Dometic 3- Way Fridge

[Davydd]

The most irritating thing about propane is you do have to turn them off at times and in a long line at some ferries many times they don't take your word for it. You had to get out of your van, get down on your knees, reach under and show the attendant you had turned it off. Harassment I tell you. I have no propane on board now.

[avanti]

The primary reason why propane use is prohibited in underwater tunnels has little to do with exploding RVs. Rather, it is that such tunnels are "U" shaped--i.e., lower in the middle. Since propane is heavier than air, gas from a leak tends to accumulate at the bottom, creating an obvious hazard.

[booster]

I think everyone needs to remember that propane is not an all or nothing type of thing. We hear about driving with the propane on, for and against, regularly, but in the real world that is ONLY because of the absorption frig. Nothing else needs to run while you are driving so you just leave the propane off until you need heat, hot water, cooktop, or gas grille. We have camped this way for years, and I would guess our propane gets turned on maybe 2% of the time we are away from home and only when parked. The compressor frig allows that very nice option and safety way of use,

[rowiebowie]

Originally Posted by rtbill

The thing that provides the greatest benefit with absorption fridges in Roadtreks, and likely other Bs with similarly poor fridge installations, is better draft over the coils, specifically the finned coil at the top. These fridges are intended to have a tall chimney above to create natural draft. The RT installation sorta kinda follows Dometic's guidelines, but results in the fridge only sorta kinda working.

Long ago I installed a bank of computer fans to blow air over the coils. The fans are installed on the lower removable panel, and the edges around the fans are taped to prevent air escaping. On the upper panel, the two lower vents are taped closed so that all the air is forced to blow over the finned coil.

With the fans running our fridge works spectacularly well. It has coped with 100+ temperatures in the southwest. It holds temperature on DC when driving.

I measured the duty cycle of the heater over several consecutive hot summer days. On AC the duty cycle dropped from 76% with the fans off to 61% with the fans on. On DC the duty cycles were 89% with the fans off and 72% with the fans on.

The fans draw about 200 mA, which is well worthwhile compared to the fridge's consumption.

You've hit on the ventilation problem, only it applies to compressor fridges as well. Airstream likes to cover everthing inside with a nice panel. Looks great. But in my Avenue it prevented them from providing the Novacool recommended 30 square inches of lower ventilation (they only missed by 30 square inches). Zip, zero, nada provided below. And while they may have come close to providing the recommended 30 sq. inches in the upper ventilation, the air must pass over the rear of the microwave, then make a 90-degree turn to exit around the front of the microwave grill.

My solution was to install a small squirrel-cage fan that evacuates hot air from the cabinet into the living area via a 1-1/2" hose. The fan has an off-on switch and a 100 degree on/85 degree off button thermostat. It works on the principle that he fan would exchange cooler coach air for the hot air in the cabinet. It might be needed it the weather is as warm as today and I've tested it. Fan does the job it was intended.

But, we've never actually used the fan since we've only camped in mild weather.

[GallenH]

rowie (and others who have dealt with this): So compressor fridges require cooling via air movement around the exterior workings of the unit. Makes sense. My question is why wouldn't you draw air past the exterior of the unit and vent it to the outside as opposed to returning it to the inside.

1. It's day. Hot outside. You pull air from the presumably cooler inside and vent it past the fridge to the outside. Eventually you have to draw in air from the outside to replenish that air. Is the air cooler than the air expelled from the fridge or hotter?

2. It's night. Now cool outside. Isn't the exterior air sucked into the RV through an open window certainly cooler than the air coming back through the fridge compartment?

Not arguing. I don't understand. Need some schooling!

[avanti]

Originally Posted by GallenH

rowie (and others who have dealt with this): So compressor fridges require cooling via air movement around the exterior workings of the unit. Makes sense. My question is why wouldn't you draw air past the exterior of the unit and vent it to the outside as opposed to returning it to the inside.

1. It's day. Hot outside. You pull air from the presumably cooler inside and vent it past the fridge to the outside. Eventually you have to draw in air from the outside to replenish that air. Is the air cooler than the air expelled from the fridge or hotter?

2. It's night. Now cool outside. Isn't the exterior air sucked into the RV through an open window certainly cooler than the air coming back through the fridge compartment?

Not arguing. I don't understand. Need some schooling!

1) Without those ugly penetrations, you van is stealthier and arguably nicer-looking. And, over time, all non-factory penetrations tend to encourage rust unless done very carefully, which is rare.

2) All that heat coming out of the back of the fridge represents "cold" that has leaked out of the cooled box of the unit into the interior of the van. The "heat" and the "cold" exactly balance each other, so they are not affecting the interior temperature of your van one way or the other. The only exception is the electricity used to run the compressor, almost all of which is converted to heat. But, these units are quite efficient, so this is not particularly significant.

[GallenH]

Sorry, Avanti. #2 went right past me. Are you saying that when you open the fridge door, some cool goes into the van and this compensates for the heat that you're putting into the van by venting the fridge into the interior?

I'm still at a loss as to why someone wouldn't vent the hotter air from the fridge out of the RV to the exterior. Outside of some potential for future rust, what would the downside be?

Again: I'm just trying to figure this out.

[booster]

Originally Posted by GallenH

Sorry, Avanti. #2 went right past me. Are you saying that when you open the fridge door, some cool goes into the van and this compensates for the heat that you're putting into the van by venting the fridge into the interior?

I'm still at a loss as to why someone wouldn't vent the hotter air from the fridge out of the RV to the exterior. Outside of some potential for future rust, what would the downside be?

Again: I'm just trying to figure this out.

We did our compressor frig install a long time ago compared to most at nearly 10 years ago. The venting issues were not really known about then, or at least not publicized and the sellers just said to drop them in place and it will be wonderful. We learned the hard way as we tried to get the frig to work properly until the venting issue got recognized and fixed. Ours is an Isotherm, so no big rear coils, just a box of coils around the the compressor with a s small computer fan blowing through it sitting in a notch in the upper rear corner of the cabinet. Our Roadtrek already had external vents so that is what we used. They have plenty of high and low area, but needed to be ducted properly for gravity to work right in the airflow to the unit as the fan blows toward the inside of the van not the outside as it is a marine frig they all vent to the inside of the boat. That gave us heat looping. As rowiebowie, Avanti and most others have found out, getting decent heat evacuation from the coils in back of the frig into the interior of the van often requires some kind of mechanical ventilation like fans because of all the cabinets involved in the airpath. Heating of the area behind the frig from sunload can also be an issue in the behind the frig area which contributes to the need for extra ventilation.

The heat coming off the compressor is not a huge amount, but does count up. If you use 48ah per day, which is probably typical for most of the larger 5cf size units (ours is a 3cf so uses less) that is 2ah per hour on average, or about 25wh per hour. That would be like running a 100 watt light bulb for 15 minutes per hour. As Avanti pointed out, the frig is in a "sealed" system if venting to the inside of the van, so the full amount of heat off the coils is balanced by the heat that leaves the interior of the van by going back into the frig interior. I don't recall exactly how much of the energy is directly offset, but Danfoss does list that spec in some of their technical papers and efficiency charts for various compressors, at various speeds. The spec is listed in watt hrs per btu of heat removed IIRC and in metric units also. I posted a couple of those charts in a thread about compressor frig efficiency and speed controls some time back and that information may be worth looking up. I think we all have walked into a small unoccupied kitchen and found it warmer than the rest of the rooms because of the running frig so the do heat areas some, but in a van it is probably small compared to other heat source from high outdoor temps and poor insulation and solar through the glass.

Of course, if you are outside vented most of the heat that the frig gains back to the interior is actually removed from the inside of the van and dissipated to the outside, so it works like a, very small, air conditioner to some extent. Outside venting will also be a bit quieter on the inside of the van.

If I were building a van from scratch I think I would not be cutting the outside vents in it for practical reasons, but if they were already there I would certainly use them instead of venting inside.

[booster]

I found the thread that listed the btu removal per watt hour of power used.


https://www.classbforum.com/forums/f...pecs-9459.html


At the speeds and amp draws we are talking about the Danfoss will remove about 4 btu per watt hour. Conversion chart shows 1 watt hour about equal to 3.4 btu.


The 3.4 btu per watt hour times the 25wh per hour the frig is using is 85 btu per hour. That all would go into heating the van.



The 4 btu per watt hour removed from the frig would be 4X25 or 100 btu per hour of heat coming off the back of the frig after being removed from the interior of the frig and that would be the recoverable portion as it was just moved around from in to out of the inside of the frig.


If you vent to the outside, you would not get the 85 btu/hr gain in inside heat, and you would also remove 100 btu/hr from the van or about 1% of the capacity of a 10K btu/hr AC unit .


If I did the numbers correctly, the bottom line would be that the frig vented internally is going to heat up the van similarly to running 2 of the 10 watt halogen lights (that we all love to hate and most don't have any more) continuously.

[eric1514]

Originally Posted by GallenH

rowie (and others who have dealt with this): So compressor fridges require cooling via air movement around the exterior workings of the unit. Makes sense. My question is why wouldn't you draw air past the exterior of the unit and vent it to the outside as opposed to returning it to the inside.

1. It's day. Hot outside. You pull air from the presumably cooler inside and vent it past the fridge to the outside. Eventually you have to draw in air from the outside to replenish that air. Is the air cooler than the air expelled from the fridge or hotter?

2. It's night. Now cool outside. Isn't the exterior air sucked into the RV through an open window certainly cooler than the air coming back through the fridge compartment?

Not arguing. I don't understand. Need some schooling!

When I replaced the absortion unit in my van the holes were already in the van side so I used them. Unless I'm running the A/C, it's always cooler outside than inside the van.

I first glued a piece of rigid insulation along along the inside of the van between the two vent cut outs to try and keep the heat off the fridge and to narrow the "chimney". Two fans come on when the compressor turns on, one from novakool blowing across the compressor and electronics and one piggybacked to the same terminals blowing upwards across the evaporator coils. I have a third fan blowing out of the upper vent that turns on if the chimney temp rises above 90 degrees F.

I would think it's also quieter than venting inside.

[booster]

As mentioned, we were flying blind in our install, as I couldn't find any that had been done mentioned anywhere. One thing I knew I didn't want to do unless absolutely necessary was adding fans and their power use.

The Isotherm and others like Vitrifrigo that use the compressor and condenser block in the corner actually turned out to be easier to address than many of the other designs.

I made a foil faced foamboard duct to the fan inlet on the assembly, sealed tight so all the fan air had to go through the duct.





The duct extends down far enough to get air only from the lower vent.








It all stays with the frig and is installed complete, along with the foamboard on the frig itself to fill gap to the cabinet and give a bit more insulation.

The air comes out of the compressor assembly from the top and side and has to go out the top vent almost horizontally. To get that to happen reliably and not loop down to the duct inlet, I put a couple of layers of reflectix on the van skin and stuffed the area between the frig assembly and the skin with fiberglass bats to completely seal the vents from each other.

The fan has to take air from the lower vent as that is the only air available, and the air coming out of the compressor assembly has to go out the top vent as there is no place else it can go. This type of setup could also be used for an inside vent setup pretty easily as the fan could be fed with a duct from an area of cool air and then just have openings to let the air back out into the van.

[GeorgeRa]

Are you using a factory capillary thermostat or an electronic one like for example the Isotherm Smart controller?

[booster]

Originally Posted by GeorgeRa

Are you using a factory capillary thermostat or an electronic one like for example the Isotherm Smart controller?

I am still using the capillary one because they never had a smart controller I like until recently, and it won't work with the board in our frig as it is so old. I do have the sensing bulb repositioned, though We do have a homemade adjustable speed control, though, that I built when I put in the frig all those years ago. We never have to take it off the lowest speed unless we want to, even when we were in 100* heat with the sun on the frig side. If we ever have to replace the board or the frig, we will upgrade the thermostat.

[GeorgeRa]

Originally Posted by booster

I am still using the capillary one because they never had a smart controller I like until recently, and it won't work with the board in our frig as it is so old. I do have the sensing bulb repositioned, though We do have a homemade adjustable speed control, though, that I built when I put in the frig all those years ago. We never have to take it off the lowest speed unless we want to, even when we were in 100* heat with the sun on the frig side. If we ever have to replace the board or the frig, we will upgrade the thermostat.

I replaced the capillary thermostat in 2016, one of key benefits were reduced noise and lower energy consumption. Compressor runs on lower speed, no more capillary electromechanical clicking. I also replaced the original fan with a very quiet one. It was a good modification.

I am surprised that manufacturers still use capillary controllers. Perhaps I shouldn’t be, pneumatic temperature controllers are still used in office buildings and only recently an electronic interface was added. I have a very old mercury thermometer/thermostat with precisely controlled electrode via magnetic clutch touching the surface of the mercury in the capillary to close the circuit, ancient. Wow, found its picture in the museum of University of Queensland – another antique in our house not counting myself. https://physicsmuseum.uq.edu.au/merc...ermostat-0-50c

[GallenH]

Thanks to everyone here for providing some excellent information. You answered all of my questions and clarified what Avanti wrote. Homeschooling
during the pandemic does work!

[OK: I still wish I was on the road]

[booster]

Originally Posted by GeorgeRa

I replaced the capillary thermostat in 2016, one of key benefits were reduced noise and lower energy consumption. Compressor runs on lower speed, no more capillary electromechanical clicking. I also replaced the original fan with a very quiet one. It was a good modification.

I am surprised that manufacturers still use capillary controllers. Perhaps I shouldn’t be, pneumatic temperature controllers are still used in office buildings and only recently an electronic interface was added. I have a very old mercury thermometer/thermostat with precisely controlled electrode via magnetic clutch touching the surface of the mercury in the capillary to close the circuit, ancient. Wow, found its picture in the museum of University of Queensland – another antique in our house not counting myself. https://physicsmuseum.uq.edu.au/merc...ermostat-0-50c

In some ways, I am a fan of some the older controls for heating and cooling type stuff. The newer style thermostats often have +/-.5* control so are short cycling all the time if you don't have a multistage furnace or AC to slow them down. Very few let you set that in many industries. Startup and shut off inefficiencies can really add up from short cycling.


I actually just finished designing and building a relay based control for out new HRV ventilator that had unreliable, inflexible, electronic controls on it. Programmable chip on it, but the don't make any access to changing things at all, so we couldn't run it like we wanted. Used 5 SPST and 2 DPDT Omron DIN track relays, a 12v DC power supply for the controls, 75/90/115 volt AC transformer for motor speeds, and relay output electronic sensor thermostat to handle the defrost feature. Works great, simple two switches and 4 LEDs on the remote control panel, does exactly what we want and easy to make do other things.

[rowiebowie]

Originally Posted by GallenH

Thanks to everyone here for providing some excellent information. You answered all of my questions and clarified what Avanti wrote. Homeschooling
during the pandemic does work!

[OK: I still wish I was on the road]

In my case, the fan does remove heat generated by the fridge, but that is minor. It mainly helps to exhaust the huge amount of heat generated by sun on the outer skin of my van which is painted black there to simulate wrap-around windows. So yes, I'm pulling some heat from the outside into my living space. I am essentially cooling the small area behind my fridge with the air in my living space. But . . .

I did not want to drill any holes to the exterior as that side has none (see pic). And yes, my driveway's pretty steep.

Plus I have mitigated outside heat by adding triple insulation to the wall behind the fridge/microwave cavity. But mainly, when it's hot we just stay at home inside with our a/c. And in the event we camped in he heat, the roof a/c more than cools the interior (15K btu's - long story). So the extra heat brought into the coach is not noticed.

Yeah, it doesn't totally seem logical even to me. But that's my story and I'm sticking to it.

[engnrsrule]

Just took first post COVID trip and wanted to report out on the fridge performance as it is also first really hot (into the 90's) weather since adding the auxiliary cooling fans.

Getting the fridge cool seemed ok, got right down to 34 and freezer to -2 within a day of starting. However in use it krept up and hovered around 40-46, and while no food actually spoiled, that is warmer than I would prefer.

I am considering three more modifications to see if I can do better. (1) install a fan in the refrigerator; (2) Increase the size of the auxiliary cooling fans; and (3) explore the notion of opening between the freezer and fridge space.

The dometic is a single temp control box. so the same cooling covers both the freezer and fridge spaces. The temp difference is achieved by segregation of the space and concentration of the cooling coils in the freezer space. It would seem that allowing some communication by propping the freezer door open, or drilling some holes in it would result in a warmer freezer space and cooler refrigerator space. After all, I note that some RV fridges offer a total fridge or fridge/freezer space, and some even allow alternating between the two. Does anyone have any experience with this kind of modification?

[ChicagoTom]

I've been preaching forever, the outside fans help, but what really works is a rechargeable small 3 speed fan on low, top shelf & pointed at the fins. I've been in 100 degree weather and it still stays cold. Such a simple solution. No need to do #3. Please report back. Ex-HVAC tech, long time ago, but still in my head. Link below or something similar
https://www.amazon.com/JoySusie-vent...-garden&sr=1-2

[Doneworking]

I have used this fan for years. You can find it at Walmart, on Amazon, etc. It takes up little room and right underneath the fins works and will run for over a month continuously on two A batteries. I bought a different model of a rv refrig fan and it didn't work as well as this old reliable. The fact that it only takes a few cubic inches of space makes it an attractive possibility. It will not, of course, stir up air as well as what ChicagoTom posted above, but space is a real priority for us in our Roadtrek refrig.

I live in hot weather country and this little fan and a small fan I installed on the outside for the coils have never given us any problems and we keep the refrig at the proper temps.

https://www.walmart.com/ip/Valterra-...l-Fan/55355999