Adding fan behind refrigerator...worthwhile??

[SJ-Chris]

I have 3 RVs and I seem to be addicted to adding stuff to them....lol.
They each have a standard Dometic 2852 absorption refrigerator. They seem to be working as expected. I did replace the cooling unit on one about 2 years ago and it has been working flawlessly.

My question is this... Under NORMAL OPERATION how hot does it get behind the refrigerator in the cabinet area and venting area out through the roof? And does adding a fan back there do much of anything to help with refrigerator operation (keeping interior of the refrigerator/freezer cool)? I'm not talking about preventing overheating and/or fires...just wondering if it helps the refrigerators perform better.

Something like this with some sort of thermostat such that it's not running all the time:

This particular fan moves ~38CFM at 45dB (might be louder than I'd like, but behind a refrigerator would probably muffle the noise quite a bit). Maybe I could find something quieter...
Here is a nice but simple 12v thermostat that could be programmed to turn the fan "on" when it hits a certain temperature and then turn it "off" once the fan cools it down to some lower value.
https://www.mpja.com/Single-Zone-Intelligent-Thermostat-Temperature-Controller/productinfo/34757+MP/

Let me know what you think. Total waste? Or significant increase in summertime cooling inside your fridge? I sometimes camp or lend out my RVs in 100+ degree summer temps.

Does anyone have any "before and after" data points or personal observations after having added such a fan?

Thanks!
Chris

[ferndaleflyer]

I added a fan to mine 2 years ago and it has performed flawlessly ever since.  Before always trouble when it was hot.

[pianotuna]

One side benefit of having a mask with fans at the top is reduced air flow in the winter. This helps prevent the fridge from freeze up, but is not enough to assure it.

I do know that without the fans first cycle from ambient (65 f) was about 8 hours on electric. After the fans that dropped to about 4 hours.

I have an adjustable thermostat which I tweaked over several trips.

[fj12ryder]

FWIW, I feel that having fans at the top, and setting it up so they work to pull air through the coils, will inhibit the air flow if the fans aren't running, so you'd have to run the fans all the time. It may work more efficiently, but IMO the fans would have to run constantly to keep temperatures down. I have my fans down at the bottom just for that reason.

[Community Alumni]

you need less fans as it is a more lamaner air flow.  when a fan isn't turning there is still plenty of flow alowed through it, especialy since you might be putting one or two 4X4" fans at a opening that is about 8X16"  its kind of like pushing a rope vs pulling it, the smoothest way to direct air flow where you want it is to put the one side in a vaccum and using baffels to direct it, the other way is to put a bigger or multi fans below and force it at the baffel but not all the air will go throuhg the baffel which is why you need more.

[fj12ryder]

But if you don't completely cover the opening you're trying to pull air through, you're just going to pull air from around the sides of the fans, not up through the opening. That's why fans are shrouded. And, I agree you'll get some air through the fans if they're not running, it's not going to be very much as the fan blades will block any straight through air flow.

[Community Alumni]

you keep getting fixated on a fan that is the same size as the opening. once again one small fan in a large space is not going to block any airflow.   don't forget also that if you read the instalation instructions for most fridges the back of the condenser is suposed to have a baffel extending from the wall to within 1/2 to 1/4" of the baffel so you are restricting the flow right there. I have a factory fan in my fridge that is hooked to a thermostatic switch so it comes on when ever the temp gets above a specific value(don't ask me what it is haha.)  they are not at the top, they are just above the baffel.  the intent is to draw a smooth flow of aire accross the fins of the condenser.  turbulant flow is less efficient at removing heat.  mind you , there is a way to do it with the fan/fans at the bottom and that would be to build a wide linear nozzel that the fanse are sealed to that direct the air through the fins, but thats a lot of work.   the easiest way is to remove the cover on your top vent and then reach down and mount it just above the baffel blowing up.  before even adding a fan though make sure that baffel is in place.  I have seen several units that were having fridge issues and when we looked the baffel was either mossing or installed to far away from the fins alowing air to bypass them instead of flowing throuhg them.

[fj12ryder]

And you keep missing my point. 🙂 One small fan in a large space won't block any air flow, but it also won't increase air flow much, if any, because air will simply come in around the side of it. That's why fans are shrouded. The baffle doesn't restrict air flow so much as direct the air flow, which is why there are issues when the baffle is missing, or placed incorrectly. We'll just have to agree to disagree. 🙂

[StirCrazy]

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@fj12ryder wrote:

And you keep missing my point. 🙂 One small fan in a large space won't block any air flow, but it also won't increase air flow much, 🙂

haha ya, but I can tell you if I diable the one 3 or 4" fan that keystone installed inside the space for minethe temprature in the fridge will start to rise and not work very good, but I turn it on and it starts to cool right down again.  playing with this I installed one in my camper to see what it would do and I can tell you one tiny fan in a large space dropped my tempratures in the fridge 8 degrees.

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[SJ-Chris]

ernie1 wrote:
SJ-Chris: Thanks a bunch for the info and clearing a few things up for me as I also am trying to gain more efficiency from my Dometic 3 way fridge.

I'm not going to hijack this thread but I feel compelled to mention only briefly what I have done to gain more efficiency from my fridge. After noticing that the refrigerator in my 2018 PW would always run warmer by about 8 deg.F when it was on propane versus 120v shore power or inverter. Puzzled by this since normally when on propane a refrigerator will usually perform better than when on 120v ac I decided to experiment. I took the jet out of the burner and had it drilled out .003" and reinstalled it. The refrigerator performed much better but I stopped the experiment with this jet because I felt there was too much heat blowing into the burner. This a seat of the pants feeling only.

Next, I bought a new original sized jet for $70 and drilled it out only .001" and reinstalled it . So far I tested it for weeks at a time and I am now at a point where when I'm running the fridge on propane it's about 8 deg.F cooler than when on 120v ac and I'm really happy but am still experimenting. I think I've gained 16 deg.F of cooling. Oh yes, I have three fans in the outside rear and two inside the cooling area.

So that's it. So that I don't hijack this thread and put unproven experimental ideas into others heads which will cause a dangerous condition, I done discussing this experiment of mine.

Interesting.

I learn so much from these forums... I hadn't really thought much about refrigerator performance DIFFERENCES when using propane vs 120v. Would others agree that refrigerators generally run cooler on propane vs 120v? I hadn't heard that. On my RVs, I have the option to run on gas even if connected to shore power....I wonder if that is why the option exists (it's on a standard Dometic refrigerator).

If there is a difference in performance, does anyone know fundamentally WHY? My current understanding would make me think that the primary (only?) difference would be how HOT the boiler gets. Is my thinking correct?

One of my next DIY projects is going to install a simple high-temp thermostat to protect the boiler from getting too hot (overheating, as in when out of level). My understanding is that under normal operation it runs between 380F-400F. Now I will have to put on my to-do list to run an experiment to see how hot the boiler gets when on propane vs on 120v. We'll see...

-Chris

[SJ-Chris]

StirCrazy wrote:

SJ-Chris wrote:
Added a 3rd fan and repositioned a few things. Cleaned up the wiring. I'll be taking it out in a week and we'll see how it does. This next trip will only be in ~80 degree weather, so I don't know if the fans will come on often and/or be of much help. The real test will be the next time I am boondocking in 95F+ temps.



These 3 fans will move 15cfm x 3 = 45cfm in theory. Running a test right now in ~94F temps while in storage.

-Chris

Depending on the baffle setup those fans may not make a difference. The normal way to install them is at the top vent sucking out and to insure you have the baffle setup properly around the fins on the heat exchanger to the air is directed through the fins. the reason the top sucking out is more effective is a bunch of laminar airflow and thermal dynamics stuff that would take me fore ever to type but essentially if you baffle the top exchanger section properly and just have one fan drawing out (maybe two depending on the space size) this will direct the air flow over the fins and ensure proper heat exchange. Just using a bunch of fans at the bottom and forcing more air in will force more air through but you're not controlling where you want it or getting a smooth high velocity air flow. you're getting chaotic and slow flow which won't cool as effectively.

Thanks for the feedback.

If I had the choice (translation: If it were easy enough to do), I would rather have a fan or two at the top roof vent sucking air from the behind the fridge area below and blowing it out the roof vent more directly, as I believe that would be more effective at forcing hot air out of the refrigerator area. But installing down below on the side panel was a lot easier.

The way that my side vent fans are set up is that they are sucking air from right next to the side vent slot openings. I believe this will pull cooler air in from outside, blowing it up behind the backside of the refrigerator. There is nowhere (practically speaking) for the air to flow but up out the roof vent. In doing so, it will pull heat from everywhere behind the refrigerator. This will aide in better cooling.

I ran a somewhat controlled test with just 2 of the fans installed. Ran the fridge for 24hrs on back to back 90F days. One day without the fans on. One day with the fans on. When the fans were on, at 3pm at the peak heat of the day, the freezer and fridge were both 6F cooler on the day the fans were running. So it seems like they are helping. I added a 3rd fan just because and I'm hoping that helps even more. We'll see.

Happy Camping!
Chris

[ernie1]

SJ-Chris: Thanks a bunch for the info and clearing a few things up for me as I also am trying to gain more efficiency from my Dometic 3 way fridge.

I'm not going to hijack this thread but I feel compelled to mention only briefly what I have done to gain more efficiency from my fridge. After noticing that the refrigerator in my 2018 PW would always run warmer by about 8 deg.F when it was on propane versus 120v shore power or inverter. Puzzled by this since normally when on propane a refrigerator will usually perform better than when on 120v ac I decided to experiment. I took the jet out of the burner and had it drilled out .003" and reinstalled it. The refrigerator performed much better but I stopped the experiment with this jet because I felt there was too much heat blowing into the burner. This a seat of the pants feeling only.

Next, I bought a new original sized jet for $70 and drilled it out only .001" and reinstalled it . So far I tested it for weeks at a time and I am now at a point where when I'm running the fridge on propane it's about 8 deg.F cooler than when on 120v ac and I'm really happy but am still experimenting. I think I've gained 16 deg.F of cooling. Oh yes, I have three fans in the outside rear and two inside the cooling area.

So that's it. So that I don't hijack this thread and put unproven experimental ideas into others heads which will cause a dangerous condition, I done discussing this experiment of mine.

[StirCrazy]

SJ-Chris wrote:
Added a 3rd fan and repositioned a few things. Cleaned up the wiring. I'll be taking it out in a week and we'll see how it does. This next trip will only be in ~80 degree weather, so I don't know if the fans will come on often and/or be of much help. The real test will be the next time I am boondocking in 95F+ temps.



These 3 fans will move 15cfm x 3 = 45cfm in theory. Running a test right now in ~94F temps while in storage.

-Chris

Depending on the baffle setup those fans may not make a difference. The normal way to install them is at the top vent sucking out and to insure you have the baffle setup properly around the fins on the heat exchanger to the air is directed through the fins. the reason the top sucking out is more effective is a bunch of laminar airflow and thermal dynamics stuff that would take me fore ever to type but essentially if you baffle the top exchanger section properly and just have one fan drawing out (maybe two depending on the space size) this will direct the air flow over the fins and ensure proper heat exchange. Just using a bunch of fans at the bottom and forcing more air in will force more air through but you're not controlling where you want it or getting a smooth high velocity air flow. you're getting chaotic and slow flow which won't cool as effectively.

[SJ-Chris]

ernie1 wrote:
Am I to understand that the cycling temps of the cooling fans are 95degF on and 130degF off?

No.

I'm not sure what the best on/off temps are, but I'll share my findings and thoughts...Over the coming year I'll test out a few different strategies with these fans to see what works best.

Assumption for my situation: I am ONLY trying to make it so that my fridge temps stay under 10F in the freezer and 40F and below in the fridge section. I am not trying to reduce propane use (although that could happen a bit) and I'm not worried about battery usage (I've got more than enough solar and the fans are low power).

First off, When the ambient temps outside are 60-70F it seems the behind the fridge fans are not even needed. Inside temps of the freezer and fridge are fine (cool enough). The cooling fans are really only NEEDED when the outside ambient temps are probably 80-85F or higher.

Data point: Without the fans running, and with outside ambient temperature of 90F, I notice WHERE I HAVE MY THERMOSTAT PROBE POSITIONED, the probe reads a max of 120F (after the boiler has been on for its cycle). Note: This will be HIGHLY dependent on where you put your thermostat probe. So I would suggest that if you add cooling fans like this, you must first determine what the NORMAL (non-fan cooled) temps are behind your fridge on a hot day with the fridge boiler on and your probe placement. For me and my placement, that temp is 120F max (in 90F ambient temps). So it gets 30F higher than ambient outside temps.

With my 120F high temp data point in mind, I have currently set my cooling fans to go on at 98F and turn off at 96F. How did I pick those temps? Let me explain... I only really want the fans to be on and running when the temps BEHIND the fridge are above 98F. So on a warmish day and the boiler on, it will heat up in the space behind the fridge and I want the cooling fans to turn on and move some of this hot air out. What I have found is that when the boiler is on (which lasts for many minutes) the cooling fans don't cool the behind the fridge cavity as much as you might think. It might cool it 5-10F compared to what it would be without cooling fans. But moving this hot air out helps the fridge to cool better and lower internal freezer/fridge temps. That is the goal afterall.

I have the cooling fans set to turn OFF at 96F. The reason for this is because I don't want to hear the fans in the morning or in the evening and the temps where I live/camp will be down to the 60-80F range when the sun isn't at its warmest/highest (mid-day).

Another data point: Overnight, with night time temps of ~60F and nobody going in/out of the fridge/freezer, (and I assume no cooling fans going on based on my 98F setting), my freezer in the morning was at 5F and my fridge was at 33F.

Hope that helps.
Chris

[SJ-Chris]

enblethen wrote:
Have you checked the distance between the cooling unit and the rig's outside wall? Normally installation direction say it should be only about 3/4 of an inch.
I closed mine up with foam board attached with Liquid Nails type material.

On my RVs, the spacing looks correct.