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

[carlgorski]

How about installing a fan to blow directly on compressor or a vent to have air while you are driving blow over it?

Originally Posted by gmtech16450yz

https://pdfs.semanticscholar.org/ea3...8748bbe044.pdf


I really like reading tests like this, you can learn a ton from them. For those that don't like reading technical gobbly gook like this, here's a quote from the conclusion...



This little quote could explain why a lot of you had older coaches that cooled fine, but now have similar sized newer coaches that don't.

So I stand corrected on a comment I made about these RV roof air conditioners not changing much in the last 3 or 4 decades. I was wrong. They got LESS EFFICIENT, mostly in very hot conditions. I don't know about you guys, but I usually want my AC to work at it's best when it's really hot. lol. When we need these roof AC's to work is when it's super hot and there's a ton of heat load. That's when people are complaining about their AC performance. The surface temps on the deck of my boat on that 100 degree day were as high as 165 degrees. I didn't go up on the roof of my RV, but I'm betting the temps in that roof air unit were well above that. I'm willing to bet the compressor temps were easily nearing 200 degrees, if not over. High and low side pressures are ~ 500psi/250psi on these 410a systems, which is insanely high. Automotive R12 and R134a systems generally run high and low pressures around 175psi/35psi. High pressures create high heat. That heat has to be dealt with in order for the AC system to properly cool the inside air.

Wanna talk about heat loads? Somebody needs to calculate the BTU's created by these generators running under the coach and the inefficient roof air conditioner compressors pumping out tons of heat simply to build these high pressures (not even mentioning the actual heat extraction process). The whole process is WILDELY INEFFICIENT. You have a generator turning mechanical energy into electrical energy. Then you have a roof air turning that electrical energy back into mechanical energy. (Twice actually, one being the compressor and the second being the fans.)

Think about this as an alternative, IF the RV equipment manufacturers cared to invest R&D money into this stuff... The generator could easily directly drive an AC compressor AND fan system for the air distribution mechanically, skipping the entire mechanical/electrical/electrical/mechanical conversions and their inherent losses.

That scenario would be the ultimate in efficiency. The next level would be something like the residential split systems. (I'm still seriously considering grafting a mini-split system into my Vegas BTW.) They are FAR more efficient than what companies like Coleman and Dometic are putting out for the RV community. The next level would be what RV roof AC systems USED to be, with a refrigerant that worked better when you really need it, when temps are very high. (Obviously that's not possible, those "bad" refrigerants are gone. Just stating it because it's a level that was above what we now have.) The very last level of efficiency... What we have on our newer coaches. It's cr@p.

Yes, everything you guys have mentioned about the actual heat loads of the RV are valid. Yes, it's a major factor. But the bottom line, IMHO, is that these roof AC units aren't even close to as efficient and strong as they easily could be.

BTW, as far as my particular RV's heat loads go, I've got more insulation in mine than you guys do, I can guaranty that. Somebody mentioned the front cap being empty behind it. Yeah, If you guys have been keeping up with my mods, I GUTTED THE ENTIRE CAB of my Vegas and insulated all of that stuff. I even gutted the rear cap, which also has NO insulation, and insulated it. I've got literally thousands of dollars in insulation and sound deadening in my Vegas. If mine's getting hot, a stock one would be getting inferno hot. lol. That 99 degree reading on my inside temp picture I posted? You can bet that number would be significantly higher on a bone stock Vegas or similar coach. Mine's not stock.

Here's some pics of my Coleman 15k for you guys. So much of what I see here makes me just shake my head. Like the squirrel cage fan and how the area it's in is designed. WTF??? Just plain inefficient/old/poor design and cheap components.

[gmtech16450yz]

Originally Posted by carlgorski

How about installing a fan to blow directly on compressor or a vent to have air while you are driving blow over it?

Smart guy! I already ordered a couple pancake fans and got them a couple days ago! I'll be putting them alongside the condenser fan to aid airflow through the condenser. I did that on the roof air "project" I had those few decades ago and I could see the difference in efficiency like night and day. I added a 12 volt electric cooling fan to the condenser box on that old Coleman roof air. I used 12 volts because my little 600 watt generator was already maxed out running the 110 for the AC. I had plenty of 12 volt power available. I had that extra fan on a switch, I could turn it on and literally see the amperage the AC was pulling drop and the outlet temps also drop. It helped. It's one of the things I'm trying on this 15k before I toss the whole thing in the trash. haha.

I'll take pics of the fans for you guys. They will actually blow directly on the compressor too.

BTW I'm reading more about 410a vs. r22 and r12. (I am professionally trained, certified and know a sh!tload about r12 and 134a automotive systems, but know very little about r22 or 410a.) To get evap temps of roughly 40 degrees these are the approximate pressures needed...

r12- temp and pressures run close to each other- about 40 psi.

r22- roughly 70 psi.

410a- roughly 120psi.

Bottom line... the change to the more "friendly" refrigerants has resulted in WAY higher compressor loads. If your condenser temps are around 130 degrees, that means that little compressor has to put out almost 500psi on it's high side! That's a lot of work, and builds a lot of heat.

Hey carlgorski, our daughter lived in Fairbanks for 3 years (military) and loved it!

[carlgorski]

I put a fan in outside ref compartment to get air flow so I figured it should work on a/c. Electronics Tech for 40 years so I know heat is not our friend.

Originally Posted by gmtech16450yz

Smart guy! I already ordered a couple pancake fans and got them a couple days ago! I'll be putting them alongside the condenser fan to aid airflow through the condenser. I did that on the roof air "project" I had those few decades ago and I could see the difference in efficiency like night and day. I added a 12 volt electric cooling fan to the condenser box on that old Coleman roof air. I used 12 volts because my little 600 watt generator was already maxed out running the 110 for the AC. I had plenty of 12 volt power available. I had that extra fan on a switch, I could turn it on and literally see the amperage the AC was pulling drop and the outlet temps also drop. It helped. It's one of the things I'm trying on this 15k before I toss the whole thing in the trash. haha.

I'll take pics of the fans for you guys. They will actually blow directly on the compressor too.

BTW I'm reading more about 410a vs. r22 and r12. (I am professionally trained, certified and know a sh!tload about r12 and 134a automotive systems, but know very little about r22 or 410a.) To get evap temps of roughly 40 degrees these are the approximate pressures needed...

r12- temp and pressures run close to each other- about 40 psi.

r22- roughly 70 psi.

410a- roughly 120psi.

Bottom line... the change to the more "friendly" refrigerants has resulted in WAY higher compressor loads. If your condenser temps are around 130 degrees, that means that little compressor has to put out almost 500psi on it's high side! That's a lot of work, and builds a lot of heat.

Hey carlgorski, our daughter lived in Fairbanks for 3 years (military) and loved it!

[Beau388]

Most AirXcel A/Cs use the condensate to wet the evaporator with a "sling ring". You could run a water line up to the condenser and spray a very small amount of water through the fan and onto the condenser. They actually sell such a device for home heat pumps. Supposedly, it increases the SEER considerably, or so the adds state. I would guess very soft water is a necessity.

[Long & Winding road]

GM Tech, great info. Let us know what you come up to mod the factory AC unit without re-inventing the wheel. Most of us don’t know half of what you know about automobiles and refrigeration......Perhaps we can add a solar fan to the top of the outside shroud? That sounds like an easy fix that might help some.

What insulation materials did you use for the front and rear caps. Also where on the rear cap did you insulated? Was it the small spot under the ceiling (on an angle to the back wall)? If so how did you open it up without tearing it up? I like to keep my RV factory looking and would love to insulate that area as well.

Thanks. Keep the updates coming.

I find myself checking this tread 5 + times a day to see what the latest info is.

[Long & Winding road]

Originally Posted by Beau388

Most AirXcel A/Cs use the condensate to wet the evaporator with a "sling ring". You could run a water line up to the condenser and spray a very small amount of water through the fan and onto the condenser. They actually sell such a device for home heat pumps. Supposedly, it increases the SEER considerably, or so the adds state. I would guess very soft water is a necessity.

Great Idea. I was wonder about the same thing myself. I wonder if the AC would spit out enough water to make this work well enough to be worth it? I hate to climb on the roof to fill a reservor or have to pump up water. Like to keep the mods farily simple but yet effective to my AC unit if possible.

[Pete'sMH]

Wouldn’t it just be easier and cheaper to add a second air conditioner and be done with it?

[Long & Winding road]

Originally Posted by Pete'sMH

Wouldn’t it just be easier and cheaper to add a second air conditioner and be done with it?

Yes but that defeats the purpose of this tread. We are trying to maximize the cooling (for the average joe) of the current unit. I have made improvements to mine where I don’t feel I need a 2nd unit so I was trying to pass that along to every one else so they can "try" some easy fixes before they spent $800 + on a 2nd unit.

But GM Tech has the knowledge and is going way above and beyond what I ever thought of (or have the skills for). So in his case - Yes - I would just get a 2nd unit before I took my off the roof and totally upgraded to something Colman would have never done. But he likes to tinker with this stuff and had valuable info for us so I applaud him.

[JamieGeek]

Originally Posted by Pete'sMH

Wouldn’t it just be easier and cheaper to add a second air conditioner and be done with it?

But not nearly as much fun!

[Beau388]

Let's see - $2,000 for a second A/V vs $20 for plastic hose and fittings plus a tube of Dicor to hold the 1/4" tubing on the roof.


I changed my coach over primarily for the second, quiet bedroom A/C and that with the 15,000 btu A/C and water heater on, I could not turn on any other appliance. Substitute microwave for water heater, or toaster, or induction hotplate or ice maker+ charger. I have three 15 amps, 120 volt circuits of duplex receptacles (13 total) in my coach, on cold mornings, I want to be able to max them out. I got tired of managing my electrical loads.
In January in Fredericksburg, I manged to get 39 amps on leg 1 and 41 amps on leg 2 according to the PI EMS. We were warm and toasty inside at 26 degrees outside, cooking breakfast. We used no propane for the two weeks we stayed in the area.

[Long & Winding road]

Originally Posted by Beau388

Let's see - $2,000 for a second A/V vs $20 for plastic hose and fittings plus a tube of Dicor to hold the 1/4" tubing on the roof.


I changed my coach over primarily for the second, quiet bedroom A/C and that with the 15,000 btu A/C and water heater on, I could not turn on any other appliance. Substitute microwave for water heater, or toaster, or induction hotplate or ice maker+ charger. I have three 15 amps, 120 volt circuits of duplex receptacles (13 total) in my coach, on cold mornings, I want to be able to max them out. I got tired of managing my electrical loads.
In January in Fredericksburg, I manged to get 39 amps on leg 1 and 41 amps on leg 2 according to the PI EMS. We were warm and toasty inside at 26 degrees outside, cooking breakfast. We used no propane for the two weeks we stayed in the area.

Thats good - not sure how to go about changing the legs of power like you have - do you have 50 amp service now but one is at 39 and one is 41? Confusing to me. No propane is awesome for the wintertime.

I think If I had an RV that was 29 foot or longer and I didnt have a 2nd AC installed at factory I would consider it. But for our small 25 - 27 foot Axis/Vegas we are "trying" to make due with our current units and having fun "trying" different things.

I like the Idea of adding a 2nd unit to an RV with only 30 amp service (and only 4000 watt Genny) but have another plug in the you used just for the extra AC unit when your at the camp grounds. That way you dont have to update the entire fuse panel to 50 amp esp. when you only need more juice just for that AC.

[halfprice]

I'm glad I added my second ac. We run the bedroom during the day, keeps the livj g room quite and living room at night, quite in bedroom. Both cool great.

With that said I like to tinker too which us why I'll add the insulation on the box and pipe. I'll get to it soon. Been to busy gearing up for a fishing trip. I want to get a 200lb blue fin tuna

Jerry

[Long & Winding road]

Originally Posted by halfprice

I'm glad I added my second ac. We run the bedroom during the day, keeps the livj g room quite and living room at night, quite in bedroom. Both cool great.

With that said I like to tinker too which us why I'll add the insulation on the box and pipe. I'll get to it soon. Been to busy gearing up for a fishing trip. I want to get a 200lb blue fin tuna

Jerry

Have fun fishing! Let us know your results when you add the insulation.

[gnach]

Originally Posted by Beau388

Let's see - $2,000 for a second A/V vs $20 for plastic hose and fittings plus a tube of Dicor to hold the 1/4" tubing on the roof.

I changed my coach over primarily for the second, quiet bedroom A/C and that with the 15,000 btu A/C and water heater on, I could not turn on any other appliance. Substitute microwave for water heater, or toaster, or induction hotplate or ice maker+ charger. I have three 15 amps, 120 volt circuits of duplex receptacles (13 total) in my coach, on cold mornings, I want to be able to max them out. I got tired of managing my electrical loads.
In January in Fredericksburg, I manged to get 39 amps on leg 1 and 41 amps on leg 2 according to the PI EMS. We were warm and toasty inside at 26 degrees outside, cooking breakfast. We used no propane for the two weeks we stayed in the area.

Your expanded circuits would be a worthy project for many of us, even those with the 30a service that Vegas, Axis and Cs have. Would you care to start a new thread on how you went about that. Two weeks without propane is enticing. TIA.

[halfprice]

To hot to climb on the roof to insulate the box but it was a good day to fill in the ducting. As I stated earlier my ducts continue past the vents to the front and rear caps where ac isn't needed

First I used expanding foam for doors and windows. This foam is low expansion. Now that probably would have done the job but I had a little piece if dynamat left over. So I cut it kind of the shape and shoved it in the ducting. So now the air will bounce off the dynamat and go down the vent where needed.

Still waiting for a somewhat cool day to climb on the roof. Hopefully soon

Pics of ducting with insulation added


Jerry

[Ron1Z]

Installed new Coleman/ Mach digital thermostat today $73.00 on ebay. Its a hell of alot better than the old stock one that gave us a 7to 8 degree swing off to on. The new one has a 2 to 3 degree swing.

[blw2]

Originally Posted by Chance

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.

Several reasons, but yeah, primary is that it's a 30A rig..... yeah, I can run my AC on a 15A circuit...for a little while under good conditions..... so prob could run two 15kbtu units at 30A...but I wouldn't want those sort of limitations.

Other reasons....
I just don't think all that much more capacity is needed. I haven't actually sat down and dusted off my old HVAC design text book to calculate an estimated thermal building load.... I know it's big.... but for my needs where I am in the SE, this whole thing is really only a big problem a relatively small amount of the total time.... and I'm not full timing with it either.
There is actually a pretty big down side to having an AC system with too much capacity.
It's only really needed a small amount of the time
The idea is only to supplement the front AC by cooling down the bedroom.

I suppose an alternative idea might be to replace the 15Kbtu with something around about 20Kbtu or 22kbtu...but the 15k is enough much of the time.

and side note...yeas, by front nose cone is similar to that earlier photo showing the thin outer wall with no insulation... and the back wall where it's radius up to the roof is the same way. Quite a few areas that could be better insulated...would help with the road noise too....

[Beau388]

Originally Posted by gnach

Your expanded circuits would be a worthy project for many of us, even those with the 30a service that Vegas, Axis and Cs have. Would you care to start a new thread on how you went about that. Two weeks without propane is enticing. TIA.

It is relative easy if you tour the Thor plant 750 and see where the original wiring harness are routed. They won't let you take pictures, but I had a steno pad and made voluminous notes. Basically there are three 120 volt circuits: inverter, GFCI and normal 120 volt. I rewired the GFCI circuit for 20 amps by using a CAFI in the power center, pulled 12 ga Romex from the previous junction box behind the inverter to the bathroom, to the kitchen/outside duplex receptacles. As I have three unused CBs in the PD4500 and I need another kitchen circuit to be completer later.... I left the normal 120 volt circuits the same for the dinette and the couch, except I change changed out the magnetic 15 amp circuit breaker for a 15 amp CAFI one. I did add a dedicated 20 amp CAFI circuit to a single duplex receptacle for an electric heater on the dinette wall. The final mod was to use another 15 amp CAFI CB to the inverter bypass relay and pulled 14 ga Romex wire from the inverter junction box to a new duplex receptacle in the dinette wall for the portable ice maker. I still have to change out the remaining 15 and 20 amp CBs to the CAFI type which I will get to this fall when it we quit traveling.

[Chance]

Originally Posted by blw2

.....cut....

I suppose an alternative idea might be to replace the 15Kbtu with something around about 20Kbtu or 22kbtu...but the 15k is enough much of the time.

......cut....

As I mentioned before, two smaller A/Cs seems to be a growing trend for mid-size motorhomes that are a little too large for one 15k A/C. The ones offered so far that I’m aware of all have two 11,000 BTU/hr Power Saver A/Cs.

If done from the factory, I’d guesstimate that total added cost would not be more than about $1,000. These units retain the base-case 30-Amp service and 4-kW generator, so they appear to only add a second A/C and needed power management to automate what can run simultaneously. I think it’s a great idea if done from the onset. Retrofitting may get a bit more expensive though.

[JamieGeek]

Having the RV home for a sec I took off one of my forward A/C vents and found exactly the same thing everyone else has (and what halfprice mentioned a few posts above): The channels don't stop at the vents and continue forward at least as far as I can stick my hand in. On our Axis they probably are attempting to cool that big, empty, uninsulated space in the front cap.

I didn't have any expanding foam handy, nor any dynamat or anything else. So I reverted to using the Mythbusters favorite building material: Duct tape.

I used about a dozen 6" strips of duct tape to fashion a ramp to direct the air out the vent and stop it from going forward. With the vent back on it was really easy to feel the difference between the untaped vent and the taped vent.

It doesn't look pretty, but it works:

(With the vent on you can't even see it.)

I proceeded to repeat the procedure for the other 3 vents at the end of the duct work.