Performance of 3-way absorption fridge - Fiberglass RV


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Old 06-08-2008, 09:58 PM   #1
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I spent the last few days looking after our old fridge. We have the original Dometic RM24 3-way fridge in our Trillium, and we recently had it checked and reloaded with ammonia. It worked fine in our basement, but now that I reinstalled it in our trailer, I decided to put a thermometer inside and see how it behaves while the trailer is parked in our driveway, with no food inside. Today was one of those hot and muggy days (well above 32C ~ 90F) and the trailer does sit in the sun for part of the afternoon.

I currently have it set to it's at or slightly below freezing point when I check it in the morning. But in the middle of the afternoon today, I got temperatures inside the fridge reaching 12-15C ~ 54-59F. It basically means that it's only able to keep about 20C ~ 68F below ambient temperature (in somewhat harsh conditions). Once it reaches that temperature, it will take forever to cool back down once the outside temperature cools down, even if I crank it up to its MAX setting. And if I happen to open the door for, say, more than a few seconds, it will not start recovering until much later in the evening. In more normal conditions (70-75F outside) it manages to stay around 41F in the afternoon.

My question is this: I am not really comfortable keeping milk or meat at those temperatures. If I was to replace it with a new one, could I expect better performance, or is this as good as they get? What do you guys with actual hot weather (not the Canadian sissy warm, but real southern warm) do, other than dump the fridge and use an ice box?

I would like to keep the 3-way ability if I can find a decent model. Thanks for any input...
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Old 06-08-2008, 10:36 PM   #2
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I currently have it set to it's at or slightly below freezing point when I check it in the morning. But in the middle of the afternoon today, I got temperatures inside the fridge reaching 12-15C ~ 54-59F. It basically means that it's only able to keep about 20C ~ 68F below ambient temperature (in somewhat harsh conditions). ...

My question is this: I am not really comfortable keeping milk or meat at those temperatures. If I was to replace it with a new one, could I expect better performance, or is this as good as they get? What do you guys with actual hot weather (not the Canadian sissy warm, but real southern warm) do, other than dump the fridge and use an ice box?

I would like to keep the 3-way ability if I can find a decent model. Thanks for any input...
Not the Canadian sissy warm? It was well over a 100 with the humidex reading today!

You did not say what you were running the fridge off of today. The guy that serviced my fridge says they work best off propane.

These fridges will not cool like one in the house, they are affected by the outside temperature. You can make them more efficient by making sure you have a good air flow with proper venting of the fins on the rear. Some people have made significant improvement by adding 1 or 2 computer type fans to the back of the fridge, some add a small fan to the inside. I am not familiar with the set up of the Trillium, so I can not make specific recommendations for your trailer on what works best.
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Old 06-08-2008, 10:50 PM   #3
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Not the Canadian sissy warm? It was well over a 100 with the humidex reading today!


No kidding, eh? Both yesterday and today were pretty warm here in the Ottawa region also.

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You did not say what you were running the fridge off of today. The guy that serviced my fridge says they work best off propane.
Yesterday I ran it off 120VAC and today was on propane. Not much difference.

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These fridges will not cool like one in the house, they are affected by the outside temperature. You can make them more efficient by making sure you have a good air flow with proper venting of the fins on the rear. Some people have made significant improvement by adding 1 or 2 computer type fans to the back of the fridge, some add a small fan to the inside. I am not familiar with the set up of the Trillium, so I can not make specific recommendations for your trailer on what works best.
I understand there's a difference compared to a house fridge, but what I'm trying to figure out is how far away from optimal my fridge is running, and if I would gain anything with a newer unit. Any idea how significant the improvement could be with a small fan as you're describing?
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Old 06-08-2008, 11:11 PM   #4
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Hi: Our local R.V. fridge Tech told me that in hot humid weather with little or no breeze you can't expect these fridges to cool very much. Adding a fan to the vent to draw air away from the coils would greatly improve the cooling effect of the boiler system. The next big improvement to your fridge would be a new door seal or make sure the old one is clean and still closes tightly!!! Google www.gammonsrv.com for a complete story on R.V. fridges.
Alf S. North shore of Lake Erie
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Old 06-08-2008, 11:50 PM   #5
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I understand there's a difference compared to a house fridge, but what I'm trying to figure out is how far away from optimal my fridge is running, and if I would gain anything with a newer unit. Any idea how significant the improvement could be with a small fan as you're describing?
Daniel,

I'm not sure of the actual #'s. I know the more you can cool the fins on the outside the more efficient the unit becomes and better at cooling the inside. I know it is almost a $1000 to get a new fridge installed. My Dometic fridge was taken from a VW Westfalia. The PO installed it in my Boler. A botch job, but I fixed up the venting as suggested by my propane guy & hope to post pictures soon. My info came from searching the Westfalia sites for specifics on my fridge and fixes. Their fridges come with a small fan hooked up to a thermo couple to start the fan once the outer fins reach a certain temperature. My propane guy agreed that adding extra fans would help.

The best anology I can think of is the new cars with the electric fans. Those cars have rads, and the fans kick in at certain temperatures to help cool off your engine when needed.

I do know that making the appropriate changes to allow for proper air flow made a significant difference in my fridges operation. Essentially I've created a chimney type airflow that helps cool the upper fins. Have a look at this service manual in the documents section. It will give you an idea on venting. I know I spent many hours searching for my fridge. I've got a general diagnostic manual by Dometic as a PDF that has a lot of good info in it. Including how these things work and some diagnostic flow charts. Send me your email and I'll email it to you. It is about 2MB in size for a 52 page PDF file. Maybe one of the moderators here will accept it in an email and post it for all to access in the documents center.

We actually made ice in it last summer post mod. I disconnected the built in fan because the thermocouple did not seem to be working and the fan ran all the time. I've sourced a new thermo couple and will give it a try later this summer as I continue to do the electrical system. But I've also designed my mod to take 2 computer fans to increase efficiency. They draw very little power at 12 V. Those fans will be manually switched.

Roy
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Old 06-09-2008, 12:26 AM   #6
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I do know that making the appropriate changes to allow for proper air flow made a significant difference in my fridges operation. Essentially I've created a chimney type airflow that helps cool the upper fins.
OK, I'll bite on this fan idea. I already have the service manual for my fridge, which may as well have been created before fridge fans were invented (35-ish years ago), but the basic principles will apply for sure.

Surfing for fridge fans produced these results, among others. Some even claim to have a built-in thermostat.
Fridge Vent Exhaust Fan, 12V
Solar Fridge Exhaust Fan

I'm willing to give it a shot, and am leaning towards the simple computer fan type. I guess all I need now is to figure out the strategic spot to place it for my specific situation.
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Old 06-09-2008, 01:07 AM   #7
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Talking

Check out this link:
Gas/Electric Fridge Venting

Many small under-counter fridges are set up like the last example: "Bad Venting 2"

Hopefully, you can get your setup more like "Almost Good Venting"

THEN adding the 12 volt exhaust fan will help in hot weather when static convection alone doesn't work.

I use both an internal FridgeCool Fan plus computer fans mounted in my upper fridge exhaust vent.
Attached Thumbnails
Upper_Fridge_Vent.jpg   Upper_Fridge_Vent_Open.jpg  

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Old 06-09-2008, 02:04 AM   #8
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There are some physics at work here, foremost it requires 144 times the btu's to remove one drop of water from one cubic foot of air over dry air... also the moist air will not drop in temp until the moisture is removed first...all that energy pulled out and all you get is the drop of water...not cooler air... So gasket seals are crucial.

Next, there is sensible heat and in-sensible heat...sensible heat can be managed by refrigeration systems, insensible heat like infrared cannot be moved...but it can be reflected.

The only way to manage insensible infrared is to reflect or use a barrier, I line the cabinet with a auto firewall highly reflective surface to reflect infrared heat away which will penetrate any box if allowed warming the contents and then warming the ambient air in the box.

I live in Arizona, my fridge can manage 45 degrees.

The advantages to a new box is it will be built to reflect heat more, it will move it from inside to outside the box faster as fin design has got better...its design will be double carbureted being more tolerant to a less than perfect un-level operation, and they are lighter.

The trills do not use a highly efficient convected drafting design, like the 13' scamps they have a low intake louver and a higher louvered exhaust...these are not really the best design from the point of view of the equipment manufacturer, the new ones you would buy would void the warranty if you use that set up today, manufactures often insist you use their vent designs.

To highlight in short, I see three issues. 1. moisture "check seals"
2. infrared load "surround with reflective barrier"
3. method of convected heat discharge
"a power assist may help"

Lastly when you test a box, test the box full, not empty.

Harry
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Old 06-09-2008, 08:16 AM   #9
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Check out this link:
Gas/Electric Fridge Venting

Many small under-counter fridges are set up like the last example: "Bad Venting 2"

Hopefully, you can get your setup more like "Almost Good Venting"
Good link Frederick!

I went from something a little worse than "bad venting 2". There was only one vent that was too high on the bottom and too low on the top.

I sealed off the fridge from the inside of the trailer since it was wide open under the cabinets. Then tried to get as close as I could to the "almost good venting" including the baffle. The baffle creates what I reffered to as the "chimney type airflow". A major difference in temperatures inside the fridge.

Like I said before, I'm still working on ressurecting the trailers electrical system. I'm hoping that reconnecting the built in fan with a new thermocouple and adding two manually switched computer fans will make even more of a difference.

Roy
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Old 06-09-2008, 08:19 AM   #10
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Originally posted by www.fridge-and-solar.net/fridge_vent.htm
[b]Fans. Theoretically, perfect venting will create a draft that will remove heat from the cooling unit in even the warmest conditions. However, perfect venting isn't always that easy to achieve. The purchase of an add-on fan can solve a lot of problems in border line venting, but is not a cure all for terrible venting. The important thing to remember is that the fan should be installed above the cooling unit, preferably right at the roof vent. The purpose of the fan is to [b]improve the draft, not to blow air onto the cooling unit.
Great information here. I am probably somwhere between "Almost Good Venting" without the baffles and "Bad Venting 2". Looks like there are a few things I can do to greatly improve the air flow.

Quote:
To highlight in short, I see three issues. 1. moisture "check seals"
2. infrared load "surround with reflective barrier"
3. method of convected heat discharge
"a power assist may help"
Where exactly do you put the reflective lining? Do you cover the inside of the enclosure before inserting the fridge? Any additional places? Would aluminium foil do the trick? I had already insulated a few open areas around the fridge cabinet with some styrofoam I had lying around, but nothing reflective.

Thanks!
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Old 06-09-2008, 02:01 PM   #11
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I just purchased a new frig from Norcold and quickly discovered that my current frig enclosure was poorly designed. There was not a floor in the frig enclosure and the original frig was simply suspended on rails above a drawer and cabinet space. The lower vent was entirely too high and too big and baffles were nonexistant. Now I'm going to need to lower the frig enclosure (the wall curves too much as it approaches the roof and the enclosure narrows significantly enough that the frig won't fit otherwise), add an insulated floor, and install another vent below the bellyband at the appropriate level.

My question is. . .
since the original wall vent was fairly large and I'm not up to doing a large fiberglass-fill of the vent space, what do ya'll suggest I use to seal the original vent grill? Of course, ideally, I'd like the solution to not only be visually appealing, but additionally allow me to open the access panel if need be. Could I use butyl tape and a reflectix panel cut to size to cover the vent holes? Does anyone have a more elegant solution to my dilemma?

Thanks!
Lisa
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Old 06-09-2008, 08:23 PM   #12
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http://cgi.ebay.com/ebaymotors/AUTOMOTIVE-...emZ140239721907

This is the vendor I use, it arrives fast and creates a fireproof protective closet.

Harry


Where exactly do you put the reflective lining? Do you cover the inside of the enclosure before inserting the fridge? Any additional places? Would aluminium foil do the trick? I had already insulated a few open areas around the fridge cabinet with some styrofoam I had lying around, but nothing reflective.

Thanks!
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Old 06-09-2008, 08:33 PM   #13
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Another thought I had over looked yesterday was this...if there is inadequate venting of the output heat "btu's" from the rear of the unit you can "cook" the whole enclosure in the compartment designed to hold the unit in place...I would be interested in knowing the night time temp of that compartment...The reading taken at night eliminates infrared heat temp readings having an influence on the thermometer...I also would like to know the outside of the trailer temps when the inside reading was taken...this reading should be near the top of the compartment.

Harry
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Old 06-09-2008, 08:44 PM   #14
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I would look at fiberglass reinforced board (avail home depot or lowes) it is water proof, rot proof and a panel might be cut to fit behind the louvered panel and silicon glued into place...The silicon glue resists heat and moisture...when you do decide to repair the hull permanently there are some tricks that make the idea less scary and the results are very good. Also a how to video is available for this type of repair.

Harry

Quote:
My question is. . .
since the original wall vent was fairly large and I'm not up to doing a large fiberglass-fill of the vent space, what do ya'll suggest I use to seal the original vent grill? Of course, ideally, I'd like the solution to not only be visually appealing, but additionally allow me to open the access panel if need be. Could I use butyl tape and a reflectix panel cut to size to cover the vent holes? Does anyone have a more elegant solution to my dilemma?

Thanks!
Lisa
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