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InterBlog · 07-28-2016, 11:19 AM

Quote:

Originally Posted by galvestongrl

... I wonder if a 100 lb 62 year old woman (me) could do this repair?

I reckon you could do whatever you put your mind to. I'm not quite as old as you and not quite as small, but the Interstate has taught me to do things that I never would have attempted previously in my life. It's been a royal pain in the a$$ at times but there's also an immense feeling of satisfaction that comes from realizing my own expanded competencies. I learned major new stuff as a gray-hair. It's never too late to take on new challenges.

southjk · 07-28-2016, 11:58 AM

They did the mod in ft worth which is a 5 hr drive from where I live. Since I needed to do a lot of testing and it takes a while to know whether or not the refrigerator is working I did several tweaks myself with the guidance from the nova kool guys.
They sent me the 1500 ohm resister to replace the 500 ohm resister which was in the original mod.
It is a bit awkward removing the refrigerator , if you can, by all means let the dealership handle this. I've got a background in electronics so I didn't mind doing it.

amirm · 07-29-2016, 11:51 AM

Coming late into this discussion

. What is the resistor mod for? I know about the problem of the two units cooking the air behind them but what is the resistor supposed to do?

73shark · 07-29-2016, 03:55 PM

Quote:

Originally Posted by amirm

Coming late into this discussion

. What is the resistor mod for? I know about the problem of the two units cooking the air behind them but what is the resistor supposed to do?

I suspect it biases the compressor to run longer but that's just a guess.

Sounds like a real fix is exhausting the air from the condenser, preferably outside the RV.

amirm · 07-29-2016, 04:19 PM

Quote:

Originally Posted by 73shark

I suspect it biases the compressor to run longer but that's just a guess.

Thanks. You reminded me that the schematic is in the manual and it shows this:

Seems to me the resistor controls the speed of the optional fan.

73shark · 07-29-2016, 04:24 PM

Doubt that'll help much based on someone's measurement of the exhaust temp of 102*.

Protagonist · 07-29-2016, 04:50 PM

Quote:

Originally Posted by 73shark

Doubt that'll help much based on someone's measurement of the exhaust temp of 102*.

The important thing is not the 102°F temperature. It's the 13°F temperature difference between inlet and outlet air. The same post that cited 102°F at the outlet also cited 89°F at the inlet, meaning they weren't running the air conditioner and it was already uncomfortably warm inside the van anyway.

If the A/C was on and the interior temperature was only about 75°F, then the exhaust from the fridge/freezer coils would probably be closer to 88°F, still more heat than one would like to have pumped back into the van, but not as extreme as the southjk experienced during his test.

73shark · 07-29-2016, 05:02 PM

If what you say is accurate, then does that mean that all of those reporting problems maintaining acceptable temps in the frig and freezer are not keeping the interior cool enuf?

Protagonist · 07-29-2016, 06:15 PM

Quote:

Originally Posted by 73shark

If what you say is accurate, then does that mean that all of those reporting problems maintaining acceptable temps in the frig and freezer are not keeping the interior cool enuf?

Not necessarily. I'm just saying that while the 102°F exhaust temperature was a real attention-getter, it has to be taken in context. So maybe in this case, but not in all cases.

Cooler air at the inlet would help in this one case. The coils are a heat exchanger. Using an extreme example— If the air is already just as hot as the coils, no heat will be exchanged and then no cooling happens inside the box. The greater the difference in temperature between coils and the surrounding air, the more efficient the heat exchange will be and the better the fridge will cool.

All other things being equal, the cooler the air at the inlet, the more efficient the coils will be at taking heat from the box and transferring it to the air outside the box.

So what is the optimum outside air temperature for the inside of the fridge to get as cold as it needs to be for safe preservation of food? I don't really know. Lewster might know, since he's the resident Danfoss compressor guru. But generally speaking, if the air temperature outside the fridge is already pushing 90°F, it will be hard to get the temperature inside the box down below 40°F, no matter what you do to increase air flow. Fans circulating the air do keep the air at the coils from getting even hotter, but if the air starts off hot before it even reaches the coils, fridge efficiency will still be reduced.

That does not mean running the rooftop A/C is the solution to every Grand Tour fridge problem. Just that it should help in southjk's case. He might want to run another test, when the air inside the van is cooler, just to see if it makes a difference.

73shark · 07-29-2016, 07:09 PM

Couldn't find any specs re: operating space temp. vs interior temp.

From the Nova Kool manual:

The cavity where the condenser is located
MUST be adequately ventilated.

Double door units
Require 60 sq.in. INLET and 60 sq.in. OUTLET

and

4. Compressor Speed:
All Nova Kool units are supplied with the silent and efficient BD series variable speed compressors from Danfoss. Nova Kool has supplied your unit preset at the optimum speed for most “normal” applications. The speed is controlled by a resistor in the thermostat circuit. 0 ohms(no resistor) runs the compressor on its slowest speed, a 1523 ohm resistor will provide you with the fastest speed. The slower the speed, the lower the amp draw, and the lower the capacity. The chart provides four examples; the resistance must be between 0 ohms and 1523ohms.

Protagonist · 07-30-2016, 05:30 AM

The principle of heat exchange is basic thermodynamics. But even though I have a basic understanding of thermodynamics from my college engineering courses, I never learned how to teach thermodynamics to laymen. So I don't know how to explain to you how temperature gradients and the rate of heat transfer are interconnected, without breaking out the equations and graphs. And for that I apologize.

What I can tell you is that the temperature of the air around the coils on the back of the fridge has an effect on the efficiency of the fridge, just as does the volume of air passed around those coils, or as does the speed of the compressor motor. So if you start with cooler air around the coils, the fridge will get cooler inside the box, too— all other things being equal. But when you start changing the compressor speed and the volume of air passing over the coils as well, then all other things are not equal, and it becomes more difficult to predict (or explain) exactly how the temperature inside the fridge will be affected.

So I'm saying that southjk needs to experiment, and see if the fridge gets colder if the air in the van isn't nearly 90°F. It's a simple experiment to do, just repeat what he did before, but with the air conditioner running as well as the fridge and freezer (or even just at a different time of day), and measure the temperature inside the fridge, at the vent inlet, and at the vent outlet.

With experimental data in hand, he'll be able to either prove me right or prove me wrong. And he'll have a better handle on what to expect in his travels, because I very much doubt he plans to live in a van that's a constant 89°F inside as it was when he performed his previous test.

If he proves me wrong about the effect of air temperature on fridge efficiency, then I'll want to know, so that I'll get a better understanding for next time.

GETRIDAONE · 07-30-2016, 08:26 AM

The bottom intake 12 x 6 opening is only 47.5 square inches when the effective free area of the grille is taken into account. The top two 7 x 3 openings are only 31.6 square inches free area with the grilles in place. The installation instructions point out the fact that the grille will reduce the actual free area of the opening. The 60 square inch minimum is reduced by 21% on the bottom and 47% on the top !! As said in most of the posts the heat has no place to escape.

73shark · 07-30-2016, 08:27 AM

I understand the basic principles of freon-based refrigeration and realize that the higher the ambient temperature, the harder it is for the system to maintain the interior temp. A good example is to watch the current draw on the rear A/C as the ambient temp. rises. On a 70* day mine drew about 16 amps w/ fan on Hi and at 95*, it approached 20 amps. My point was that if you put the frig inside a box w/ little ventilation, it's not going to cool very well because it can't get rid of the heat. Sure, if the van is cooler inside, the frig should be able to get the interior temp. down to 40* but not if there's not adequate ventilation in the box the frig is in.

Mine exhausted thru the grill over the microwave and even w/ the interior of the van at 75*, that exhaust air was very hot.

southjk · 07-30-2016, 06:52 PM

When the coach/inlet air is less than 85 f I have no problems getting the fridge to hold 38f.

southjk · 07-30-2016, 06:55 PM

Quote:

Originally Posted by 73shark

I suspect it biases the compressor to run longer but that's just a guess.

Sounds like a real fix is exhausting the air from the condenser, preferably outside the RV.

According to nova Kool the resistor speeds up the compressor, the fan seems to run at a constant speed. I did notice it sounded faster when I started it up.

73shark · 07-31-2016, 03:29 PM

Quote:

Originally Posted by southjk

According to nova Kool the resistor speeds up the compressor, the fan seems to run at a constant speed. I did notice it sounded faster when I started it up.

Yep . . . >>>Post #30

While this will help, restricted air flow will cause the compressor to work harder, draw more current, and probably wear out sooner.

Dalwyn · 08-02-2016, 07:27 AM

I have been following the discussions and want to try testing with my fridge before I take it in to service tomorrow. Since it is hot and humid here today, it will be ideal to test by cooling the coach to see if it makes a difference. My concern is running the A/C with my unit in the garage. Does anyone know if I can do damage to the A/C by running in the garage?

73shark · 08-02-2016, 09:23 AM

Won't hurt the A/C but will put a lot of CO in the garage if it's closed up. I'd leave the garage door open.

Dalwyn · 08-02-2016, 10:14 AM

Thanks 73shark. I have the side door open. Garage door faces east and the sun would just make the coach hotter and work the A/C more.

Boxster1971 · 08-02-2016, 11:21 AM

Quote:

Originally Posted by Dalwyn

Thanks 73shark. I have the side door open. Garage door faces east and the sun would just make the coach hotter and work the A/C more.

I recommend you open the garage door a foot or so to get some airflow in the garage. With just the side door open and running the generator it could cause a CO buildup in some spots. That stuff can be deadly.

- - Mike
2013 Interstate Lounge EXT on 2012 Sprinter