficient on our rigs is to reject as much heat as you can before it gets into the RV.

If your in one spot all day (or longer) try and use reflective bubbles.

I know my window tint helped tremendously esp driving down the road.

Plus adding the clear tint to my shower skylight and the fat mat to the tops of the roof vents helped further. I did a write up on this about a week ago.

On Saturday I was working on my Axis and I put my hand on the inside wall with the full sun on it and it was very warm. No wonder the AC struggles. If they just made the wall 1/2 thicker with foam insulation that would help tremendously. Plus if we had double pane windows.

I thought about this on SAT as well..... It would be nice to have an awning on the driver side that way you can put it out for the afternoon sun to keep the entire side of the RV in shade. I also thought about if you mounted hooks on the side of the RV about in inch below were the walls meet the roof you could attach a 20 x 8 sunshade of some sort and anchor to the groud with stake or weights of some sort. You might be able to use some of those industrial suction cups (the kind with the lever that holds incredibly well).

Also if we have an option on the way the RV is parked - it would be much cooler to have the rear of the RV in the direct sunlight (esp in afternoons). Not only is the wall much smaller compared to your side (only 94 - 101 inches on most RV's) it wont have the front cockpit windows to let the heat in. Put refective bubble in the rear windows would be help as well.

Let use know if you come up with any more answers. Thanks for your time and relentless pursuit of perfection.