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Old 13-05-2013, 13:16   #1
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Fridge Thermostat Replacement

Appears the thermostat died on my fridge recently. I can bypass and compressor chills fine. Picked up a physically similar thermostat but cant get the bugger to work.

Compressor is a Danfoss BD35 w Danfoss t'stat. Potential replacement I found is Ranco K59.

Anyone aboard w a strong knowpedge of t'stats?
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Old 13-05-2013, 13:56   #2
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Re: Fridge Thermostat Replacement

Be patient and I'm sure an expert will respond.

Until then, which version of the thermostat do you have? There are three versions (A,B,C). It matters.

I assume you are connecting between terminals 3 and 4; is that correct?

Greg
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Old 13-05-2013, 14:36   #3
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Researching versions now...will post when I have that sorted out.

Yes, using pins 3-4 for compressor. On the K59 appears that 6 is an accessory pin for fridge light.
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Old 13-05-2013, 14:51   #4
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Re: Fridge Thermostat Replacement

While you have the thermostat in hand, it is a simple matter to test it. Since the operating temperature range is -32 to 6 degrees C, at room temp the resistance between terminals 3 and 4 should be zero (i.e. it will be shorted in order to run the compressor). You should test this with a digital multimeter on the resistance (ohms) setting. The Danfoss controller uses the thermostat circuit to control compressor speed as well as on/off; when the thermostat is closed (conducting with no resistance) and a specified resistance is in the circuit the motor will operate at a different speed. Values other than the defined ones might cause problems.

As for pin 6, it is used for an auxiliary switch, not the lamp. If I understand correctly, versions A and B have this switch built into the rotary switch; turning the knob all the way in one direction will open that switch and prevent the compressor from running if connected between terminals 3 and 6.

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Old 13-05-2013, 15:12   #5
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Re: Fridge Thermostat Replacement

Looking more closely at the specs, it probably doesn't matter which version you have. Version B was the one I was concerned about because it has an internal heater (resistance). However, this resistance is so high (82kOhm) that it would probably be seen as an open by the Danfoss controller. In any event this resistance would only be visible to the controller when the thermostat is off (open) so the symptom would be that the compressor would not turn off - not that it wouldn't turn on. So if the compressor is not running at all I would be looking for a resistance as I just described between 3 & 4.

Greg
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Old 13-05-2013, 15:26   #6
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Re: Fridge Thermostat Replacement

As should be clear, I am not anything like knowledgeable re: thermostats. I can find and view a data sheet and read a schematic. And since I wired a custom control panel including speed control for my Danfoss compressor I understand that part. What I don't know, and what is not in the data sheet, is whether this device (thermostat) requires line power to operate. The fact that it is spec'd at 250V 50Hz and not 120V 60Hz or 12V DC would imply that there is some circuit besides just a bimetallic contact; the question that needs answering is whether this requires 220-250V AC to operate. For that we may need an expert. Until then, it may suffice to know the contact resistance.

Greg
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Old 13-05-2013, 16:42   #7
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Yes, Ive tested the original thermostat and it never goes open circuit regardless of temp...thats how I noticed the problem...compressor was running continuosly. Even disassembled thinking it might be something simple like dirty contacts, but they were like new. Tested the thermal bellows...size never changes with temp as measured w micrometer..so that appears to be point of failure. Very simple mechanism...bellows expands/contracts and trips a mechanical switch..no electical power required to operate and it will function for any voltage up to its max rating.

Found two tstats locally. A Ranco K59 and a Bev Air 502-139A. Not having sucess w either. I think the base temp ranges are not quite what I need.

Simple fix w the right part, but of course exact match not available here in paradise.
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Old 13-05-2013, 17:26   #8
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Re: Fridge Thermostat Replacement

But have you tested the Ranco for resistance? The temperature range is correct for a fridge, although I don't like the operating mode for this application. I am guessing that the original thermostat had a hysteresis adjustment, which set the degrees between cut-off and cut-in (e.g. if it were 3 deg F then you might operate between 35 and 38 F). The Ranco has a fixed cut-in, apparently at 6 C ( about 43 F), so that the box always defrosts every cycle. This is normal for a home fridge, but is not so good for keeping food for a long time.

I keep coming back to the voltage spec, and the units of operating range of "K", as indicative that this thermostat requires 240V AC to operate. I am guessing that the "K" refers to ohms (why can't they provide the units?). Further, I am guessing that the adjustment knob is a potentiometer (variable resistor) and that this device works on relative resistance, and needs that line voltage.

As I wrote earlier, a measure of the resistance across the 3-4 terminals will tell you if this will start the compressor. A resistance of 0 Ohms will run the compressor at 2000 rpm, 277 Ohms at 2500 rpm, 692 Ohms at 3000 rpm, and 1523 Ohms at 3500 rpm. If the resistance goes substantially higher than that I don't know what happens, but my guess is that it will never turn the compressor on.

Until someone more familiar with refrigeration joins in, my only advice is a) test the resistance, and give up on this thermostat if non-zero, and b) look for one rated for 12V DC operation. In the US or Europe they are available, particularly from eBay and rparts.com. In Belize I think you are going to have to find a way of shipping one in if there is no marine fridge service.

Greg
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Old 15-05-2013, 06:39   #9
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Found a workable solution in a Subco GC506 thermostat. It is a "constant differential" type meaning that the temp diff between cut-in and cut-out is fixed but you can vary the temp range w the knob. Fixed diff is 13F which is wider than I would like, but its working.

The way my system was originally set up was w a tstat that was simply open or closed at cut-in/cut-out w no in-line resistor (most danfoss systems I have seen use an in-line resistor to set motor speed) so the motor would run at its default speed.

Never got the Ranco to function "properly". When switch closed it showed about 65 ohms resistance w no variation as I rotated the knob and/or temp changed. I think these tstats Ive tried are mostly knock offs so who knows whether it functioned properly out of the box.

Ive learned way more about tstats than I ever wanted to know...never knew there were so many variations and variables. Will order a more suitable one down the road (need to redo the fridge box anyway).
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Old 15-05-2013, 14:34   #10
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Re: Fridge Thermostat Replacement

I recommend replacing the thermostat as soon as is convenient; the 13 deg F hysteresis will not safely preserve food. The desirable range is 2-3 degrees F.

Looking around on the internet, simple mechanical thermostats are not all that common, and cost $40-50. There may be cheaper ones out there, but quality matters. (I have had to throw out several electric heaters because they don't last more than one season - the cheap thermostats are the most common problem.) For $15-20 it is possible to buy 12v electronic controllers with digital temperature display and electronic sensor. The only downside is a small current drain.

My system has a small freezer and a fridge area. The thermostat can control one or the other area, but not both; I have to adjust the insulation between the two with the seasons. A better choice is to have a thermostat to control the compressor for the freezer, and a second for controlling a fan or shutter to cool the fridge. A proper digital controller with two temperature probes and at least two relay outputs is the ideal solution. Carel makes such a controller; the downside is the small current draw and the $200 price.

Good luck,

Greg
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Old 16-05-2013, 06:50   #11
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Yes 13F diff is more than I would like but is working for our purposes.

Next season will probably rework the whole fridge/freezer set up. Probably go w a digital controller so I can program its behavior and have an external display.
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Old 17-05-2013, 06:14   #12
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Quote:
Originally Posted by CarinaPDX View Post
As should be clear, I am not anything like knowledgeable re: thermostats. I can find and view a data sheet and read a schematic. And since I wired a custom control panel including speed control for my Danfoss compressor I understand that part. What I don't know, and what is not in the data sheet, is whether this device (thermostat) requires line power to operate. The fact that it is spec'd at 250V 50Hz and not 120V 60Hz or 12V DC would imply that there is some circuit besides just a bimetallic contact; the question that needs answering is whether this requires 220-250V AC to operate. For that we may need an expert. Until then, it may suffice to know the contact resistance.

Greg
Disassembled the K59 and there is no circuitry...it is a purely mechanical switch. So it should work for any voltage up to its max rating. There is nothing powered in the device. The only energy it uses is the force of the thermal belows expanding and contracting.

Internally it has 3 adjustment screws. These appear to me to effect cut-in, cut-out, and cut-in range. So I think it could be adjusted to almost any operating characteristics needed. I suspect that each model is simply calibrated differently at the factory. The diff w doing this in the field is not having a precise and easily variable temp source...would require lots of trail and error using the fridge.
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Old 17-05-2013, 11:20   #13
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Re: Fridge Thermostat Replacement

In most cases it is best to replace refrigerator thermostats with the same thermostat type and differential (Hysteresis) setting. There are four basic types of thermostats used in pleasure boat refrigeration, Zone or area thermostats for coolers and spillover boxes, Refrigerator, Freezer and full range temperature controller with adjustable differential.

Differential is the difference in temperature between when 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 AC electrical compressors would over load and heat up if there was not enough time between stopping and restarting compressor so this along with energy considerations determined compressor off cycle times. Modern air conditioning and large refrigeration units are equipped with timers to prevent short off cycling times.


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 providing an area in box to store or freeze a few items like ice.
  • The Most important reason to stay with evaporator controlled thermostats is they are more energy efficient than area thermostats. Snap action or electronic thermostats controlling a zone or area will tend to overpower the evaporators ability to absorb heat efficiently increasing daily amp-hrs consumed.
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Old 17-05-2013, 12:15   #14
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Richard, thanks for posting. I was just looking at your web site and books. Will likely be ordering books soon. Any chance they are available in electronic format?

My plan is to replace w approproate tstat as soon as I can, but selection here in Bocas del Toro Panama is rather limited and I dont want to spend more time here waiting on parts (ah...the cruising life).

Thanks.
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Old 17-05-2013, 14:49   #15
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Re: Fridge Thermostat Replacement

belizesailor, when changing thermostat on BD 35 and BD50 compressor system the amperage in that circuit is very small from 2 to 5 milliamps depending on size of resistor installed in thermostatís wiring. With no resistance in this circuit 5 milliamps will operate compressor at minimum speed of 2000 rpm. With a 1500 ohm resister in thermostat wiring compressor will run at 3500 rpm.

Over the last 28 years I have produced five technical books on boat refrigeration. Technical books are little value if they are not revised regularly so I decided 13 years ago to maintain a web site and forum to improve on the information I provide. I answer ten to fifteen emails per day and ship books twice a week. As long as a boater can contact my web site or email me on the internet he does not need my book.
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