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Old 21-06-2018, 12:18   #1
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Running a fridge on high or low?

Hi,

upfront, we will eventually replace the household fridge we have on board with a more efficient 12V based system with top load.
I am aware of that what we have right now is not very efficient.

Still, until then, we run our exsisting household fridge on 220V via a small inverter from our batteries.

Here is the question. (Simple answer please) :

We can set a temperature inside with settings from 0=Off to 7=Max cooling.
Is it better to keep it constantly at, let's say 3 or is it better to set it to 7 and after a while turn it off for a couple of hours?
Setting and forgetting would be easier but if it takes a lot more power that's probably not very clever.

Thanks, as always, Franziska
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Old 21-06-2018, 12:37   #2
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Re: Running a fridge on high or low?

I would set it and forget it. If you turn it off and then on later, you stand to have the temps drop too low. Once turned back on the fridge runs constantly until the temp is raised again. Setting it will allow the fridge to turn on and off with a tighter threshold. Therefore it will run for a much shorter period to reach the same temp.

I believe you average run times over the same time period will be shorter by using the internal thermostat, rather than you're memory and a clock as a manual process.

Never mind the "Oh @!#$&" I/we forgot to turn it back on before I/we fell asleep last night" factor.
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Old 21-06-2018, 12:41   #3
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Re: Running a fridge on high or low?

I donít think its going to make much difference and this is why.
Setting it low then turning it off will take advantage of running it constantly and not going through the inefficient on and off cycle.
However when it gets real cold, your going to gain much more heat as your trying to maintain a larger difference between the inside and outside.
Itís my belief that likely the additional heat gain will offset any efficiency increase in the longer run times.
Itís just not going to make much difference I donít think, and all it takes is to forget one time to ruin your food.
I suspect that if you find a good efficient AC fridge that a 12V is not going to be all that much more efficient, if properly sized there just isnít much loss in a good sine wave inverter, and there is nothing ďbetterĒ about DC power.

I know it goes against our common sense, but top load is not all that more efficient, reason is there just isnít all that much mass in air, so itís not holding all that much heat. Yes there is some, just maybe not as much as you might think.

You ought to look up Sailor Chics threads on an AC dorm fridge, difference in efficiency could easily be made up with one small Solar panel if I remember correctly, and for a whole lot less money.
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Old 21-06-2018, 13:47   #4
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Re: Running a fridge on high or low?

Good advice as usual. Thank you.
Will set it to permanently "on" than at a low setting.

Mind you eventually we are up for a new fridge and inverter. Both are about 20 years old and technology has evolved.

Despite this I feel though that I dislike throwing away working stuff. So maybe replacement will happen if & when it fails.
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Old 21-06-2018, 14:59   #5
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Re: Running a fridge on high or low?

Franziska

You might want to have a read of this thread by Sailorchic34 who found an interesting solution:


Sailorchic34's new Fridge
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Old 21-06-2018, 16:01   #6
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Re: Running a fridge on high or low?

I sometimes turn our freezer to full power when the solar charge controller goes into float.

Just seems like I'm using power that would otherwise be "wasted".
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Old 22-06-2018, 14:55   #7
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Re: Running a fridge on high or low?

@Pete
Thanks for the link to Sailorchicks Thread. Good info!

@Cat44
Interesting idea. I'll try that! Thanks!
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Old 23-06-2018, 20:44   #8
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Re: Running a fridge on high or low?

The key to low energy use in any system is a well insulated box, with a gasketed top-loading lid.

We have had excellent results with our Isotherm system using the following:

Set the thermostat so the contents are just above freezing in the main section. Once you find the sweet spot, forget about it. I haven't touched our thermostat in two years.

If you don't already have one, install a small biscuit fan to circulate the air inside the box.

Place a piece of 1/2" blue foam insulation over the area where you store delicate vegs like lettuce to prevent inadvertent freezing.

We have a 6 cu ft box with 5" of closed-cell blue foam. 200 watts solar with MPPT controller (average input to the batteries is around 80 AH/day). All lighting is LED. We have fairly low electronic loads - VHF, GPS plotter, FM radio. The compressor runs 2.5 - 3.5 hrs/day @ 7 amps, ~25 AH/day. Our batteries are usually fully charged by mid-afternoon. We do have a high-output alternator/regulator on the engine, but rarely need to use it.

We resisted installing solar for many years because of the bulky unattractive appearance of aluminum framed panels. We now have 2 semi-flexible panels mounted on a light mahogany lattice frame on top of the cockpit Bimini. Most folks have to be told we have solar because the thin panels are almost invisible.

Compared to the inefficiencies of your old system, I'd be worrying most about things like good box insulation and a circulating fan. Set it and forget it for the thermostat
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