too much ventilation in fridge - Fiberglass RV


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Old 09-18-2012, 11:39 PM   #1
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Smile too much ventilation in fridge

Hi,
I wanted to report that I have finally solved the fridge issues I was having ever since we bought our trailer 2 years ago. It took more than a year of frequent experimentation to discover and solve the problem. I am certain, since we are the 3rd owner and the prior owners were not especially handy, that the problem existed since 1980 but they just lived with it. Our fridge is the Dometic RM211 3-way fridge that came with the trailer. It now works perfectly in all modes and will keep the fridge at 33 F when the outside temperature is in the mid 90s, whether stationary or driving, without using my installed fan. The fridge is installed with upper and lower side vents, on the driver side of the trailer.

The conventional wisdom is that the more ventilation the better. However, I have now found out that it is possible to have too much ventilation, so much that the fridge pipes cool below their operating requirements and the fridge wonít work.

The initial problem was that, when running on propane, the fridge flame would not stay lit when driving at highway speeds. The second problem was that, while driving at highway speeds, the fridge would not cool whether it was switched to gas or 12V, even though the 12V was working properly. In fact, if I started at 33 F, the fridge would warm up ten degrees in about 10 minutes of highway driving. The fridge would work very well when the trailer was not moving. It would also work sometimes in city driving where the speed was low, but not always.

I did all the maintenance fixes for the gas and electric circuits, and the fridge was running correctly. I also insulated the gaps and sealed the entire back of the fridge with butyl tape and/or metal HVAC tape around the edges so that no air would get from the back to the top or sides, or into the fridge.

From experimentation with a leaf blower, I discovered that the main problem was that too much wind was blowing into the upper vent while driving. Most wind fixes focus on the lower vent, since the upper vent is fastened into place and canít be adjusted. I had to remove the upper vent several times by drilling out the rivets and then re-riveting it back into place later. Needless to say, I have figured out a way to secure the upper vent well enough for testing with only 3 rivets, after removing it several times over the past year.

The issue is this: The boiler (from whatever source of heat) heats up the ammonia solution which travels up to the water separator and condenser at the top of the fridge. The water separator, which is the very top pipe on the back of the fridge, gets very hot under normal operation. This prompts the evaporator to exchange heat, which cools the fridge. However, on my fridge when travelling, the water separator and condenser were dead cold after travelling for only a few minutes, even with the heat source running normally. This means that there was no heat at the top of the fridge, and therefore no cooling action in the fridge. When the trailer stopped moving it would again start to cool properly after 20 minutes or so, which is normal operation since it takes that long for the boiler to heat up the unit to begin to work.

So, I had to prevent the wind from cooling the top of the fridge too much while driving, while at the same time allowing good venting for operation while stationary.

I installed the almost-good venting as described in various places online and surrounded the fridge with fibreglass insulation so that there was no air gap. I will cut short the various things I tried, and only say that I ended up with the bottom third of the upper vent closed off, the rest baffled by a piece of metal roof soffit riveted to the inside of the upper vent to slow the wind down, and two vertical metal panels sticking up between the upper vent and the fridge, which each came to within about 2 inches from the top of the exit chamber. Even this did not work properly until I taped over almost all of the bottom vent, leaving only a vertical opening about 1 Ĺ inches wide at the rear end. There is still enough wind getting in the top vent to provide very good cooling, but not so much that the fridge wonít work. When driving I close the bottom vent, when stopped I open it a bit. I also put in a piece of plexiglass that covers the rear half of the bottom vent, about a half inch from the vent when closed. This cuts the wind so that the flame does not blow out when camping in very windy areas. I can still reach around it to light the fridge.

A couple of pictures show the soffit riveted on to the upper vent, and one of the metal fins sticking up in the upper vent opening.
Rick G
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Old 09-19-2012, 12:00 AM   #2
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Rick,
You are the MAN! This is seriously good work! I feel like I learned a bit more about the operation of absorption refrigeration. Understanding exactly how this works is on my bucket list. I suspect that the top loop that is getting too cool is a distillation column of some sort. The little kink in the bottom of the tube is a water dam. I assume that the extra cooling was causing something else to drop out along with the water. Ammonia?

I really am not getting this part though:
Quote:
Originally Posted by Rick G View Post
two vertical metal panels sticking up between the upper vent and the fridge, which each came to within about 2 inches from the top of the exit chamber. Even this did not work properly until I taped over almost all of the bottom vent, leaving only a vertical opening about 1 Ĺ inches wide at the rear end.
Can you make a sketch or something?

I have an RM36E in my trailer, so the geometry would be totally different, but there are a number of other 4500's out there with an RM211.
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Old 09-19-2012, 12:51 AM   #3
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I have a 211 and took my maiden voyage recently. Needless to say i encountered all of the above. I am anxious to try this.

Thanks for the two pix and if you have any more please do post!

Cheers
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Old 09-19-2012, 06:30 AM   #4
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Nice bit of troubleshooting. I was told Trillium used an over sized upper vent so the emboised Trillium symbol could be completely covered, thus not interfering with sealing. I guess there were unintended consequences. What are the effects on performance when stationary? Any concern with higher temperatures causing higher pressures on an older system with thinned pipes due to rust? Raz
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Old 09-19-2012, 07:54 AM   #5
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Nice work.

My take is that in order to work, part of the piping needs to get hot and stay hot, part needs to be cooled by airflow. I noticed that on my Norcold 323 the chimney and heater box are wrapped with glass wool. Is that something that could be done on this model? Or perhaps more defined baffling to prevent a blast of air over the parts that need to stay hot.
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Old 09-19-2012, 09:44 AM   #6
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Quote:
Originally Posted by Thomas G. View Post
Nice work.

My take is that in order to work, part of the piping needs to get hot and stay hot, part needs to be cooled by airflow. I noticed that on my Norcold 323 the chimney and heater box are wrapped with glass wool. Is that something that could be done on this model? Or perhaps more defined baffling to prevent a blast of air over the parts that need to stay hot.
The RM211 has a large glass wool filled box that the heater stack is in, I think the part that is getting too cool is the first radiator across the top. While it is supposed to cool, apparently if it get too cool, it doesn't work.
I have an RM211 in my first 1300, and now 3 spares.
I suspect part of the problem is that his RM211 is in a hole designed for a larger fridge. The RM36E in my 4500 barely fits. The extra space around the back of the fridge probably allows for much more air flow when he is driving. Mind you, I have not run my fridge while driving yet. I may have the same problem.
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Old 09-19-2012, 11:02 AM   #7
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Quote:
Originally Posted by P. Raz View Post
N What are the effects on performance when stationary?
Good question on my old fridge its performance in high temps improves greatly with more air movement - ie leave the bottom vent open and put a fan on to move air over the upper vent area. Its not an RM211 though but almost as old.
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Old 09-19-2012, 11:11 AM   #8
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Quote:
Originally Posted by David Tilston View Post
Rick,

Can you make a sketch or something?
Ok, here is a cross-section sketch. I hope it helps. The fridge is the square box with squiggles on the left to represent the cooling unit. I have left gaps in the drawing for the upper and lower vents to show where they are, but the lower vent is actually hinged at the top to make it easy to open. I am not showing the plexiglass half-cover on the bottom vent that helps keep the gas lit on windy days, because I canít figure out how to put it in the diagram. I will take a picture when I get home later.

I first made the big ďDĒ shaped aluminum baffle, coming very close to the cooling unit (within less than an inch). This forces the vent air into the coils rather than around them during normal stationary operation. This, and the insulation, was enough to make a noticeable difference in the efficiency of the fridge when operating while stationary. However, I suspect it made the wind problem worse since there was now a really good wind tunnel when travelling. The upper compartment of the trailer was curved from the factory, but I added some sheet metal at the top to make it a better curve extending right to the top of the fridge so that warm air would not get into the gap between the top of the fridge and the top of the space in the trailer. I donít show that bit on the diagram in order to keep it simple.

My fridge opening was larger than the fridge by about a half inch on each side and about 4 inches at the top. These gaps were filled and insulated with fibreglass insulation and sealed off using metal HVAC tape.

The top of the ďDĒ on the left shows that I made the upper vent opening smaller by closing off the bottom third or so of the top vent. That made a narrower channel for the air to move in, but still quite large. It seemed to me from looking at the service manual for the fridge that the upper vent could be smaller, as Raz mentioned above.

The two vertical lines coming from the top of the ďDĒ shaped baffle near the fridge are the two vertical fins I installed to cut the wind. They are solid sheet metal, not perforated. They come fairly close to the top of the opening, within about 2 inches for the middle one, a bit more of a gap for the one near the fridge. I tried numerous other configurations of angled brackets and perforated metal before finding this one. I figured that, since hot air rises anyway, it would not interfere with the normal operation of the fridge much, if at all, which proved to be correct.

The upper vent with a squiggly shape attached is the perforated baffle made of roof soffit that I showed in the picture. It is mounted using its natural bends so that it is only a half inch or less from the vent. This cut the wind significantly, but again allowed warm air to escape during normal operation.

In addition, I also mounted a metal baffle at the top of the bottom vent (near the flame box and only about a third of the width across, not shown in diagram) to cut the amount of wind reaching the flame box, and the plexiglass half-cover for wind while stationary. I also taped over almost all of the bottom vent. This resulted in very good cooling while travelling, for which my theory is that it actually does not matter where the wind comes from as long as it cools the top water separator pipe enough but not too much.

To answer the question on operation, these changes made the fridge work much much better. It now works, as I said initially, at least up to the mid 90s of outdoor temperature and the fridge remains at 33 F (I have a thermometer mounted inside the fridge with a readout outside). That is with the thermostat set at about 2-3 out of 9 on both gas and electric, so there should be sufficient room for hotter climates. I havenít been camping anywhere above the mid 90s yet, although I hope to in the future.

I found that, while stationary, the lower vent door had to be open a bit to allow air in the bottom, which makes sense because the lower vent is mostly taped over and not much air gets in there unless I open the vent door an inch or so. I donít mind leaving the door open a bit when camping, although I am now experimenting with how much of the lower vent I can uncover before there is too much wind again and the fridge stops cooling while travelling. The current cooler fall weather here in northern Alberta helps this experiment, since if it works with a cold wind it will also work while travelling in warmer weather.

I am also attaching a diagram of the cooling system taken from the service manual. I ran the fridge for a time while it was out of the trailer on my bench, and then took (incredibly accurate!) measurements by putting my hand on several components to see how hot they get under ideal cooling conditions. As you can see, only the water separator at the very top gets really hot, burning hot to the touch. I took these measurements while the fridge was actually cooling down and had been running for a couple of hours so that I would get the temperatures while it was in cooling mode, not once it had reached the set temperature.

Hope that helps, sorry for the long explanation. I am surprised that there were no references I could find to this problem anywhere on the internet. I assumed that I was the only one experiencing it, which seemed odd given the number of Dometic fridges in use in the past 60 years. However, I was buoyed by the assumption that my fridge is supposed to work while travelling like it did for everyone else, and it was just a matter of me finding the proper configuration.

Rick G
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File Type: pdf cross-section fridge baffles Sept 12.pdf (9.7 KB, 98 views)
File Type: pdf fridge operating temp.pdf (13.2 KB, 72 views)
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Old 09-19-2012, 01:25 PM   #9
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Good Stuff... I'm sure it's been referenced elsewhere, but there's this, too:

http://www.dometic.com/FileOrganizer...rigerators.pdf
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Old 09-19-2012, 09:03 PM   #10
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Here is a picture of the lower vent of my fridge. It has a plexiglass cover attached right by the vent door that goes about 40% of the way across. This prevents the wind from blowing out the flame when camping on very windy days. I can still reach in to open the viewing port so I can tell if the flame is lit. I also have installed a brass cover on the flame box to restrict the wind more. Also, inside the cavity on the right side you can see a white metal piece that is installed horizontally across part of the opening near the flame box, again to restrict the wind flowing down from the upper vent. I found that this didnít make any noticeable difference, but I left it in anyway since it didnít seem to do any harm. You can see that I have applied duct tape over the whole vent cover except for a small part on the right side. I will be experimenting with that over the next few weeks to see how much I can remove and still have not too much wind for the fridge to operate.

Rick G.
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Old 06-16-2013, 12:06 AM   #11
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I am going to have to try this with my RM211. I sat here and installed 3 old PC fans for MORE ventilation and it seemed to do worse. I'm going to try keeping the fans turned off (I put them on a switch) and see if it does better. I may also try blocking it part way like you did to see if it helps.

What made me think about this is how the back side is barely warm. I can put my hand flat in the cooling unit and leave it there! It's not that hot at ALL and the fidge just isn't cooling like it should.

With the fans off, after 10 minutes, then cooling unit is much hotter but I can still tolerate keeping my hand on it. I guess once it's burning hot, then it's working right.
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Old 06-16-2013, 08:55 AM   #12
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Good work Rick , its nice to find something that works.
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Old 06-16-2013, 09:03 AM   #13
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I put some aluminum foil over the outside top vent last night and some electrical tape over 1/3 of the bottom vent. The fridge got to about 41-42 degrees over night which is the peak lowest temp I have seen it in the past, I guess the fridge has a clog in the lines since it's as level as you can get without breaking out 1/4" pieces of wood to shim under the tires.

I guess I'm going to just have to pull it and put in a dorm fridge, as much as I don't want to. Or just save up and put in another RV fridge... or do the dorm fridge until I can afford an RV fridge ..

But I'll keep testing this one in the meantime..
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Old 06-16-2013, 12:18 PM   #14
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Cubey, thanks for bringing this thread back to life. I missed it the first time around. 41-42 degrees sounds good, it is a fridge not a freezer. Somewhere here someone has posted the anticipated maximum temperature differential.

Rick, thanks for taking the time to document your efforts. It gets us to thinking on how to make our fridges work better in varying conditions. I believe Tom [Thomas G.] got it right in posting "in order to work, part of the piping needs to get hot and stay hot, part needs to be cooled by airflow."

The pdf link that Gary [fusedlight] posted contains a lot of information. Most notably:
"The boiler section utilizes the ammonia-water liquid solution in the absorber and as it is heated, turns the solution into a strong ammonia vapor, which is needed to operate the system. This strong ammonia vapor rises from the pump tube to the condenser coil.
Any deviation from the listed amount of heat to the absorption system will alter the
ammonia to water ratio, which, in turn, will decrease the cooling unit’s overall efficiency."

The RM182B has a thermoswitch attached to the condensor unit as can be seen by following the yellow and blue wires up from the fan in this picture.

Dometic is using the temperature of the condensor to control the cooling of the absorber via the thermoswitch on the condensor and the fan below the absorber.
I see that Rick is inadvertantly doing this with his vent design. He is effectively altering the temperature of the boiler, condensor and absorber. I may have done the same with the sizing and location of the baffle.

With that in mind, the pdf above says:
"After the unit has been operating for
approximately one hour, carefully touch the unit
at the boiler box and the absorber area. These
areas should be approximately the same
temperature, regardless of the ambient
temperature. Equal heat between the boiler and
the absorber indicates the fluid circulation within

the unit is good."

If I'm reading this right, to get our fridges working best we would want to vent and/or control air flow such that we are providing maximum cooling at the uppermost fins (condensor) and controlling the temperature of the absorber unit. Rick might be onto something with the apparent split in venting at the top and controlling cooling at the boiler. It would be nice to know what the best temperature is for the condensor area. Maybe the specs of thermoswitch on the RM182B would give us some indication of what the Dometic engineers consider optimal.
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