Dometic 2351 RV Fridge not cooling well on LP

[theoilchanger]

Hello! My Dometic 2351 cools nicely on AC but not so well on LP gas. I have read the very excellent thread at https://community.fmca.com/topic/12815-dometic-fridge-50-deg-on-propane/ but continue to be completely baffled (no pun intended).

I have checked everything: flame quality, the baffle is there and in good shape, the burn tube cap is on, cleaned the burn tube, checked and cleaned the gas jet, you name it, we checked it!

The top of the burn tube is much hotter running on LP than on AC which probably makes sense since the AC heating element is attached directly to the boiler and shouldn't send off as much extra heat as the LP flame.

OK here's the thing that is driving me crazy: the temperature at various points of all the refrigerator tubes show about the same temperature when running on either AC or LP! For example, at the same point on the top of the evaporator stack we register about the same temperature--226F and 233F when running on AC and LP, respectively--so we know the proper amount of heat is being sent into the system.

The only thing I can think of is that the flame is creating so much heat outside that it's heating the refrigerator back up.

Can anyone think of what on earth can be wrong here? Thank you!!

 

[rayin]

You might be right, the insulation surrounding the boiler and chimney is very important to proper operation. I decided some time ago if the insulation deteriorated on mine, I will pull the fridge out and replace the old wrapped insulation with rigid steam-pipe insulation, it is the same thickness and much better R value.

[dickandlois]

Oilchanger, Welcome to the FMCA Forum!

Could you post if the refrigerator is located in a slide out or next to a none moving wall and if there are fanes mounted behind the unit to help remove the excess heat?

Excess heat behind the refrigerators does cause a reduced cooling range. 

Rich.

[manholt]

Welcome to the Forum!

What type and age of RV is your fridg in?  Yes, fan/fans are important to regulate flow and prevent hot spots.  I remember having a battery operated small fan inside the fridge to keep air moving!

[theoilchanger]

Hey, thanks, all! Great to hear from you.

This is a 2011 Skyline Nomad with no slide-outs. The fridge is mounted on the starboard side of the rig in the rear quarter and has properly spaced and sized top and bottom panels. Factory installation.

There are no fans installed. The fridge works fine on AC power without fans but I could see how the LP exhaust could heat the whole bay more than the more compact AC element. I'll put a small fan in the bay tomorrow and start her up on gas to see if there's any effect. Meanwhile....

I did go back again yesterday and measure the temperature all over the tubes and I found that there is actually a significant temp differential in many areas. Seems the overall circuit is running cooler on LP (where it ought to be hotter).

So now I have two theories:

1. The gas pressure is below 11 inches of water column
2. There is a crack between the boiler and flue that's affecting heat transfer

For #1 I'm going to make my own u-tube manometer and test the pressure tomorrow. If the pressure is good then I'm going to shove some tin foil between the flue and boiler to increase the contact surface area between both. I'll yet y'all know. This is keeping me up!!! LOL

 

 

[manholt]

I just looked at all the floor plans for non slide...didn't see one with the fridge in rear quarter.  What length is yours?  The reason for asking, the longer the run from gas bottle to fridge/stove, could affect the flow...you could also have a loss fitting, elbow. Is your gas line copper or tubing?  To me the bow of a pull behind is at the hitch. 😀

[theoilchanger]

Hey Manholt--it's 21 feet from the pin to the rear bumper and I went ahead and made my own pressure tester.

Sure enough, the pressure was well below the 10-11 inches of water column! Turns out the regulator is adjustable. After unscrewing the cap I was able to turn a disc which, wouldn't you know it, pushed the pressure right on back up there. Going to give it a run and see how it goes now for the next 24 hours.

Certainly read in many places to check the regulator. Never suspected that the regulator was adjustable. That means I've been running at below pressure for years since I put in the replacement reg. I bet our coffee will be ready faster in the morning now. Too funny. I'll report back here with cooling results against the electric benchmark.

 

 

[manholt]

Sir!  You done good. 😁

[wolfe10]

Excellent job with the diagnostics.

But, on a 9-10 year old propane regulator that was way out of spec, I STRONGLY recommend replacing it. 

They are under $25.

[wildebill308]

10 minutes ago, wolfe10 said:

But, on a 9-10 year old propane regulator that was way out of spec, I STRONGLY recommend replacing it. 

I would second that. I had one rupture on a old 1978 Executive I had and when I lit the stove I had flames up inside the vent a hood. Plenty of excitment. 

Bill

[theoilchanger]

Happy to report that the tests have check out at the Dometic 2351 is fully functional. Lesson learned here is there is always a variable that you didn't check until you did.... In this case, all the information I found on the web talked about replacing the regulator but no one mentioned that RV regulators can be variable regulators and may very well be adjusted downwards from the standard 11" of water column.

The good news is that I am now an expert (in my own mind) on RV fridges. I took the thing completely apart to verify integrity and, along the way, uncovered the mystery of the absorption fridge. It is an amazing piece of engineering.

For my final tests I kept the thermistor disconnected to put the fridge into "Flying Dutchman" mode where she just keeps on cranking regardless of internal temp. The first photo below shows what it looks like when the LP burner runs all night; the second shows what it looks like the next morning with the thermistor re-installed. The last image is a graph of the data I collected monitoring the remote thermometer that was placed outside to measure ambient temperature in the fridge box.

I will now switch to AC to see if the numbers are any different.

Big take-away is that these don't work at high outside temperature so we'll purchase a portable 12V compressor fridge if we end up camping in warmer climates.

The best news of this all is that this drove me to become part of this great community. I look forward to participating and and learning from all of you...and I bet there will be a few laughs along the way.

============

After overnight run in Flying Dutchman mode:

After overnight run with thermistor connected to lower control board

Temperature over time on a particularly hot day for the SF Bay Area

 

[rayin]

The chimney design was not addressed, this will affect running temperature and overall operation. Download the installation manual for your fridge and double-check the chimney design and specs to  make sure it is correct. If it is any comparison, my Norcold 1200 will maintain 34° in the fridge section on a 95° day. I did add a thermostatic blower-type fan  blowing upward over the tubing.

It is not uncommon to read of improperly installed chimneys in RV's.

[theoilchanger]

Thank you, sir. I will check on that. Interestingly today is the same hot day and the system is cooling better on AC than on gas. The temperature of the top of the boiler seems consistent regardless of gas or electric operation and the temperature of the back of the compartment is also similar. Ran a fan yesterday at the bottom pushing air upwards and still got the relatively high reading. Let me continue to log and I will review the installation tolerances.

[jleamont]

One personal experience with the chimney designs. I have yet to see an RV manufacture follow the refrigerator manufacture's recommended chimney design or refrigerator installation. I would download the refrigerator installation manual for your model and follow those.

[theoilchanger]

 I did go ahead and add a loop to form an air trap for the drain.

@RayIN @jleamont confirmed that the installation is according to the manual: 28" from top of top side vent to bottom of bottom side vent. Also confirmed that the top bulkhead is curved as shown in the manual and that the vent assemblies are also Dometic.

Final step will be to install one or 2 4" thermostatically-controlled 12v fans as shown in the manual. That is to say, above the absorber and below the condenser, blowing upwards.