Pull-down time for residential unit

A few months ago I got a Kenmore bottom-freezer (looks same as LG model LRDN22734) and I'm suspicious of the pull-down time.

I put a temperature logger in it and found that after adding a couple of small bowls of room temperature (72F) spaghetti and sauce it took 5 hours to pull down from 45F to 39F. The cabinet was at 39F before putting the spaghetti in it. So I had their service come by and he had no idea what the pull-down time should be. He said that since it was able to reach a reasonable temp that it worked OK. He also said that modern "energy-star" fridges have tiny compressors and that's likely why it took so long.

So what's a reasonable pull-down time for a modern residential fridge?

With so many variables (specific heat of Spaghetti?, amount of Spaghetti? does it have sauce on? etc) it would be difficult to answer with any accuracy.

A better method would be to do an experiment with known quantities, say 1 or 2 litres of water.

Place the water in the fridge, noting the temperature.

Allow a time lag, say 1 hour intervals and measure the water temperature, making a log (time/temp). After a time you will get the final temperature as the fridge cycles on the stat.

Look on the back of the fridge and find the compressor rating in Watts.

From this information we can do a calculation to determine the refrigerating effect and compare it to the expected perfomance of the compressor.

Mate, you should change your name to " toomuchtime"....LOL

Does the freezer section freeze?

Does the fridge section chill?

If yes to both those answers then I wouldn`t worry too much about "pull down times".

Its only a domestic freezer, it is not capable of freezing amounts of mass other than some ice cubes of some fruit from the garden.

Yes I know it will freeze legs of chicken or whatever, but it is not specifically designed to do so. The only reason it does freeze those items is because it runs at -20deg C.

Wait a mo...just reread your post...youre talking about the fridge section..

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I put a temperature logger in it and found that after adding a couple of small bowls of room temperature (72F) spaghetti and sauce it took 5 hours to pull down from 45F to 39F. The cabinet was at 39F before putting the spaghetti in it

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3.3deg C in 5 hours for a domestic is quite good.

Same argument here though, the chiller section is not designed to chill amounts of anything, it just holds items at a temperature.

(I left the 1st part of my post in case you started asking questions about the freezer section)


Chillin:) :)

Hi Frank,

Good idea. I just put in a liter of 79F water and will log its cooling and post the results.

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Originally Posted by frank

With so many variables (specific heat of Spaghetti?, amount of Spaghetti? does it have sauce on? etc) it would be difficult to answer with any accuracy.

A better method would be to do an experiment with known quantities, say 1 or 2 litres of water.

Place the water in the fridge, noting the temperature.

Allow a time lag, say 1 hour intervals and measure the water temperature, making a log (time/temp). After a time you will get the final temperature as the fridge cycles on the stat.

Look on the back of the fridge and find the compressor rating in Watts.

From this information we can do a calculation to determine the refrigerating effect and compare it to the expected perfomance of the compressor.

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Hi Chillin,

Yuk! Sure freezer freezes and fridge chills, but how can I tell if the system is built to reasonable specs? The 5 hours worried me, so I just want to do a sanity check.

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Originally Posted by chillin out

Mate, you should change your name to " toomuchtime"....LOL

Does the freezer section freeze?

Does the fridge section chill?

If yes to both those answers then I wouldn`t worry too much about "pull down times".
...

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Hi Frank,

OK, I put a liter of 79F water in the fridge and logged the temperature over time. It took 5 hours to come back to 38F. Attached is the graph. It's hard to pull out the fridge to look for a compressor rating and there's nothing in the spec sheet about it, so I'll do that later.

It looks to me like the problem is that it's cycling even though it's above temp. Notice the sawtooth pattern. If it kept pulling down without cycling it looks like it would have reached temp in about an hour.

What do you think?

Was the temp logger in the water?

I would have expected to see a steady decline in temperature, not the saw tooth pattern which suggests that the compressor is cycling, as you say.

Perhaps you ought to determine if this is the case.

I don't work on domestic fridges but I believe some of them cycle on stats attached to the evoparator plate. Perhaps the saw tooth pattern we see is related to this type of control?

I'll do the calcs and post back

Hi Frank,

It's an air logger that I put on the shelf a few inches away from the water bowl. This fridge has two thermistors (could be more I guess) with one at top of fridge and one at top of freezer compartment.

Perhaps it's only using the freezer sensor to cycle, and ignoring the fridge sensor?

A repair guy was here and ran all it's diagnostics and they passed...

I've reread the post.

To do the calculation I need the water temperatures - not the cabinet temps.

You need to measure the water at the starting point and ending point and give me the time elapsed. :)

I can't put the cabinet temps into the formula Q =mcdt/time

The sawtooth is because the fridge is measuring propably the evaporator temperature which isn't the air temperature.

You allways see this pattern when you regulate the temperature with the evaporator temperature.

The plate is responding different and faster to the temperature of the air that the evaporator is cooling.

Pull down is determined by entering temperature, kind of of product, way you store it (in a closed recipeint or spread open on a plate)

A domestic freezer has to run almost 2 days if you fill it complete with fresh meat!!

Yep, you will get a saw-tooth effect measuring the air temp. as the sensor is measuring the evap. temp. Placing the probe in the flask of water will give a more realistic reading & should show a steady decline in temp. with little or no saw-toothing.

God am I glad I don`t do domestics anymore!!!!!

Having to put up with people putting bloomin temp logs in a domestic....mate there is more to life..:eek: :eek:

As I have said before, as long as it cools and freezes...forget about it.

I don`t mean to sound like a knob...but think about it, you just called an engineer out for nothing:eek: an absolute waste of time for him. And don`t you just know what he called you as he pulled out the drive.

Posting this type of question on a forum is ok, because it costs nothing to anybody, but calling an engineer to your home???

If you have any more problems with anything, new car, new phone, new mp3 or new or even old anything, find a forum 1st and ask your questions there.

Hope you satisfy yourself with your queery.

Chillin:) :)

Hi Frank,

Sorry I wasn't more clear, but the water started at 79F and ended at 38F. That's with a separate thermometer measuring the water only at those two times. The log is air temperature.

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Originally Posted by frank

I've reread the post.

To do the calculation I need the water temperatures - not the cabinet temps.

You need to measure the water at the starting point and ending point and give me the time elapsed. :)

I can't put the cabinet temps into the formula Q =mcdt/time

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Hi Phil68,

Sure that'll smooth out the curve, but I don't think it changes the results and thus the conclusions. It seems clear that the air temp is elevated due to the above temp water. So a water probe would show something like the time average of the air temp. It's taking this fridge 5 hours to cool a liter of water! It has no trouble putting out plenty of ice (at other times).

Quote:

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Originally Posted by phil68

Yep, you will get a saw-tooth effect measuring the air temp. as the sensor is measuring the evap. temp. Placing the probe in the flask of water will give a more realistic reading & should show a steady decline in temp. with little or no saw-toothing.

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Hi Chillin,

Geez, take it easy... you're supposed to be chillin!

I originally called the service folks because of the fridge pull down problem. They found that LG had put both the thermistors up in the insulation (probably during the foaming process) and forgotten to lower them into the sensor clips in the fridge and freezer. He sait this is a known manufacturing problem with LG. A month later, after the problem persisted, I got a temp logger to see if I could tell what's going on. Then I called the service folks again as described in the opening post.

You're shooting at the messenger. I'll return the fridge as defective (or poorly designed) if it can't do what my previous fridge did with no problem. The manufacturer pays warrantee costs, so this is likely to be a case of field service engineers helping manufacturing find their problems.

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Originally Posted by chillin out

God am I glad I don`t do domestics anymore!!!!!

Having to put up with people putting bloomin temp logs in a domestic....mate there is more to life..:eek: :eek:

As I have said before, as long as it cools and freezes...forget about it.

I don`t mean to sound like a knob...but think about it, you just called an engineer out for nothing:eek: an absolute waste of time for him. And don`t you just know what he called you as he pulled out the drive.

Posting this type of question on a forum is ok, because it costs nothing to anybody, but calling an engineer to your home???

If you have any more problems with anything, new car, new phone, new mp3 or new or even old anything, find a forum 1st and ask your questions there.

Hope you satisfy yourself with your queery.

Chillin:) :)

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Hi Peter,

Interesting. Thanks for your comments.

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Originally Posted by Peter_1

The sawtooth is because the fridge is measuring propably the evaporator temperature which isn't the air temperature.

You allways see this pattern when you regulate the temperature with the evaporator temperature.

The plate is responding different and faster to the temperature of the air that the evaporator is cooling.

Pull down is determined by entering temperature, kind of of product, way you store it (in a closed recipeint or spread open on a plate)

A domestic freezer has to run almost 2 days if you fill it complete with fresh meat!!

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Hi!
I don't have much experience in domestic fridges. Still from your data logger results it seems your cooling system is working for 10 mins and then stops for 10 mins. Once it's working the cooling is quite fast. but then the temp rises again for 10 mins until it starts again.
Are you sure it's not some control on your compressor doing this? Maybe it's overheating and some protection device is cutting it off everytime??

With a starting temp of 78F (22.77C) and a final temp of 38F (3.33C) it took 451W of refrigeration power to cool 1kg of water in 5 hours.

The next step is to compare this to your compressor power (assuming you know the COP) to see if you are happy with the performance.

As most domestic fridges are rated at about 650W it is not a million miles out

Hi Frank,

That's the first solid basis I've heard for what's going on. Does this mean that a compressor power (or overheat) limiter is the likely cause of the sawtooth cooling pattern?

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Originally Posted by frank

With a starting temp of 78F (22.77C) and a final temp of 38F (3.33C) it took 451W of refrigeration power to cool 1kg of water in 5 hours.

The next step is to compare this to your compressor power (assuming you know the COP) to see if you are happy with the performance.

As most domestic fridges are rated at about 650W it is not a million miles out

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Hi Frank,

It looks like your calculation is off by a factor of 100. When I do the calc it came up at 90 btu which is about 5 Watts.

Quote:

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Originally Posted by frank

With a starting temp of 78F (22.77C) and a final temp of 38F (3.33C) it took 451W of refrigeration power to cool 1kg of water in 5 hours.

The next step is to compare this to your compressor power (assuming you know the COP) to see if you are happy with the performance.

As most domestic fridges are rated at about 650W it is not a million miles out

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You are quite right. I get the answer to be 4.51W - must have slipped with the calc button :o

No, it doens't mean that...your fridge is controlling on a different base as you're measuring.
I can show you hundreds of similar sawtooth graphs.
As Frank roughly calculated..it's functioning normal.

if its one of those frost-free things dont they go on defrost? by using a timer the coils in the freezer just gussing dont do alot of domestics but defrost would be either timer or a bulb sensor have units here that the stat for the cooler side only controls a flap that opens or closes based on how u set it,so if timer, would maybe account for those times there is a temp. rise:cool:

Hi Peter,

But Frank's calculation (now) shows about 5 watts, which is about 100x smaller than what'd be expected. Plus the freezer has no trouble making ice at a decent rate. It makes about 100 cubes a day, as advertised in the user manual.

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Originally Posted by Peter_1

No, it doens't mean that...your fridge is controlling on a different base as you're measuring.
I can show you hundreds of similar sawtooth graphs.
As Frank roughly calculated..it's functioning normal.

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Hi Frank,

There's a sticker on the cabinet that says: "Design pressures 140 psi low, 400 psi high, Full load Amp 3.5A, R134a 4.05 oz" and this has a 115 volt service. I guess that means the compressor is 403 VA at full load. Does that help at all?

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Originally Posted by frank

You are quite right. I get the answer to be 4.51W - must have slipped with the calc button :o

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