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Old 03-07-2023, 12:32   #1
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Fridge Compressor Control

Our boat has a very large fridge/freezer space access from the top. I replaced the freezer thermostat with the fridge version so the whole space is just a fridge. Both compressors easily keep things at 36 deg F, but one gets stuck with most of the work. Is there any reason I can’t make a “y” from one thermostat and control both compressors with it? Thanks!
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Old 03-07-2023, 13:53   #2
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Re: Fridge Compressor Controll

It would depend on the design of the space (I assume divided into 2 spaces) and the insulation between them.
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Old 03-07-2023, 14:05   #3
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Re: Fridge Compressor Controll

It’s common space. I removed the ineffective wall between what used to be the freezer and the fridge. Thanks!
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Old 03-07-2023, 15:15   #4
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Re: Fridge Compressor Controll

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It’s common space. I removed the ineffective wall between what used to be the freezer and the fridge. Thanks!
Just curious, was the "inefective wall" a spill over from freezer to fridge?
Also, depending on the type and size of your compressor, you may not need both for just a fridge.
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Old 03-07-2023, 15:21   #5
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Re: Fridge Compressor Controll

It was not a spillover wall. It was meant to provide separation between sides. It didn’t work well enough as a discreet freezer space. Needed to be smaller. And the whole thing needs more insulation.

I was really just hoping for a response regarding the feasibility of controlling two compressors from one thermostat
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Old 03-07-2023, 17:30   #6
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Re: Fridge Compressor Controll

It is feasible, but without some testing of the unit or more information on how the control box works, I can't tell you how to do it. (I am assuming you have a BD35/50 compressor)

There is a good chance that simply paralleling the to control boxes to one thermostat will work. However, there are two resistors in the circuit that set the RPM the compressor works at and the low voltage cutout. I would need to test the box to know what to do with those. If the 2 control boxes are not the same, you could damage something.

I would try it with just one compressor running. If that works, you would probably use less power to keep your box cold.
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Old 03-07-2023, 18:59   #7
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Re: Fridge Compressor Controll

I’ve tried with one compressor running full time at 2,000 rpm (most efficient). It achieves about 40 degrees at the far side of the box.

I need both compressors running, or a smaller, better insulated space for just one.

Thanks for your input….
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Old 03-07-2023, 20:46   #8
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Re: Fridge Compressor Controll

Quote:
Originally Posted by SVArcticTern View Post
It was not a spillover wall. It was meant to provide separation between sides. It didn’t work well enough as a discreet freezer space. Needed to be smaller. And the whole thing needs more insulation.

I was really just hoping for a response regarding the feasibility of controlling two compressors from one thermostat
I wish I could help, but most likely you will need to provide the size of both boxes and compressor specs to get the information you need.
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Old 03-07-2023, 20:54   #9
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Re: Fridge Compressor Controll

To be safe, you should probably drive a small double pole relay with the thermostat, one relay pole for each compressor - this allows you to put the series resistor in each one to set the compressor RPM and "keep them seperaate" but with one control. Also, good practice to put a flyback catch diode across the relay coil (cathode to positive, anode to negative) to prevent arcing of the thermostat contacts from the inductive kick when de-energizing the coil.
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Old 03-07-2023, 21:02   #10
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Re: Fridge Compressor Controll

Hmm. That sounds way more complex than our situation deserves, even though I would likely learn a lot in the process of doing it Thank you for the thoughtful reply.

Our compressors are both Frigoboat BD 50s with speed boards. I can’t tell you what the total volume is; I’ve looked through Island Packet 40 info online and can’t find anything. I probably won’t try to fill the space with water to measure it, either. Ha, ha.
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Old 03-07-2023, 21:07   #11
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Re: Fridge Compressor Controll

The answer is yes. Assuming Secop (Danfoss) compressors, The current controlled by the thermostat is small such that one thermostat would not be overloaded or damaged by controlling two compressors.
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Old 03-07-2023, 21:10   #12
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Re: Fridge Compressor Controll

Thank you. I actually tried it a few months ago and nothing bad happened. Then I thought about it, wondered if it could wreck something, and removed my little jumper setup.
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Old 03-07-2023, 22:16   #13
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Re: Fridge Compressor Controll

You can buy 2 electronic temperature controller type W1209 (google it) on-line for about $10 each. The controller can be set for temp cut in, cut out, time to restart etc very handy device, put one in for each compressor control unit (Assuming Secop or Danfoss BD35/50 units. Then you can fiddle around until you get the desired results. We installed one on a fridge / freezer and it keeps the freezer at 6F and fridge at 37F
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Old 04-07-2023, 01:24   #14
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Re: Fridge Compressor Controll

Thank you. I’ll take a look.
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