RV Refrigerator Design/Rating

[Sailorguy7]

Because of the significance for this info, I decided to start a new post, in the hope more people would see it and therefore better understand why they are having problems! The topic of Absorption Refrigerator's poor performance in Temperatures above the mid 80s is well documented, along with and all sorts of alleged fixes, none of which, IMO really help. We are replacing our Dometic RM8505 in our 2018 PW-Ascent with an Isotherm Cruise 115 Freeline Compressor Fridge. The reason we are having problems, which I suspect is true for a lot of us, is the fact that the OE Refrigerators supplied by the RV Industry are not DESIGNED/RATED high enough. For example our current Dometic RM8505 has a "Climate Class SN" of 50-90 degrees, the replacement Isotherm Cruise 115 Freeline has a "Climate Class ST" of 61-100.4 Degrees. The "Bottom Line," is no amount of modifications will fix a Refridge that is not intended to operate in higher Temps! BTW, the "Climate Class" can be found on your Spec Sheet inside your Refridge.

[booster]

Along with the specs, you also have to look at what conditions they are rating at for in the frig and freezer temps as that can be a big factor. The frig rating would also probably be done at the standard as shipped compressor speed, and capacity can be increased by raising those temps, especially the freezer if it is the kind, like ours, that have the evaporator used as the freezer box.



There is a thread on this forum that shows the btu/hour rating for the BD35 compressor units in relation to evaporator temperature and compressor speed so you can see how much it changes, which is a lot.



The heat gain of the frig is normally quite constant over time at the same temp unless it is opened, so the temperature achievable will be related to ambient temps and what temp the evaporator is running at, plus compressor speed.


We, and others have found that simply removing the drip tray can be a large improvement as it helps the cold air get to frig section and allows the thermostat to be turned to a warmer setting. That setting is evaporator temp, approximately, in a unit that has the sensor in freezer.


If you are using a frig that has one of Isotherm's frig sensing digital controls, the unit can get into a death spiral in relation to capacity. With frig sensing, if the cold air can't get out of the freezer area fast enough, which appears to be pretty common (ours and another poster have both seen the issue), the control turns down the thermostat to get to frig temp. This starts the spiral because the freezer gets colder and running at less capacity because of being colder, per the ratings mentioned earlier. Eventually, the loss of capacity causes the frig to quit getting colder. We found this happening at much less than the rated temperature, so they messed something up, probably be adding a shroud to freezer box and redesigning the freezer door to fit tighter and those things make the cold air unable to get to frig section well enough.


Ours is a Cruise 85 Elegance. We had a previous Cruise 85 with old door, tray, and no shroud and it worked very well at over 100*F with very slow compressor speed.

[Davydd]

I've got the Isotherm Cruise 115 Freeline but I don't know anything about Climate Class. So far I am happy with it, it performs better than my previous refrigerators, I bought because I like the design layout, it has just enough storage for our historical use and then fortunately fit the design best I allowed for a refrigerator.

[Dreadnaught]

As unmodified, I’d agree with the rating. Once we put a fan array to move the air through the back vents, we had no problems. It’s all just a matter of heat removal. It’s a sealed insulated box. Get the heat out and stuff gets cold. That takes heat transfer. As built, depending on natural airflow the system can’t get the heat out fast enough. Changing the stock configuration by moving more air makes the Dometic work well enough and many have done so successfully. The rating isn’t the same with additional airflow.

In short, trying the attached solutions is far cheaper and easier than replacing the whole unit. It also reduces waste and material use.

https://www.smartrving.net/best-rv-refrigerator-fans/

[mkguitar]

Originally Posted by Dreadnaught

Get the heat out and stuff gets cold. That takes heat transfer.

That's my experience. I can pretty much count on getting 60º below ambient temps on propane with the $15 of fans I added

That doesn't do much in phoenix


But camping in temperate climates it works fine.


I certainly understand people seeking better fridge, but my electrical system wouldn't support it.... it would cost thousands

[Winston]

Originally Posted by Dreadnaught

In short, trying the attached solutions is far cheaper and easier than replacing the whole unit. It also reduces waste and material use.

https://www.smartrving.net/best-rv-refrigerator-fans/

Some of the fans your link discusses have curious packaging . . . where do these fans get mounted?

[ChicagoTom]

Originally Posted by Sailorguy7

Because of the significance for this info, I decided to start a new post, in the hope more people would see it and therefore better understand why they are having problems! The topic of Absorption Refrigerator's poor performance in Temperatures above the mid 80s is well documented, along with and all sorts of alleged fixes, none of which, IMO really help. We are replacing our Dometic RM8505 in our 2018 PW-Ascent with an Isotherm Cruise 115 Freeline Compressor Fridge. The reason we are having problems, which I suspect is true for a lot of us, is the fact that the OE Refrigerators supplied by the RV Industry are not DESIGNED/RATED high enough. For example our current Dometic RM8505 has a "Climate Class SN" of 50-90 degrees, the replacement Isotherm Cruise 115 Freeline has a "Climate Class ST" of 61-100.4 Degrees. The "Bottom Line," is no amount of modifications will fix a Refridge that is not intended to operate in higher Temps! BTW, the "Climate Class" can be found on your Spec Sheet inside your Refridge.

105 in Vegas a few weeks ago and mine is fine. I'll stick with the propane. Glad you said "IMO" nothing works, but my interior fan pointed at the evaperator has been a game changer. Ex-Refrigeration Tech for what it's worth. JMO

[peteco]

Originally Posted by ChicagoTom

105 in Vegas a few weeks ago and mine is fine. I'll stick with the propane. Glad you said "IMO" nothing works, but my interior fan pointed at the evaperator has been a game changer. Ex-Refrigeration Tech for what it's worth. JMO

Please describe you setup and mods to get your fridge to work so well. Thanks.

[ChicagoTom]

Originally Posted by peteco

Please describe you setup and mods to get your fridge to work so well. Thanks.

Very simple. I have two rechargeable fans, one pointed at the evaporator and one charging. The fan runs on low for about 6 hours. Switch it before bed, when I get up, and maybe 1-2 times during the day.
It's also important to load it with cold beverages and let it cool down before loading perishables. I also then put ice cream etc. in the frezzer. Also, don't over pack it. Air needs to circulate it. I keep the setting on the coldest. I had it run non-stop for over a year, no issues and no frost build up. Link to fan I use
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1