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Old 22-07-2014, 12:22   #1
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Electronic Frig Thermostat

I have a AB spillover system in my boat. The AB cools the freezer and there are 2 openings between the freezer and frig and a spillover fan in one of the holes. Both the AB compressor and the spillover fan are controlled by mechanical thermostats.

The freezer works fine and the system maintains temperatures just fine. But the frig side sees huge temperature swings from being above 40 degrees to freezing stuff and I just can not seem to get it set. It seems the temperature differential of the switch just isn't small enough to run the fan in such a way to maintain the frig temperature where I want it (40-35 or so).

Has anyone replaced/installed one of the small electronic thermostats in a spillover system to run the spillover fan? Did it work to keep the temperature more even? Did you also have to install a fan in the frig box to circulate the air more evenly in it at the same time?

This would be the typical low cost PITA projects but if it works I'm up to doing it.
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Old 22-07-2014, 12:48   #2
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Re: electronic frig thermostat

I have found that a constant running fan, a really small one, like a laptop fan, is a great help.
I also have installed a digital programmable thermostat AKOSYS - Industrial refrigeration components
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Old 22-07-2014, 12:51   #3
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Re: electronic frig thermostat

You sure it's the thermostat? I have the same system with a mechanical thermostat and it works fine but I have to check the two holes from the freezer frequently. They sometimes plug up with frost from the freezer. The hole with the fan is the main culprit. If the fridge starts warming up I have to clear the fan hole.
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Old 22-07-2014, 13:07   #4
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Re: electronic frig thermostat

Yes sometimes the holes frost up, but I've long caught on to this problem, which is normally due to a seal or latch problem with my system.

But I know it is the thermostat. When the temperature is either too warm and the fan is off, or too cold and the fan is running, I can turn the thermostat till it "clicks" and the fan turns on/off. But the amount I turn the thermostat is way more that what it seems should be needed. And if the temperature is just a little too warm and I turn the thermostat till it just comes on, it will later be still running when it is too cold in the box.

And I'm manually measuring the temperature with a thermometer to verify the temperature in the box.
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Old 22-07-2014, 13:38   #5
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Re: electronic frig thermostat

Quote:
Originally Posted by sailorboy1 View Post
But I know it is the thermostat. When the temperature is either too warm and the fan is off, or too cold and the fan is running, I can turn the thermostat till it "clicks" and the fan turns on/off. But the amount I turn the thermostat is way more that what it seems should be needed. And if the temperature is just a little too warm and I turn the thermostat till it just comes on, it will later be still running when it is too cold in the box.
You have a hysteresis problem - that is the difference between the on and off set-points. You need a thermostat with tighter tolerances.
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Old 22-07-2014, 13:45   #6
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Re: electronic frig thermostat

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You have a hysteresis problem - that is the difference between the on and off set-points. You need a thermostat with tighter tolerances.
yes I know this, which is why I asked if asked if people had any experience with the electronic ones as they are programmable
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Old 22-07-2014, 13:58   #7
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Re: electronic frig thermostat

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Originally Posted by sailorboy1 View Post
yes I know this, which is why I asked if asked if people had any experience with the electronic ones as they are programmable
I read your original post and didn't see that you knew. You can get mechanical thermostats with tighter tolerances. Any decent refrigeration place should be able to sell you a part. I think you want a 'stat with 2 or 3 degree F offset.
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Old 22-07-2014, 14:22   #8
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Re: electronic frig thermostat

Your system sounds exactly like ours, except we have electronic thermostat controls. They work very well, are adjustable both for temperature and hysteresis and show you the temperature and compressor status at all times.

Ours are set at 5 degree F offset and work well with that.

You can keep the fan from freezing by making sure it is orientated so that it blows from fridge to freezer. That way, below freezing air is not drawn into it.

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Old 23-07-2014, 07:51   #9
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Re: electronic frig thermostat

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Originally Posted by colemj View Post
Ours are set at 5 degree F offset and work well with that.
5 deg F seems like a lot.

The US FDA recommends refrigeration at 40F or below. I tend to be suspicious of US regulation but 40F seems a reasonable upper limit. That leaves only an 8F range before things start to freeze (frozen lettuce - yuck). In most boxes without internal circulation there is a temperature differential between the top and bottom and between the spillover and the far side that can easily be 3 or 4 degrees. Add a couple of degrees on each side for margin and appropriate thermostat hysteresis gets pretty small.

Since you are happy with your current 'stat it would be interesting to get temperature readings in various places in your box over time to see what temperature excursions you are actually getting.

Quote:
Originally Posted by colemj View Post
You can keep the fan from freezing by making sure it is orientated so that it blows from fridge to freezer. That way, below freezing air is not drawn into it.
Good idea.
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Old 23-07-2014, 08:12   #10
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Re: electronic frig thermostat

Use one of these to control your fan or anything else, theres lots of you-tube videos showing how to set it up for various applications. They are so cheap you can buy lots of backups if the crap out . I have two of them and they work just fine.

12V Digital Temperature Controller STC 1000 with Sensor Thermostats | eBay

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Old 23-07-2014, 08:21   #11
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Re: electronic frig thermostat

Quote:
Originally Posted by Auspicious View Post
5 deg F seems like a lot.

The US FDA recommends refrigeration at 40F or below. I tend to be suspicious of US regulation but 40F seems a reasonable upper limit. That leaves only an 8F range before things start to freeze (frozen lettuce - yuck). In most boxes without internal circulation there is a temperature differential between the top and bottom and between the spillover and the far side that can easily be 3 or 4 degrees. Add a couple of degrees on each side for margin and appropriate thermostat hysteresis gets pretty small.

Since you are happy with your current 'stat it would be interesting to get temperature readings in various places in your box over time to see what temperature excursions you are actually getting.
It came from the factory set with a differential of -05 with lots of warnings about changing it only if necessary. It has always seemed to work well for us.

The only location in our box that may freeze something like lettuce is directly in front of the spillover hole blowing in from the freezer. Lettuce seems to be the only thing so far that has a problem in that spot.

While we don't have temp sensors all over, I don't have any reason to suspect that the box is not a fairly constant temperature all around (with the exception of immediately in front of the freezer air exchange). The cold air from the freezer enters the box at the top front and exits to the freezer at the top back. There is a grate on the bottom so that no food can sit directly on the bottom, and there are wire baskets holding foodstuff throughout.

I suspect the cold air entering sinks to the bottom and tumbles around the food quite a bit. The fan really forces a good airflow into and out of the closed box, so I imagine this is as good as any circulation fan added to the box.

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Old 24-07-2014, 10:19   #12
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Re: electronic frig thermostat

Differential is the difference in temperature between when the compressor stopping and restarting times. Zone area temperatures thermostats will have a narrow differential say 2 to 7 degrees while other thermostat differentials vary from 8 to 22 degrees. Originally Alternating Current (AC) electrical compressors would overload and heat up if there was not enough time between stopping and restarting the compressor so this, along with energy considerations, determined the compressorís off cycle times. Modern air conditioning and large refrigeration units are equipped with timers to prevent short off cycling times. Direct Current (DC) compressors also require an off cycle of 3 to 5 minutes to allow refrigerant pressures to equalize before attempting to restart.


Small home refrigerators and all well designed 12/24 volt boat ice box conversion units with Danfoss BD compressors use a thermostat that controls evaporator temperatures. There are three good reasons for controlling evaporator temperature instead of box temperature:
  • Box temperature is more stable from 1 to 3 degrees as compressor cycles based on evaporator temperatures.
  • Evaporator temperatures are always15 to 20 degrees colder than box temperature and provide an area in the box to store or freeze a few items like ice and food.
  • The Most important reason to stay with evaporator controlled thermostats is that they are more energy efficient than area thermostats. Snap action or electronic thermostats controlling a zone or area will tend to overpower the evaporatorís ability to absorb heat efficiently and will increase daily amp-hrs consumed.
For a more complete story on thermostats for boat ice box conversion units see Tech Tip #6 on my web site.
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Old 24-07-2014, 10:30   #13
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Re: electronic frig thermostat

Richard, the OP has a two-box spillover system. There is only the evaporator in the freezer box and the thermostat in the reefer box controls the spillover fan.

His problem is with reefer temps, so an evaporator controlled thermostat isn't applicable here.

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Old 24-07-2014, 10:55   #14
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Re: electronic frig thermostat

Besides the spill fan thermostat change I think I definitely need to install a circulation fan in the reefer box. Even though the temperature in the top of the box (where I tend to put things like meat) will be in the upper 30s, I find stuff in the lower section of the box next to the wall between the reefer and the freezer sometimes freezes.

I thought I had found the problem earlier this year when I found that the spillover fan was wired to the power duct fan terminal of the compressor, so it only ran when the compressor ran. So I moved the power for the spillover fan to the incoming power so it could run whenever the breaker was shut. But I was wrong and this only fixed part of the problem
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Old 24-07-2014, 11:35   #15
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Re: electronic frig thermostat

sailorboy-
If you can find a non-electronic thermostat with a tight range, as Auspicious suggests, I'd second that thought. Electronics are the part most prone to breakdowns in modern appliances and anytime you can find a plain "mechanical" part that will do as good a job--it will likely be way more reliable.
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