Question about absorption fridges

[reilym]

After a year of monitoring current draw in my RV, I am puzzled that when operating on DC, the Dometic 3 way (RML8330) in my Lexor uses about 15 amps continuously no matter what the dial is set at or the outdoor temperature is. I understand how these work, basically by evaporating the ammonia refrigerant with either a heating coil or LP flame. What I don't understand is why the cooling system does not seem to come on and off as necessary to maintain a temperature like a compressor fridge does. Logic tells me that when it's 40 degrees outside, the fridge should demand less power than when it's 90. Normally, I switch to LP operation when we're parked without shore power, but I figured I could leave it on battery for 8 hours while we slept in a hotel on a 40 degree night. I turned it all the way down, but still woke up to a dead battery. The fridge has not let us down so far and it stayed cold though our 2 weeks in south Florida, but I'd like to understand if something is broken or this is just the way they operate.

[eric1514]

Many fridges don't run off a thermostat in 12v mode, only when running on propane.

[GallenH]

My best understanding; I have an old Dometic. When on propane my old fridge is supposed to go to high flame to heat and when temp is achieved, change to pilot. Later Dometic fridges actually shut down the LP completely and relight automatically when thermostat demands more cooling. I suspect the same tact might persist with the electrical (12v or 120v): cool to temp and thermostat turns off the element, but I'm not sure.

The underlying problem with 12v is that it's very ineffective. It's most effective when the battery is full but as the voltage goes down in the battery I think that it requires longer times to get to the point where the internal temp is achieved. And that gets longer as the battery gets depleted. Most people use 12v only when driving and even then resist opening the fridge.

I'm giving my best guess at this. Others probably know more.

[hbn7hj]

Originally Posted by reilym

I turned it all the way down, but still woke up to a dead battery. :

I believe the Dometic 2354 12 volt heater is controlled by the
thermostat. As you found out it is intended to be used with the engine running. For me it works very well that way. With the installation of a relay controlled by the ignition that is the only way it will run.

I have a compressor fridge in the wings but with Starlink coming on I don’t want the power drain. High temps in summer travels are in the 70s so as long as it works, it stays.

I would like to have a 200 amp lithium battery charger that can be run from a 4000 watt generator. Can’t find one. Presently at 120 amps which is good but not good enough.

[Rlum]

Originally Posted by reilym

After a year of monitoring current draw in my RV, I am puzzled that when operating on DC, the Dometic 3 way (RML8330) in my Lexor uses about 15 amps continuously no matter what the dial is set at or the outdoor temperature is. I understand how these work, basically by evaporating the ammonia refrigerant with either a heating coil or LP flame. What I don't understand is why the cooling system does not seem to come on and off as necessary to maintain a temperature like a compressor fridge does. Logic tells me that when it's 40 degrees outside, the fridge should demand less power than when it's 90. Normally, I switch to LP operation when we're parked without shore power, but I figured I could leave it on battery for 8 hours while we slept in a hotel on a 40 degree night. I turned it all the way down, but still woke up to a dead battery. The fridge has not let us down so far and it stayed cold though our 2 weeks in south Florida, but I'd like to understand if something is broken or this is just the way they operate.

I think that is normal for 3 way refrigerators. When I had one. I very seldom
basically never ran on 12 volt power. It was either 110 volt or propane. My refrigerator worked OK when ambient temps were 90 or less. When it got above 90, it was struggling to keep box below 40.

[eric1514]

Quoted from the manual pg 15

"12V operation: The refrigerator operates without thermostatic control (continuous operation)."

RTFM

[reilym]

Originally Posted by eric1514

Quoted from the manual pg 15

"12V operation: The refrigerator operates without thermostatic control (continuous operation)."

RTFM

Thanks for posting that. I usually do RTFM, but I missed this - I now see it on page 16. All is explained, except for why it operates without thermostatic control on 12V - seems like that should be possible. The FM does not say anything about thermostatic controls for 120V or LP operation and the US support center could not answer these questions for me. To see how it behaves on shore power, I'll connect a current monitor to see if it modulates and report back.

Based on the feedback here, I'll stick with only using 12 V while driving and use LP when parked and not on shore power. Like I said, the fridge has never let us down - I am just trying to understand how it operates.

[themexicandoctor]

Originally Posted by reilym

Thanks for posting that. I usually do RTFM, but I missed this - I now see it on page 16. All is explained, except for why it operates without thermostatic control on 12V - seems like that should be possible. The FM does not say anything about thermostatic controls for 120V or LP operation and the US support center could not answer these questions for me. To see how it behaves on shore power, I'll connect a current monitor to see if it modulates and report back.

Based on the feedback here, I'll stick with only using 12 V while driving and use LP when parked and not on shore power. Like I said, the fridge has never let us down - I am just trying to understand how it operates.

If it's a Dometic.

And the model initials start with RM.

DON'T TRY TO UNDERSTAND IT AS IT WILL LET YOU DOWN & BREAK YOUR HEART ...

Unless of course, you never park on grades outside of 5% & don't encounter hot summers.

[GallenH]

It appears that Dometic has varying approaches to how the 12v element and the thermostat interact. On my older (RM2310) the wiring schematic shows the 12v supply clearly going through the thermostat before proceeding to the heating element.

Perhaps they found that it was pointless since the fridge never maintained cooling even when on high?

[hbn7hj]

Originally Posted by GallenH

Perhaps they found that it was pointless since the fridge never maintained cooling even when on high?

Mentioned before but I changed the 125 watt 12 volt heating element to the 175 watt element used in later RM 2354 units. The heating elements have the same profile so you could put a 225 watt element in. The manual has warnings against doing that but I figure the ARP will turn it off before I burn down and it will.

The 175 watt element works very well but it still can’t cope with crossing the Mojave desert at 118F.

[GallenH]

Originally Posted by hbn7hj

The 175 watt element works very well but it still can’t cope with crossing the Mojave desert at 118F.

I know that you're heading toward a compressor in your B. Not sure if you have one in your C. Do you predict that a compressor would fare better in your Mojave crossing? I'm still trying to figure out if compressor fridges have any similar limits. IOW if a compressor fridge is running full out, would it still not be able to cool down in such conditions.

[hbn7hj]

Originally Posted by GallenH

I know that you're heading toward a compressor in your B. Not sure if you have one in your C. Do you predict that a compressor would fare better in your Mojave crossing? I'm still trying to figure out if compressor fridges have any similar limits. IOW if a compressor fridge is running full out, would it still not be able to cool down in such conditions.

I still don’t have a compressor fridge installed but do have the compressor upgrade for an RM 2663 in the hallway of my house. I also have the aftermarket potentiometer to up it’s performance if necessary.

I don’t know that it can cross the Mojave at 118F but I am expecting to be able to adjust the performance till it does.

[reilym]

Originally Posted by reilym

Thanks for posting that. I usually do RTFM, but I missed this - I now see it on page 16. All is explained, except for why it operates without thermostatic control on 12V - seems like that should be possible. The FM does not say anything about thermostatic controls for 120V or LP operation and the US support center could not answer these questions for me. To see how it behaves on shore power, I'll connect a current monitor to see if it modulates and report back.

Based on the feedback here, I'll stick with only using 12 V while driving and use LP when parked and not on shore power. Like I said, the fridge has never let us down - I am just trying to understand how it operates.

OK, so I am continuing my education here. After studying the wiring diagrams of the fridge, I found the attached PDF that describes the operation of the V85 gas valve used in the fridge and had my moment of clarity. This is probably not news to you experts, but it certainly explained things for me. As I understand it now, the V85 uses a temperature sensor to modulate the flame height in gas mode, or the 120V heater in AC mode. Hence, those two modes are thermostatically controlled based on the dial setting and the DC mode is not. Interestingly, before I had this epiphany, I emailed Dometic technical service about this and they responded with the following uselessness:

"We don't have that information written down for us but I can tell you that the DC function allows your fridge to maintain temperature until you reach destination, but your AC & LP are you 2 power sources to fully cool the unit."

The PDF provides a good explanation of how this valve works.

[Scottbaldassari]

Looks like you have found your answers (no thermostat for DC).

I have the same fridge in my Travato, and it might be interesting to note, that when in moderate outdoor temps (70's in the day, and 40's at night), we often run the fridge on DC all day on driving days, and it gets TOO cold - we can shut it off at night (if we are not opening and closing it) and it will maintain a good temp till morning.

[reilym]

Originally Posted by Scottbaldassari

Looks like you have found your answers (no thermostat for DC).

I have the same fridge in my Travato, and it might be interesting to note, that when in moderate outdoor temps (70's in the day, and 40's at night), we often run the fridge on DC all day on driving days, and it gets TOO cold - we can shut it off at night (if we are not opening and closing it) and it will maintain a good temp till morning.

Now that you mention it, we have had a few freezing events and I'd wager that those correspond to long periods of DC operation. Now that I understand it, it makes sense. In DC mode the fridge ignores the dial setting and simply runs the DC heating element full throttle. It seems that additional control could be possible with a circuit that turns the DC heater (or even the entire fridge) on and off based on the temperature inside the fridge (e.g. on at 42 degrees, off at 36 degrees) when in DC mode. That would make the DC mode less of a hog and prevent inadvertent freezing.

I actually like the flexibility, size and configuration of this fridge now that I understand how it works I like it even more. Once it gives up the ghost, I'll revisit the compressor vs. absorption options. I appreciate all the dialog on this thread!

[JML]

Originally Posted by themexicandoctor

If it's a Dometic.

And the model initials start with RM.

DON'T TRY TO UNDERSTAND IT AS IT WILL LET YOU DOWN & BREAK YOUR HEART ...

Unless of course, you never park on grades outside of 5% & don't encounter hot summers.

Best comment on the web today!

[ChicagoTom]

The 12V element is smaller and runs continuously and IMO is useless, because many forget to shut it off and then kill their battery. I can get the same results with it off while driving and turn the propane on during breaks, lunch etc. I actually just leave it on propane, but don't want to start that debate. Ex HVAC Tech for what it's worth...Bottom line is that mine works great, but you need to understand how to load it, where to position the interior fan etc. Mine stays Ice Cold until it hits around 105 and hot during the night.

[themexicandoctor]

Just caught up to this - the size of the 12 volt element being a limiting factor makes a lot of sense - I stopped turning my unit off when buying propane after the first month or so. It's been 12 years now. So Easy to forget to remember to turn it back o.

[JohnnyFry]

The fridge does not modulate the DC Current according to temperature, it is all or nothing...mostly all because the DC heater is lower wattage than either the AC heater or heat delivered by propane. Is really only capable of maintaining the temperature and not doing primary cooling, in my experience.It probably runs continuously in order to do this,

[DaveWA]

Just wondering if you or anyone on this thread can confirm how my Dometic RML 8555 3-way fridge should perform when in Propane mode.

Fridge works perfectly on 110v mode, acceptably on 12v when driving, but very poorly on propane. What can I do to resolve the propane operation issue?
I have cleaned the burner orifice and exhaust, the burner fires up just fine and the flame looks good. I am just not certain what to expect in performance when the fridge is running on propane.

Example:

2/1/22 Went on 8 day trip to Death Valley.

With the fridge set (and left) on thermostat setting of 4 (out of 1-5 setting), prechilled the fridge on with the van on shore power 110v prior to leaving and it came down to -5F in freezer and 38F in fridge within about 6 hours. Outside temps were in the 50s. All is good.

Drove for 6 hours with the fridge running on 12v. Freezer was about 0F and fridge was about 40F when I arrived at my overnight destination. The outside temp during the drive was averaging in the low to mid 50’s F.

Plugged into shore power overnight and the freezer and fridge both dropped down a bit. Overnight outside temp was upper 40’sF.

Drove for another 3 hours with outside temps around mid 60’sF and freezer and fridge maintained temperatures.

Overnighted without shore power so fridge ran on propane. Freezer came up to 10F and fridge to 42F.

Drove 4 hours with the fridge running on 12v and freezer and fridge both dropped a few degrees, with outside temps around low 60’sF.

Trouble begins:
Camped without shore power for 5 days with fridge running on propane only. Freezer temp raised to 22F and fridge to 58F. Average outside temps were mid 40’sF and night and low 70’sF during the day.

Drove for 6 hours with fridge on 12v and freezer dropped to 18F and fridge to 50F.

Overnighted without shore power , fridge on propane, and then drove another 6 hours to get home with fridge on 12v. Freezer was at 10F and fridge at 45F when I arrive at home.

So. . . . . Is this what to expect from propane operation performance or is something wrong with the burner, exhaust, or other functions when in propane mode?

Your thoughts and experience are appreciated!