Fridge thermostat problem.

XDC

XDC

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Unfortunately a long story involving a fridge and an engineer.

I have one of these

Compressor

https://www.marinesuperstore.com/refrigeration/cooling-kits/waeco-cu-84-compressor-fridge

running one of these

Evaporator

https://www.force4.co.uk/dometic-vd-15-series-80-fan-assisted-evaporator.html?sqr=Dometic&

controlled by one of these

Thermostat

https://www.ebay.co.uk/itm/Thermost...1ca83451650ada1f23230b7ffe6c084&ul_noapp=true


Since an engineer regassed the unit (nuts had vibrated loose) and fitted a new thermostat (cautionary, he was there, thermo >10 years old, only £20, why not) the fridge cycles on and off repeatedly and holds the temperature to within 0.1 of a degree. By repeatedly, I mean on for a minute off for a minute.

It seems absolutely determined to hold the evaporator plate at an EXACT temperature.

I CANNOT get him to understand the problem. More later if necessary.

The fridge used to ON AVERAGE turn on for 10 minutes and off for 20.

In that time the temperature would change from say 6-8 degrees where my internal monitor was placed. The temperature achieved/displayed is incidental. Our food stayed cold.

Question.

What USED to happen?

My thoughts are that the evaporator used to ice over (not heavily), then the thermostat would kick in turning the compressor off, the ice would hold the temperature but slowly melt, the temperature would rise after 20 minutes, the compressor would kick in and repeat the cycle indefinitely.

There was always a bit of water in the fridge bottom, not excessive, now it’s literally bone dry.

Just to explain (actually I can’t fully) my frustration one of his suggestions is to move my sensor!

Can anyone please tell me if our > 10 years experience is the norm (on 10, off 20) and just confirm to someone whose being driven mad (that’s me) that the continuous cycling is totally wrong.

Please don’t get involved in ambient temps, seals, parts of world, fridge contents, fridge ful, half full, empty etc etc. I know these can all impact on the running time but:

Off/on, right/wrong?

Thanks.
 
Ok here a way I solved my problem, remover the thermostat, buy one of them cheap Chinese thermostat job on eBay , set it to go on and off what temp you want and keep it way from the cold plate .
After three different fridge engineers three vacuums a new dryer and two thermostats plus a few hundred ponds in engineer fee , I shorted it my self for a few pounds .
 
Last edited:
It sounds like your new thermostat has a very small differential between cut in and cut out temperatures. Is the eBay thermostat the new one? It's not clear from your post.

Many Danfoss thermostats can be adjusted to change the differential, there's a hole on the opposite side to the knob spindle giving access to an adjustment screw.
 
Thanks Vic, back to you later. PVB, the engineer supplied the thermostat, the link was reference only.
 
XDC said:
Can anyone please tell me if our > 10 years experience is the norm (on 10, off 20)
Click to expand...
Not the normal here, i made a new thermostat from an arduino which also records the duty cycle over previous 2 hours varies from 15% to near 100% depending on ambient temperature and how many warm beers were just put in :)
 
Yes - what's needed is a diff between on and off.

Could be called hysteresis.

a 1 or 2C hysteresis should do what you want. Either set manually on the twiddly thing or replace it with something electronic that actually does what you want it to!

OR you can even leave the danfoss one all wired in and ADD a second thermostat in series that will mean to turn things on the Danfoss one needs to be saying the cold plate is warm enough (so not frozen... no point turning it on if it is) and the 'cabinet' is warm enough as well to need turned on... That may need you to turn the danfoss up a bit or it will cut out the cooling as soon as it gets cold.

You should be able to get a W1209 for about £3 from a UK seller on ebay. Will let you manually set a hysteresis. Can also add a time delay if you want to force a period of time off. Doesn't come in any kind of enclosure - but look on ebay for them too...
 
sailaboutvic said:
Ok here a way I solved my problem, remover the thermostat, buy one of them cheap Chinese thermostat job on eBay , set it to go on and off what temp you want and keep it way from the cold plate .
After three different fridge engineers three vacuums a new dryer and two thermostats plus a few hundred ponds in engineer fee , I shorted it my self for a few pounds .
Click to expand...

+1

I had a faulty thermostat on a Danfoss/Isotherm unit. It heavily frosted the outside of the freezer box on shorepower, which then melted when on battery power.
The replacement Isotherm stat was a crazy price, so I replaced it with this:
https://www.amazon.co.uk/gp/product/B00HC6PDAW/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1

I removed the temperature sensor and capillary tube and fitted this.
It was fairly easy to fit, gives an accurate temperature readout, and both the temperature on and temperature off can be set, as well as a 'lag' period. I have mine set to on at 4, off at 6, and 0 mins lag, and it works perfectly.
I haven't sat and timed the time it is on or off, but it no longer freezes everything, and behaves like a normal fridge whether on shorepower or house battery power.
 
fearmhuir said:
+1

I had a faulty thermostat on a Danfoss/Isotherm unit. It heavily frosted the outside of the freezer box on shorepower, which then melted when on battery power.
The replacement Isotherm stat was a crazy price, so I replaced it with this:
https://www.amazon.co.uk/gp/product/B00HC6PDAW/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1

I removed the temperature sensor and capillary tube and fitted this.
It was fairly easy to fit, gives an accurate temperature readout, and both the temperature on and temperature off can be set, as well as a 'lag' period. I have mine set to on at 4, off at 6, and 0 mins lag, and it works perfectly.
I haven't sat and timed the time it is on or off, but it no longer freezes everything, and behaves like a normal fridge whether on shorepower or house battery power.
Click to expand...

Used the 240V version to fix my fridge at home several years ago. Still works perfectly, easy to fit and many more options than I ever needed.
 
In my day job we refer to the difference between cut in and cut out as the deadband or diff, and this is absolutely essential to keep sensible control of a thermal load.

The old fashioned bimetallic strip provided a natural deadband or hysteresis by the stiffness of the strip.

I have found this one on EBAY that does show on the front face both a start temperature and stop temperature, but reading the spiel is hard going.

https://www.ebay.co.uk/itm/Electron...m=273134185413&_trksid=p2045573.c100643.m3226
 
Shineyshoe, many thanks. You lost me at adding something in series. I’m a plug and play guy :rolleyes::rolleyes:

Vic and Fearmhuir, that seems to be the answer.

Do I just take the power from the 12 volt feed to the compressor and then feed the probe into the fridge box?

If so does the probe then send the on/off signal back to the box? And that’s it!!?? Good grief, my life is never that simple:)

Also how do you mount the boxes and would they be ok in the locker where the compressor sits. Are there lugs to screw them down or ... what? The locker is well ventilated and has lots of space.
 
XDC said:
Shineyshoe, many thanks. You lost me at adding something in series. I’m a plug and play guy :rolleyes::rolleyes:
Click to expand...
That means you'd cut into the on / off wire on the existing thermostat.

Vic and Fearmhuir, that seems to be the answer.

Do I just take the power from the 12 volt feed to the compressor and then feed the probe into the fridge box?

If so does the probe then send the on/off signal back to the box? And that’s it!!?? Good grief, my life is never that simple:)
Click to expand...

So you take 12V + and 12V - (GND) from whereever - the compressor supply seems sensible. As long as it is always 12V when the fridge is meant to be on you are sorted.

You then take the +ve wire that makes the compressor actually run and feed that wire to one of the switch terminals on the unit and the other would go to 12V


You should end up with something like this:


12V - -------------------------------> 12V -ve or GND on back
|
Battery
|
12V + -------------------------------> 12V +ve on back of box

12V + -------------------------------> Relay Switch on box


Compressor + ----------------------> Other Relay Swith connect on box


Compressor - ----------------------> 12V -ve on Battery

Also how do you mount the boxes and would they be ok in the locker where the compressor sits. Are there lugs to screw them down or ... what? The locker is well ventilated and has lots of space.
Click to expand...
The rectangular one is designed to 'slot' into a rectangle in the steel case of a fridge. Those orange bits move and hold it in place. I'd guess some 6mm ply would be OK to mount it on. Then mount that wherever you want. It doesn't need special ventilation etc.

Put the probe in the fridge air space, that is what you want to control.
 
Brilliant. I have a relation who can make anything out of wood. Providing its wood. A box to mount it in shouldn’t be a problem.

So grateful for all the answers.

Except for the engineers one. Move the internal sensor. Good grief.
 
Wouldn’t the fused switch (breaker actually) from my main panel that turns the compressor on be sufficient?
 
XDC said:
Shineyshoe, many thanks. You lost me at adding something in series. I’m a plug and play guy :rolleyes::rolleyes:

Vic and Fearmhuir, that seems to be the answer.

Do I just take the power from the 12 volt feed to the compressor and then feed the probe into the fridge box?

If so does the probe then send the on/off signal back to the box? And that’s it!!?? Good grief, my life is never that simple:)

Also how do you mount the boxes and would they be ok in the locker where the compressor sits. Are there lugs to screw them down or ... what? The locker is well ventilated and has lots of space.
Click to expand...

It's very straight forward , as shinyshoe said .
On the display you can set the starting and stopping temp and the temp the fridge is at the moment ,
 
XDC said:
Shineyshoe, many thanks. You lost me at adding something in series. I’m a plug and play guy :rolleyes::rolleyes:

Vic and Fearmhuir, that seems to be the answer.

Do I just take the power from the 12 volt feed to the compressor and then feed the probe into the fridge box?

If so does the probe then send the on/off signal back to the box? And that’s it!!?? Good grief, my life is never that simple:)

Also how do you mount the boxes and would they be ok in the locker where the compressor sits. Are there lugs to screw them down or ... what? The locker is well ventilated and has lots of space.
Click to expand...

That's about it. I took the power to the unit from the supply to the fridge, on my Danfoss there are two terminals that when connected start the compressor - I wired these to the 'load' connections on the unit. Take out the old thin pipe and the sensor, and replace with the sensor from the new unit.
I used stick-on velcros strips to mount the unit under the compressor unit which is under the sink.
You can see the current temperature on the unit, and it is easy to set the temperature parameters.
Its amazing to see the increase in temperature when you put six cans of beer into the fridge!
 
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