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

[blw2]

I just wish they made a small 5,000 btu roof unit to use in place of one of my roof vents as a secondary unit

[vegasruv24.1]

Originally Posted by 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.

I've came a long long way on my 24.1....as of now... I have seen 44 degrees at the reg...and my coach set at 75, the actual temp is 72 degrees and it will cycle off.. temp gun show bedding at 62 degrees as well as counter tops,.. and my thought agree's with yours, it not the AC but the ability to retain the cool...the coach consists of about 40% single pane glass, and add the low R value walls and such... we do use or coach sometimes for boating, but most use is winter at the dunes and off roading..then it's the opposite , try'n to keep it warm without running the heater all the time and killing the batterys..mostly we use the heater to keep the chill of and set the stat at 50 at night... then crank it up in the morning, the bubble wrap in the windows was a huge add... now instead of the glass showing 130 degrees, it has been a few degrees cooer than the wallsand also, though it could be better, it's working good enough to be very comfortable..
I'll be watching to see if I missed something..thanks… vegas24.1

[16ACE27]

Originally Posted by gmtech16450yz

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.

They use the same AC units on trailers and 5th wheels as motorhomes and nobody runs those going down the road. They're not going to make a new design just for motorhomes if the legacy design is "good enough">

[Chance]

Originally Posted by blw2

I just wish they made a small 5,000 btu roof unit to use in place of one of my roof vents as a secondary unit

I’m curious why you’d want such a small roof-mounted unit? Is it so you can run off 30-Amp service or 4-kW generator at same time as your other A/C? I’ve though about it to run off Inverter overnight without need of generator, but battery cost is still too high for my taste.

For what it’s worth, I’ve seen specs for units that small (low capacity) marketed in Europe, but they run on 220-Volts. And surprisingly, some are so inefficient that they nearly use as much power as one of our 11,000 Power Savers. Beyond that I’m not certain if they use same opening size as we do in US.

With your A/C experience and engineering background, you may be able to modify an A/C to get what you want or need.

[gmtech16450yz]

Originally Posted by Tfryman

They use the same AC units on trailers and 5th wheels as motorhomes and nobody runs those going down the road. They're not going to make a new design just for motorhomes if the legacy design is "good enough">

EXACTLY!

Hey blw2, if I was going to add another roof air to my Vegas I think I'd go with the Mach 3 Power Saver. My Onan is good for 33 amps. The 15k draws around 15 amps. The Mach 3 PS is supposed to draw around 10 amps. So as long as I don't try using the microwave or anything else big the Onan should have no problem running both of those units at the same time. If I want the easy solution on mine I might just go that route.

I'm not giving up on the 15k yet, I have it off the coach and I'm trying a few cheap/easy mods to see if it helps any. I can't rotate it 180 very easily because of the ducting, that's a bummer. So much for increasing the efficiency while driving.

[BradnKaren]

Originally Posted by Long & Winding road

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.

BINGO! This, I think, is the biggest cause of our A/C woes. We have double pane windows but our coach is mostly black. Pics from last summer, and that's with the awning out in that area.

[Chance]

Originally Posted by BradnKaren

BINGO! This, I think, is the biggest cause of our A/C woes. We have double pane windows but our coach is mostly black. Pics from last summer, and that's with the awning out in that area.

Interesting.

Walls are very thin and there is no insulation where aluminum framing is sandwiched between wall panels. It would also be interesting to see if you can detect warmer and cooler areas on wall by moving readings side to side and up and down. Of course, that also depends greatly on how warm it is in RV (air temperature).

[BradnKaren]

Originally Posted by Chance

Interesting.

Walls are very thin and there is no insulation where aluminum framing is sandwiched between wall panels. It would also be interesting to see if you can detect warmer and cooler areas on wall by moving readings side to side and up and down. Of course, that also depends greatly on how warm it is in RV (air temperature).

We will be back in the coach next weekend. I will see if I can get more readings then.

[vegasruv24.1]

Originally Posted by Long & Winding road

Here is a few pics of the insulation on the roof and temp gauge inside vent.

question??? I'm wondering about the condensation on the pipe... where does it go???and is the thought that It will cool the Freon? is it not supposed to use the air to keep the line dryer?

[Beau388]

Originally Posted by blw2

I just wish they made a small 5,000 btu roof unit to use in place of one of my roof vents as a secondary unit

Air Products (Coleman) makes a Polar Cub 9,200 btu rood A/C. I have one in my coach's bedroom. They are much quieter than a traditional roof A/C and of course use less electricity

[16ACE27]

Originally Posted by vegasruv24.1

question??? I'm wondering about the condensation on the pipe... where does it go???and is the thought that It will cool the Freon? is it not supposed to use the air to keep the line dryer?

That's the suction pipe from the evaporator back to the compressor. The insulation is preventing heat transfer to the freon to keep it cooler. If the insulation does its job, there is no condensation on the pipe.

[vegasruv24.1]

Originally Posted by gmtech16450yz

EXACTLY!

Hey blw2, if I was going to add another roof air to my Vegas I think I'd go with the Mach 3 Power Saver. My Onan is good for 33 amps. The 15k draws around 15 amps. The Mach 3 PS is supposed to draw around 10 amps. So as long as I don't try using the microwave or anything else big the Onan should have no problem running both of those units at the same time. If I want the easy solution on mine I might just go that route.

I'm not giving up on the 15k yet, I have it off the coach and I'm trying a few cheap/easy mods to see if it helps any. I can't rotate it 180 very easily because of the ducting, that's a bummer. So much for increasing the efficiency while driving.

they used to put 2 AC's on the roof of a 4k onan gen powered coach... the thermostat would only let one hard start at a time...both fans would come on,.. one compressor, then after it was running for a few minutes, the other compressor would kick in,..my thought was this... I was gonna put another AC where the front vent is,..and up the gen to a 5k,.. done it before... pretty easy, and the new 4k will sell quick..so cutting the cost way down.


but.. the other way, maybe the cheapest … would be to just change out the plug by the door to 30 amp...and just make sure not to use other stuff when both are running...

but,..with all the windows covered,..112 degree's ambient , i'm able to get 72 degrees inside and the unit cycles off..so I really don't think I'll need it...


still have to do a havasu trip to see for sure..if there is a AC that only draws 10 amps max, you could do it with the same plug... I just hope they didn't wire it w/ 14-2...

[vegasruv24.1]

Originally Posted by Tfryman

That's the suction pipe from the evaporator back to the compressor. The insulation is preventing heat transfer to the freon to keep it cooler. If the insulation does its job, there is no condensation on the pipe.

I'm not a HVAC tech.... but wanna learn here...if the AC takes the moister out of the inside...where does it go??? I have never seen an AC that didn't drip water with humid high temps..

[16ACE27]

Originally Posted by vegasruv24.1

I'm not a HVAC tech.... but wanna learn here...if the AC takes the moister out of the inside...where does it go??? I have never seen an AC that didn't drip water with humid high temps..

Moisture condenses on the cold evaporator coils and runs down to a catch tray underneath it. In RV ACs the catch tray just drains to the roof.

In some window units they have engineered a way to take that condensate and have it blown over the condenser coils to add evaporative cooling to the condenser making the AC more efficient.

[Chance]

Originally Posted by Beau388

Air Products (Coleman) makes a Polar Cub 9,200 btu rood A/C. I have one in my coach's bedroom. They are much quieter than a traditional roof A/C and of course use less electricity

You’re correct that it uses less power than standard A/C.

http://www.rvcomfort.com/pdf_documen...ture_copy1.pdf


However, the newer 11,000 BTU/hr Power Saver uses even less electricity, and more importantly has lower starting current. A couple of manufacturers are using 2 X 11,000 A/Cs with 30-Amp and 4-kW generators.

Unless they have already done it and is not reflected in specs above, it would be nice to see Coleman reengineer the Polar Cub with Power Saver design to lower power even more. With tremendous growth in van motorhome market, there should be a demand for quiet and efficient 8,000 to 9,000 BTU/hr A Cs.

[vegasruv24.1]

I wanna add this,.. the objective here is,. one air exchange,.. and two exterior R value.. and three AC efficacy..
#1... they blocked off half of the return with the stock grille, that's why the DB is so high...

#2.. putting the bubble insulation in the windows ups the R value and does the most... even in the shade...


#3 to get the most out of the AC, you need to have the right amount of return and service ducts...in stock form, they suffer..

[16ACE27]

Originally Posted by vegasruv24.1

I wanna add this,.. the objective here is,. one air exchange,.. and two exterior R value.. and three AC efficacy..
#1... they blocked off half of the return with the stock grille, that's why the DB is so high...

#2.. putting the bubble insulation in the windows ups the R value and does the most... even in the shade...


#3 to get the most out of the AC, you need to have the right amount of return and service ducts...in stock form, they suffer..

DB?

[Beau388]

Decibels - noise

[gmtech16450yz]

Originally Posted by vegasruv24.1

I wanna add this,.. the objective here is,. one air exchange,.. and two exterior R value.. and three AC efficacy..
#1... they blocked off half of the return with the stock grille, that's why the DB is so high...

#2.. putting the bubble insulation in the windows ups the R value and does the most... even in the shade...


#3 to get the most out of the AC, you need to have the right amount of return and service ducts...in stock form, they suffer..

Nice! Short and to the point, unlike my posts. haha.

BTW guys, during my 3 days in 100 degree heat we had bubble insulation in every window and foam pillows in all the vents. I'll post some pics of temp readings below.

Vegasruv, do you have an amp clamp? That's a really good way of judging restriction to airflow in fans. Yesterday I messed around with the intake grilles, you're absolutely right, they're horrible. I had already taken the filters out of mine weeks ago after seeing how hard they sucked into the opening when you pull them away. One of my mod tries I'm going to do before I give up on this 15k is to redesign the intake and outlet grilles on the ceiling panel. Yeah, what a stupid design.

There is a number 4 that should go on your list above, but it's the part we have no control over which is why you didn't include it as "moddable". It's the actual compressor/evap/condenser/capillary tube part of this all. We can't do much of anything with all that except for maybe trying to change the length of the capillary tube. (Which I was considering but it would actually be easier to engineer my install of one of those residential split units than to open up the refrigerant side of the Coleman.)

The reason I mention that is because the more airflow into and out of the evaporator core, the higher the discharge temps will be. There's obviously a point where you get too much air for it to be efficient. The discharge temps on these units are already REALLY HIGH, you're not going to get down into even the high 30's unless your inlet air is below 60, which is pretty d@mn cold for air conditioner use! The evap anti-freezing probe/thermistor is actually kind of pointless with the way they run the evap pressures so high. A well designed AC should be able to keep the evaporator temps just above freezing even at high fan speeds in high ambient temp situations. These Coleman units don't run anywhere near freezing even when inside coach temps are a cool 70 degrees.

My point is that it's a good idea to increase or optimize evaporator inlet and outlet airflow. Just monitor your inlet and outlet temps though because they might end up being too high because of the limitations of the rest of the hardware.

The mods I'm doing to give the Coleman 15k one more chance are these...

I'm adding 2 high cfm fans to the condenser box. The condenser fan/shroud assembly is truly a caveman design.

I can't turn the entire unit around to take advantage of 60mph air because it would mean swapping the ducting/inlet sides. boo.

I'm adding one or two vents to the grille panel on the outlet side.

I'm redesigning the inlet side of the grille panel to remove the bridge across the opening. (stupid)

I'm going to fill the voids around the evap core with expanding foam and improve airflow through the core/fan area.

I'm going to seal all air leaks around evap and condenser mounting/sheet metal. A fair amount of air was bypassing the evap fan. I'm going to "tune" the position of both the fans on the motor shaft for optimum airflow by using the amp clamp readings.

I'm going to clean up the horrible passageways from the outlet of the evap fan into the coach ducting. It's a mess.

Then I'm going to put it all back together and be done with trying to make a silk purse out of a sow's ear. lol.

On the subject of "Is it the AC units or simply the heat loss of an RV?", I totally agree with the heat loss side, but I know for a fact that these roof AC units are NOT VERY STRONG. The compressors are cheap and weak. The capillary tube system is cheap and grossly inefficient when out of it's optimum range (which is constantly, the way Coleman set up the sizing of the tube). The only thing I can say about these roof AC units is they have excellent airflow through the evap. The delivery side of the fan system is very strong, and with 1/3hp behind it, it takes a lot of restriction to slow it down. It's just too bad they didn't push the compressor/capillary tube/evap and condenser cores further to exploit that airflow. A stronger compressor, with an expansion valve system going through evap and condenser cores with more tube passes would make these units more than capable of cooling even big RV's. That would cost more to make though and cut into their profits greatly.

Hmmmmm, sounds like Thor's business model! Build them fast and cheap and don't worry about what happens after the sale. Welcome to America. We've learned well from the Chinese.

[gmtech16450yz]

Pics for you guys...