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Old 07-28-2013, 10:45 PM   #1
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I have a Dometic RM7651UL and the coach had been sitting unused for about 30 days and I found the fridge was not cooling properly on Propane. Worked fine on 120VAC and on 12V - it was -24C (-11F) in the freezer and generally 2-3C (35-37F) in the fridge compartment and 0C (32F) direct on the fridge finns in the cooling compartment. I have a fan in the fridge and two computer fans that extract heat from the finns on the back of the fridge. Only installed the rear fans last week. Must do a temp measurement with the rear fans on and off to see if they really make a difference.

Did temperature comparisons at various locations. Inside and at the rear. And measured ambient temps also. Ambient outside temp was 28C (82F) under all conditions.

The propane flame was visible in the burning chamber and looked clean while on propane but it would not get cold enough inside the fridge. With my IR laser guided thermometer, I measured the heat right at the burn chamber by the flame. It measured 231C (447F)

I then took the tube etc and and burn chamber off for inspection. This is from the output of the propane on/off selonoid to and including the burner. Everything looked ok, could not see anything untoward, nonetheless I took my air compressor and blew out the pipe and cleaned each orfice and tube. I put everything back exactly as it came off (there was no way I could misadjust the flame) and started it on propane again. The temp on the burner housing that was 231C (447F) now went to 320C (608F).

The flame was larger and very clean looking. I could not discern any problem with the flame before cleaning, but the difference on temp really told the story and the flame looked stronger (higher). There must have been some micro particle(s) somewhere that limited gas flow.

I gave it about an hour and measured the temp at the cooling finns inside the lower Fridge part, it was down to -2C (28F) whereas before it was +8C (46F) on propane. It measured 0C (32F) at the same point on AC and DC with the same setting and ambient temp. Freezer was below -26C (-14F) and out of range for my thermometer.

The fridge gets colder on propane now than on 120V or 12V at the same cooling setting. Just like it worked when it was new.

Also looked at the spark ignition during the inspection. All seemed fine.

I am certain many folks on this site have done similar testing and analysis to improve fridge performance but I thought I would share my experience with others just in case they have similar symptoms. I have cleaned many other absorption fridges I owned under similar circumstances with forced air but I never did such an extensive temperature measurement as I did on this occasion. Cleaning does improve performance on propane.

Hope my experience can be of value to others who might have similar problems. Glad I had my IR laser guided heat thermometer. Have used it for sooo many different functions.
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Old 07-28-2013, 11:15 PM   #2
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Very interesting, from a performance point of view , good info on the flame temp improvement . I'll have my IR handy when I check mine this month.
Just a quick question, I'm not up on all models of Dometic, does your fridge use a chimney flue baffle ?
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Old 07-28-2013, 11:53 PM   #3
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My fridge just has a small discharge chute/chimey about an inch in diameter and is well above the burner assembly and clear of everything. It is in a vary spacious housing with a lot of air space insulation and encasement around the top of the heat supply. I can put my hand on that outer housing. It is warm but it did not burn my hand. Never put my heat gun on it but I did put my gun on the top exhaust - can't recall the reading but I will do it the next time I have the top vented area open.

Will take pictures also and post them. In my opinion this is a much better installation than I had with my much larger Norcold units in my Monaco Diplomat and my Dynasty. There is a lot more free air around everything in my present setup.

I sent an e-mail to Dometic asking for general temperature readings on my fridge at the burner location. It will be interesting what I get back if I get anything back.
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Old 10-04-2013, 05:40 PM   #4
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Spiders

Good analysis!

For some weird reason spiders love to set up housekeeping in the burner tube. I've lost count of the number of times I've found webs in the burners of various propane appliances.

It doesn't take much to mess up the air flow, and thus the mixture. Lack of oxygen would definately make the flame cooler.

I clean mine every few months. More often if we've been in a wooded area.
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