3 AC Unit - Performance Test Results

[LewisB]

I have seen lots of opinions expressed regarding the use of 3 AC units in larger RV’s. There is widespread agreement that during very hot weather (+100°F), typical 1 or 2 AC unit installations are inadequate to keep a large RV cool. Living in Tucson, AZ, where temperatures can go over 110°F, we share that experience.

I have not been able to find any specific testing and/or test results on the effectiveness of using 3 AC Units (vs. 2 AC Units) in a large RV. So this thread is intended to advance some actual test results.

I am NOT a representative for Keystone. I AM an avid camper and toy-hauler owner. We have owned multiple camping units over the years. Our most recent purchase is a 2018 Keystone Raptor 353TS, which we love. The good news is that you CAN stay comfortable in very hot weather if you are capable of making some modifications (may be relatively easy) to your RV and have access to a stable 50A RV shore power supply.

Here are my test results from a hot day in Tucson. With 3 AC units running I was able to maintain 74°F inside the trailer while 109°F outside - a 35°F differential. In fairness, this was what I would call a "parked" condition - that is, I started the AC units in the cooler part of the morning.


Here are the test results from a hot day in Tucson with only 2 AC units running. In fairness, this was what I would call a "travelling" condition - that is, I started the AC units at the hot part of the day to simulate pulling in to an RV park after being on the road for some time. As can be seen, the typical 20°F was achieved in the bedroom, only 10°F in the main cabin, and the garage was nearly at ambient temperature. This makes the trailer nearly un-usable under these conditions until very late in the evening!


I will post an additional message with links to more tests, my thoughts on why and how this happens, making modifications, etc. If you are interested, just follow the subsequent links.

We struggled with the decision to make modifications to our new Keystone trailer. We also had to install a new 50A service at our house. But our trailer was un-usable as a "guest house" for 4-5 months out of the year due to hot weather. So we bit the bullet and proceeded; I can only say that this is the best modification I could have made to this trailer. I am thrilled with the performance shown here. Hopefully, this might help others in some way.

[LewisB]

Test #1 - 3 AC units started in the hot part of the day
Once your trailer gets hot, it takes a lot of energy to cool the internal mass of the contents as well as the air. This test resulted in only a 26°F differential. It took about 2.5 hours to cool to the point of being acceptably comfortable, and 3.5 hour to cool to the set point on the thermostat. However, this was significantly better than when only 2 AC units were in operation.


Additional Information:
If you have a 2018 or older "Pre-Omni Chill" AC system:
1Background.pdf

To understand more about thermal efficiency of your RV:
2Thermal Efficiency.pdf

Why does Keystone build trailers with 3 AC units and then set them up to allow only 2 units to operate at a time? OMHO:
3Only 2 of 3.pdf

If you own a reasonably new Keystone trailer with 3 AC units, but only 2 will work at a time:
4Requirements.pdf
5How To.pdf

Hopefully, you will find this information useful as you consider the use of 3 AC units on your RV.

[LewisB]

One final post: I forgot to provide information on how I conducted these tests.

• Temperature and humidity readings taken using an ACU-RITE indoor/outdoor thermometer. It's not scientific, but it appears to be repeatable.

• Zone Temperature readings and set points used an RV Comfort.ZC thermostat with 3 zone control and 3 zone temperature monitoring.

• AC current, AC volts, AC Kilowatts monitored with Surge Guard EMS and remote monitor.

[trooper253]

We had a 3rd AC installed on our 2016 Avalanche 361TG just before we went to Myrtle Beach this summer. In the past with the ACs in the two bedrooms, the main living area was almost unbearable. We have had a very hot summer here in the south and we noticed a huge difference. We have been in Stone Mountain, GA this week with temps near 100. Although this was a very expensive modification it has proven to be worth it.

[Racebug]

nice study/write up!

[Andyjman]

Great info lewis. My situation is slightly different. My carbon 403 only has 2 A/c units, and to my knowledge probably isnt prewired for a third, but I have more than enough room in my breaker panel, so I am going to add a third unit in the garage. We bought the trailer last fall and we love it. We were out at the river on the fourth of july and the 2 A/c units did not keep up. The bedroom was halfway decent, but the garage was unusable, and the living room was in the 90s when the daily high of 115 hit. The third day we were there, I noticed that the bedroom was unusually warm, so I climbed on the roof to check the A/c unit. Seems that the condenser coil decided to pop a U-bend and dump the charge and oil all over the roof of the coach, so unfortunately, we werent able to stay in the trailer the rest of the weekend. Its going in for service on thursday.

I also see that you listed that your system is controlled by the RV comfort.ZC. I have the same system, but mines obviously using 2 zones. Im curious as to whether mine could have a module added to support the third a/c, or if I need a stand alone t stat for the third unit once its added.

One more thing... in that picture of your surge guard, are the higher numbers for the amps and kilowatts the peaks? Im curious, because if the peak kilowatts is 3800, then I see no reason why you wouldnt run 3 a/c units on the 5500 genset. This is of course with optimum conditions, and chances are if you are needing to run all 3 a/c units, then itll be hotter than what the genset was tested and rated for.

What are your thoughts on that?

[Andyjman]

Also, just a small additonal tidbit. We camped at lake cachuma with our friends this year earlier in july. The campground only has 30amp services. We took our 2018 carbon 403 and our friends took their 2018 shockwave. Their trailer has 2 dometic a/c units, and ours has 2 coleman mach units. Much to my suprise, I was able to run both of my a/c units as well as tv, toaster, etc. My buddy was unable to run just the 2 dometic a/c units by themselves let alone with anything else.

[LewisB]

Andy,
Take a look at post #2 in this thread. You will find attachments (pdf files) that address some of your questions - especially regarding running 3 AC units on a 5.5KW generator. See "Only 2 of 3" attachment. Specifically, your genset must be de-rated 3.5% for every 1000' over 550' MSL and also de-rated 1% for every 10°F over 77°F. I suspect you will find that the 3 AC units put the generator very nearly at the maximum of design output. The max design operating temperature is 120°F for your gen set. So you are putting your generator in a box (your trailer compartment), running it at max output, in ambient temperatures over 110°F - seems like a good way to REALLY shorten the life of a $5K genset. That's just IMHO.

Good luck with your project. You will love having 3 AC units!
Brad

[firestation12]

Another point about the Onan generator running in very hot climates to consider, is that the generator's intake of cooling air is next to the exit point of the hot air discharge. Both are located under the genset. Thus a considerable amount of the hot air expelled, is being ingested back into the cooling intake. I had an elbow made at a HVAC sheet metal shop that attaches on the discharge side under the generator, which directs the hot air horizontally away from the intake. A 2" hole is cut in the elbow to allow the exhaust pipe to pass through.

[LewisB]

Originally Posted by firestation12

Another point about the Onan generator running in very hot climates to consider, is that the generator's intake of cooling air is next to the exit point of the hot air discharge. Both are located under the genset. Thus a considerable amount of the hot air expelled, is being ingested back into the cooling intake. I had an elbow made at a HVAC sheet metal shop that attaches on the discharge side under the generator, which directs the hot air horizontally away from the intake. A 2" hole is cut in the elbow to allow the exhaust pipe to pass through.

Excellent comment. Sounds very interesting. Can you get us a picture of this?
Thanks.

[K_N_L]

Originally Posted by Andyjman

I also see that you listed that your system is controlled by the RV comfort.ZC. I have the same system, but mines obviously using 2 zones. Im curious as to whether mine could have a module added to support the third a/c, or if I need a stand alone t stat for the third unit once its added.

Your existing thermostat can handle up additional zones, the control modules for the AC have jumpers that will be set to identify it as zone 3. (Link for the Coleman version - https://www.manualslib.com/manual/12...?page=9#manual )

I reworked my 3 AC after 1 trip to keep the kids from 'adjusting' it on their own. https://www.keystoneforums.com/forum...ad.php?t=34062 . I have 3 Mach 15's that I can run when on shore power. The EMS will show about 34 on leg 1 and 17-18 on leg 2.

[Jennifer Fuzion 414]

Originally Posted by LewisB

I have seen lots of opinions expressed regarding the use of 3 AC units in larger RV’s. There is widespread agreement that during very hot weather (+100°F), typical 1 or 2 AC unit installations are inadequate to keep a large RV cool. Living in Tucson, AZ, where temperatures can go over 110°F, we share that experience.

I have not been able to find any specific testing and/or test results on the effectiveness of using 3 AC Units (vs. 2 AC Units) in a large RV. So this thread is intended to advance some actual test results.

I am NOT a representative for Keystone. I AM an avid camper and toy-hauler owner. We have owned multiple camping units over the years. Our most recent purchase is a 2018 Keystone Raptor 353TS, which we love. The good news is that you CAN stay comfortable in very hot weather if you are capable of making some modifications (may be relatively easy) to your RV and have access to a stable 50A RV shore power supply.

Here are my test results from a hot day in Tucson. With 3 AC units running I was able to maintain 74°F inside the trailer while 109°F outside - a 35°F differential. In fairness, this was what I would call a "parked" condition - that is, I started the AC units in the cooler part of the morning.


Here are the test results from a hot day in Tucson with only 2 AC units running. In fairness, this was what I would call a "travelling" condition - that is, I started the AC units at the hot part of the day to simulate pulling in to an RV park after being on the road for some time. As can be seen, the typical 20°F was achieved in the bedroom, only 10°F in the main cabin, and the garage was nearly at ambient temperature. This makes the trailer nearly un-usable under these conditions until very late in the evening!


I will post an additional message with links to more tests, my thoughts on why and how this happens, making modifications, etc. If you are interested, just follow the subsequent links.

We struggled with the decision to make modifications to our new Keystone trailer. We also had to install a new 50A service at our house. But our trailer was un-usable as a "guest house" for 4-5 months out of the year due to hot weather. So we bit the bullet and proceeded; I can only say that this is the best modification I could have made to this trailer. I am thrilled with the performance shown here. Hopefully, this might help others in some way.

I'm sure someone finds this useful but it would have been a lot more useful if you'd done the 2 AC testing the same way as you did the 3 AC, starting at the same time each day. Doing what you did makes a comparison between 3 AC and 2 AC impossible, or were you trying to prove differences between starting a 3 AC earlier vs a 2 AC later in the day... you've really proved neither.


Jen

[sourdough]

Originally Posted by Jennifer Fuzion 414

I'm sure someone finds this useful but it would have been a lot more useful if you'd done the 2 AC testing the same way as you did the 3 AC, starting at the same time each day. Doing what you did makes a comparison between 3 AC and 2 AC impossible, or were you trying to prove differences between starting a 3 AC earlier vs a 2 AC later in the day... you've really proved neither.


Jen

FYI - The OP made that post approx. 4 years ago. He hasn't been back on the forum in about a year.

[TXiceman]

In order to keep the temperature at your desired set point, you have to have more cooling capacity than the heat gain on the RV. So, there is a 35 vs a 26 degF differential between interior and exterior temperature.

A 3rd A/C unit will definitely help an RV stay cooler. Looks like a lot of data and time for not much than the data.

Ken

[LED1974]

I hope someone can help me out. I have been reading over all of this stuff about the use of the 3 A/C units and to modify the trailer to use all 3 on shore power.

I have a 2018 Raptor model 362TS18 it has a Graystone electric fire place model W36BCFW-1. According to the instruction related to this post you can find the Power Share Switch (PSS) behind the fire place. I tried to remove the fire place and I was not able to get it out. The instructions from the heater manufacture remove the two screws on the top of the heater and tip the top forward and pull it up and out of the mounting bracket.

I can pull the C/B panel above the heater and remove 1 screw but cant get to the screw on the other side.

I hope I have explained this so it is understandable.

[LHaven]

There's a lot here not to understand.

According to the installation manual [LINK], there are three mounting options. Which one applies to your unit?

"According to the instruction related to this post"

You don't provide this. Do you have a URL for it, perhaps?

"but cant get to the screw on the other side."

Can you elaborate on why you can't access one screw when you can access the other?

[Matty Rich]

Originally Posted by LED1974

I hope someone can help me out. I have been reading over all of this stuff about the use of the 3 A/C units and to modify the trailer to use all 3 on shore power.

I have a 2018 Raptor model 362TS18 it has a Graystone electric fire place model W36BCFW-1. According to the instruction related to this post you can find the Power Share Switch (PSS) behind the fire place. I tried to remove the fire place and I was not able to get it out. The instructions from the heater manufacture remove the two screws on the top of the heater and tip the top forward and pull it up and out of the mounting bracket.

I can pull the C/B panel above the heater and remove 1 screw but cant get to the screw on the other side.

I hope I have explained this so it is understandable.

I just completed this modification today. I have a 2018 Fuzion 369. PowerShare box was behind the fireplace.

For me, the front glass just lifts off (just lift up) and then I had access to 4 brackets in each corner.

[LED1974]

Ok yesterday I was able to get the heater out and got the mod done. I was able to get the bracket I talked about disconected and it fell off. I found out the heater was in there so tight the screws on both sides made groves in the wood as i pulled it out.