Fridge thermostat Q

[LKB]

Q for the collective:

Thermostat went out on my Northland fridge (separate cooling module). Fortunately, the cold control unit / thermostat is a pretty standard model, like this:

https://www.amazon.com/gp/product/B07V2ZQ6JX/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Dead simple to replace, but kinda finicky to get the low temp and hysteresis screws adjusted just right.

After quite a bit of fiddling, I got it where it should be (with the dial set in the middle, cuts out at about 33F and cuts on at about 39F), such that it is not short cycling and the defrost cycle (completely separate timer circuit) can run.

Anyway, while I was doing all the trial and error adjustment (which required me to unplug the fridge module before messing with the screws), I removed the decorative cover that screens the cooling modules from view. (With the cover off, airflow to the units is marginally better, but the unit had run fine for 12 years with it in place before.) After all the adjustment, it ran perfectly for a week with the cover off.

Here’s the issue — when I reinstalled the decorative cover, the unit froze up in hours (fan clipping ice buildup and not holding temp). Removed the cover, unplugged the unit, let it defrost, plugged it back in — runs perfectly for several days. But when I reinstall the cover — it freezes up again.

i’ve done it several times and it’s repeatable. Cover on / off is the only variable. And again, prior to the old cold control failing, unit ran fine for over a decade with the cover on.

So, any ideas? Do I need to decrease the hysteresis?

 

[WorthFlorida]

Is the freezing occurring on the condenser coils on top of the box or the evaporator coils inside the refrigerator.
When refrigerators freeze, (the evaporator coil), the heating element (wire) went bad, a connection (plug) opened the circuit, or the timer to switch off the compressor and switch on the heater went bad. All refrigerators use a heater to keep the coils from freezing. It maybe 1-2 hours per day. Keeping an eye on the condensate drip pan can be used as a clue.

Why it is not freezing with the cover off? The unit went into defrost mode. Usually the fan will shut off and the compressor, and the heating element turns on. If the heating elements do not turn on, the ice will not melt but with the covers off, enough warm air from the refrigerator compartment from warm food and opening the door, there is enough ice to melt most or all of the ice. With your Northland model, the evaporator fan may never turn off on any cycle. Some refrigerators have a temperature sensor on the coils so should the temperature get below 32ºF, it'll switch off the compressor and turn on the heater.

If it is freezing on top, on the condenser coils or compressor, like a central air unit, lack of air flow over the coils can cause it to freeze. Lack of air flow over the evaporator coils will cause the coil to ice up, just like an AC air handler with fully clogged up air filter restricting air flow.

 

[Sponsor]

 

[LKB]

The freezing / ice build up is on the evaporator coils, and as previously indicated when it happens you can hear the evap fan hitting the ice build up.

On the Northland, the compressor, condenser, evaporator, evaporator fan, etc., are all contained in a cooling module (removable) that mounts atop the fridge (evaporator stuff is in an insulated box with an open bottom). The fridge has an insulated hole in the top that mates with the bottomless insulated box containing the evaporator on the cooling module — setup is comparable to what you see with a Ready-Ice unit you see with bags of ice outside a convenience store.

Evap fan does NOT run constantly, only when compressor is on.

Defrost mode is on a timer, not tied to temp. The *exact* same cooling module is on the Northland freezer — only mechanical difference between Northland fridge and freezer is that the cold control of the freezer is set a lot lower (and the freezer has an ice maker). Indeed, when I originally installed the new cold control unit, it cooled the fridge down below 25F (and was still running) until I adjusted the low temp up by a couple of turns of the adjustment screw. Despite temp being well below freezing, there was no freeze up of the evaporator (cover was off).

The thought that there is a fault somewhere in the defrost circuit is confounded by the fact that it works fine with the cover off. Evaporator, defrost element, evap fan are all essentially inside the fridge.

Additional data point: condenser coils and surrounding area were completely cleaned / dusted a couple of months ago when I had the evaporator replaced on the cooling module.

Like I said, this is a real puzzler. Any ideas?

 

[Fitter30]

Why are u running a fridge that cold? 35° is very close to evap freezing. Freezer have electric heaters for defrost fridges usually air air temp for defrost.

 

[LKB]

33-39 is the ideal fridge temp. A large hysteresis minimizes the number of starts/stops on the compressor.

As indicated above, the module has a defrost heater and a fan that blows air over the evap coils. The exact same model of cooling module is also used on the matching freezer, so running it even well below freezing isn’t the problem (the freezer doesn’t ice up).

The heater (which is on a timer) is working.

Again, the only variables are:
(1) the fridge has a new thermostat. It has only two adjustment screws: cut on and hysteresis
(2) when the cover is off, it runs perfectly. But with the cover on, evap coils ice up. But the fridge did not do that before, nor does the freezer now. So all I can think is that I need to set the hysteresis either higher or lower — but why that would have this effect is beyond me.

any ideas?

 

[Fitter30]

A grill will affect air flow wide open has use of all square inches with 60 - 80%. Sure the heater is working?

 

[LKB]

A grill will affect air flow wide open has use of all square inches with 60 - 80%. Sure the heater is working?

Yup, I can hear it kick on, and in the first couple minutes of operation you can hear the “hiss” of it defrosting.

Once again the fridge is working perfectly without the cover. The matching freezer (same model cooling module, behind the same decorative cover) is running fine with or without the cover, so apparently the slightly reduced airflow is leading to increased icing on the fridge but not the freezer. The only variable between them is the new t-stat in the fridge. Since the low temp setting doesn’t matter, the only thing I can think of is the hysteresis setting.

very very puzzling . . . .

 

[Fitter30]

model number of unit.

 

[LKB]

model number of unit.

Cooling module is Northland NM11B (sometimes shown as SNM11B; no longer in production). It was used on Northland’s “SubZero” style fridges and freezers (discontinued several years ago, but parts are still available).

 

[Fitter30]

Found no parts are available from Northland for either model. Found one manual covered all refrigerators 34° to 40° To check charge disconnect evap fan run unit for ten minutes with the door open then check to see if a lite frost covers the evap. They don't recommend hooking gauges to the system and being a critical charge and only holding 3 oz a correct field charge might would take special tools. Sorry couldn't help more.

https://www.google.com/url?q=https://www.marvelrefrigeration.com/pub/media/marvel/service_manuals/SERVICE_MANUAL.PDF&sa=U&ved=2ahUKEwjqnJOln4b8AhVsjYkEHRPeBWYQFnoECAkQAg&usg=AOvVaw2Liuf87ZNin86s2GWtZAHP

 

[LKB]

Thank you for looking at this.

I was able to order a replacement evaporator and capillary tube from Marvel a couple of months ago (my AC tech was able to replace those, pull / test a vacuum, and fill with the correct amount of refrigerant . . . but as you note that took his specialized tools and skill set). So at least some parts for the NM11BF *are* still available, but in this case they did not have a replacement cold control unit. (Fortunately, it was an off the shelf product I was able to find on Amazon for $12.)

The manual you link to is for current Northland / Marvel products that use an electronic thermostat + thermistors for cold control. Mine uses the old-fashioned mechanical thermostat that uses a capillary tube as a sensor.

The problem isn't that the unit isn't cooling (it is, and it runs perfectly fine as long as the cover is off). The mere fact that the cover is on/off cannot be the fundamental problem either -- an identical unit (on the adjacent matching freezer, running MUCH colder) is working just fine regardless of whether the cover is on or off.

The only remaining variable I can see is the hysteresis setting on the mechanical fridge cold control -- but was hoping to get some ideas as to whether increasing / decreasing it would impact this and why.

 

[Fitter30]

Old t stat do still have it? Maybe can contact the control manufacture. Its long shot because its so old probably won't be in their system. By increasing hysteresis will the unit more defrost time. Back when i first got into the trade 1970 commercial refrigerators didn't use electric heaters they used refrigerant pressure to start and end defrost. pressure set the temp in the box.

 

[LKB]

Old t-stat was virtually identical to this:

https://www.amazon.com/Refrigerator-Thermostat-Refrigeration-Temperature-WPF-20/dp/B07V2ZQ6JX/ref=sr_1_6?keywords=cold%2Bcontrol%2Bthermostat&qid=1671628569&sr=8-6&th=1

so that’s what I replaced it with. (I wish there was an easy way to retrofit the fridge to just use a digital controller and thermocouple/thermistor as the temp sensor.)

OK, when I have time I’ll try increasing the hysteresis and see if that solves the issue. But why nominally decreasing the airflow across the unit (i.e., reinstalling the decorative cover) is causing the evaporator to ice up in the first place is weird; logically, that would (slightly) decrease the efficiency of the unit, and thus produce *less* ice on the evap than if the system if running colder.

 

[Fitter30]

Low air flow causes refrigerate pressure to drop which drops temperature of the evaporator. Air temp is normally +10° difference of refrigerant temp.

 

[LKB]

Low air flow causes refrigerate pressure to drop which drops temperature of the evaporator. Air temp is normally +10° difference of refrigerant temp.

Ah. My intuitive thinking was 180 degrees from reality. So now I know that and it makes sense. Thanks.

Rather than increasing the hysteresis (it's already 8-9F), the easiest solution (which probably would also extend the life of the unit) will probably be to just add a couple surplus DC power supply fans (I've had some that ran 24-7 for literally years) to blow air on the condenser and/or exhaust the air in the enclosure through the baffles in the decorative cover. (Fridge is a built in, and there's about 3-4 cubic feet of empty space next to the condenser.) Got a spare outlet next to the units, so a wall wart + wire + fans and hopefully Bob's Yer Uncle . . . .