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Dometic CRX 65 Compressor Fridge, Does it cycle too often?


Quattrodave

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Evening, if anyone has a dometic compressor fridge, how often does yours cycle on and off.  Mine just feels like it's cycling far too often.  The compressor starts & runs for 3 to 4 mins then off for about 15 mins, then on again for 3 to 4 mins....??

 

My first thoughts are the cable is too small or not enough air flow behind it...

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Thank for your reply.

 

Battery's are all good, behaves exactly the same whist engines are running or on shore power, via victron phoenix charger.   Nothing on the sides or the fridge, all the pipework and compressor are on the back.  It has some space both sides and on top.

Also have a draw above it I can remove to increase air flow as a test...

Edited by Quattrodave
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Seems pretty good to me.  It is only on for about 1/4 of the time.  A typical power  audit allowance is for the fridge motor to be on about 50% of the time.

 

What temperature is the thermostat knob at?  Since the compressor runs OK for 3-4 minutes but comes back a bit quicker than is typical you may be gaining heat somewhere, or the fridge is very cold. 3 to 4 C in the middle is ideal.  Test with a meat thermo probe thingy into a bowl of water that has been on the middle shelf over night.

 

N

 

 

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12 hours ago, Quattrodave said:

Evening, if anyone has a dometic compressor fridge, how often does yours cycle on and off.  Mine just feels like it's cycling far too often.  The compressor starts & runs for 3 to 4 mins then off for about 15 mins, then on again for 3 to 4 mins....??

 

My first thoughts are the cable is too small or not enough air flow behind it...

 

 

Mine is similar and runs a total of 15 - 20 minutes per hour depending on external conditions (when the outside temp was 32C (in the shade) it was running about 20 minutes per hour0.

Door opening numbers makes a big difference and it barely (if ever) runs during the night.

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12 hours ago, Quattrodave said:

Evening, if anyone has a dometic compressor fridge, how often does yours cycle on and off.  Mine just feels like it's cycling far too often.  The compressor starts & runs for 3 to 4 mins then off for about 15 mins, then on again for 3 to 4 mins....??

 

My first thoughts are the cable is too small or not enough air flow behind it...

 

As an ex refrigeration engineer, I would be quite happy with that.

It will run more frequently in a warm environment than cold, of course.

It will also run more frequently when the thermostat is on a colder setting.

I think you are probably worrying about nothing. 👍

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I hope the OP doesn’t mind me asking a related question, since we have an ex refrigeration engineer in the room 😊

 

Given X% of compressor ‘on’ time, is it more efficient for it to run for longer or shorter spells? Eg 5 mins on / 15 mins off, or 15 mins on / 45 mins off? 
 

I’ve been playing with replacement temperature controls on my basic 230v fridge on the boat and will be able to make the hysteresis whatever I want. Obviously I don’t want the food temperature to swing too wildly, or for it to be switching every minute, but have been wondering whether there is a sweet spot for efficiency. 

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56 minutes ago, Thames Bhaji said:

I hope the OP doesn’t mind me asking a related question, since we have an ex refrigeration engineer in the room 😊

 

Given X% of compressor ‘on’ time, is it more efficient for it to run for longer or shorter spells? Eg 5 mins on / 15 mins off, or 15 mins on / 45 mins off? 
 

I’ve been playing with replacement temperature controls on my basic 230v fridge on the boat and will be able to make the hysteresis whatever I want. Obviously I don’t want the food temperature to swing too wildly, or for it to be switching every minute, but have been wondering whether there is a sweet spot for efficiency. 

 

 

That is a very good question, and one I look forward to the experts answering.

 

We have a small chest freezer that is used as a fridge in the summer months with an external thermostat thing. Obviously there is a large startup current for a compressor fridge, that lasts for a second or two, so the more often it comes on, the more it has this large startup current. 

 

The longer it is on, however, the hotter the compressor will get, so I suppose it depends on its ability to dissipate the warm air that dictates how quickly it will reach the required temperature.

 

I have our fridge set to between zero degrees and 5 degrees. I was toying with the idea of putting a large computer fan beneath the compressor to draw cool air onto the compressor. Opening the side doors, where the fridge is located also seems to disspitare the warm air above quicker, which has an effect on the fridge.

 

Hopefully someone will be able to offer a more scientific answer.

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14 hours ago, BEngo said:

Seems pretty good to me.  It is only on for about 1/4 of the time.  A typical power  audit allowance is for the fridge motor to be on about 50% of the time.

What temperature is the thermostat knob at?

 

12 hours ago, Alan de Enfield said:

Mine is similar and runs a total of 15 - 20 minutes per hour depending on external conditions (when the outside temp was 32C (in the shade) it was running about 20 minutes per hour0.

Door opening numbers makes a big difference and it barely (if ever) runs during the night.

 

Hmmmm mine often runs through the night... It runs less, but it still runs a couple of times an hour...

 

After reading the manual again I think I have the temp set lower than it needs to be and I believe I also have the 'Fast Cooling' enabled. Apparently I you hold the temp button for 3 seconds you can turn 'Fast Cooling' on and off.....

Edited by Quattrodave
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5 hours ago, rusty69 said:

 

 

That is a very good question, and one I look forward to the experts answering.

 

We have a small chest freezer that is used as a fridge in the summer months with an external thermostat thing. Obviously there is a large startup current for a compressor fridge, that lasts for a second or two, so the more often it comes on, the more it has this large startup current. 

 

The longer it is on, however, the hotter the compressor will get, so I suppose it depends on its ability to dissipate the warm air that dictates how quickly it will reach the required temperature.

 

I have our fridge set to between zero degrees and 5 degrees. I was toying with the idea of putting a large computer fan beneath the compressor to draw cool air onto the compressor. Opening the side doors, where the fridge is located also seems to disspitare the warm air above quicker, which has an effect on the fridge.

 

Hopefully someone will be able to offer a more scientific answer.

 

In a normal small fridge, the thermostat is operated by the temperature of its evaporator, usually around -18 deg C.  It cuts in and out at a fixed differential either side of that.  This is because a normal front opening fridge loses all its cold air when the door is opened.  The compressor would then run for an excessive time to cool the new warm and humid air to around 5 - 7 deg C, which in turn causes a large build up of ice on the evaporator.  This then becomes highly inefficient and progressively less effective.  Using the evaporator as the sensing point allows it to be kept relatively clear of excessive icing up, whilst still controlling the temperature of the air.

A chest freezer is quite different in that when the top lid is opened the cold air stays put.  I have never run a chest freezer as a refrigerator using its air temperature as the thermostat control, but I imagine it could work up to a point.  My concern would be  whether it caused short-cycling with the compressor running too frequently and for a short time.  It is unlikely the compressor would overheat, as it is doing far less work than it was designed for.

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On 19/09/2022 at 22:36, Quattrodave said:

Evening, if anyone has a dometic compressor fridge, how often does yours cycle on and off.  Mine just feels like it's cycling far too often.  The compressor starts & runs for 3 to 4 mins then off for about 15 mins, then on again for 3 to 4 mins....??

 

My first thoughts are the cable is too small or not enough air flow behind it...

We had the smaller CRX50 and it ran for a very similar amount of time to that you are stating.

 

Never found it to be a problem and never found the tiny amount of power it used to be a problem.

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14 hours ago, Rebotco said:

In a normal small fridge, the thermostat is operated by the temperature of its evaporator, usually around -18 deg C.  It cuts in and out at a fixed differential either side of that. 

 

 

Interesting. I've been trying to figure out how my own 12v fridge works. The thermostat is busted and the sensor tube does not have a bulb on the end like most other capillary tube sensing thermostats. The capillary is about 18" long and just shoves into a 6mm diameter plastic tube embedded in the insulation. Appears to go nowhere near the evaporator.

 

My new generic 'universal' thermostat is the same but the sensor tube is about 3ft long and I can't work out if it senses just at the end, ar all long, or what! Do you have any idea perhaps? (As you seem to know a lot more about fridges than me!) Thanks.

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