The 'Super Fridge' (upright freezer conversion)

Principle 12: Creatively use and respond to change

I'm one for trying out different things, and I like to see what we can get away with before committing to something bigger and 'better'. Fridges and freezers are one of the biggest energy consumers in the household - usually behind heating / cooling and hot water systems.

Our upright freezer to fridge conversion in action

We've trialed a number of fridge systems here before settling on the upright freezer conversion.
  • First we used an old 150lt bar fridge that used around 670Wh per day
  • Then we bought a 150lt chest freezer (using around 466Wh per day) for preserving bulk food - with the idea of a possible conversion.
  • Our bar fridge died two weeks later so we used ice from our freezer to create a 'ice box bar fridge'
  • We then tried a smaller old Engel fridge (about 50lt) that I had in the Kombi that used about 420Wh per day.
After not being entirely satisfied with any of the above I finally decided to try out what I always wanted to and convert a freezer to a fridge. I first read about this in Renew magazine (I think) many years ago, along with this article about a chest fridge that captured my imagination. Freezers are super insulated compared to a fridge and the motor wouldn't work as hard, so it should (?) last longer - we will see. The chest fridge idea makes a lot of sense, as the cold air can't easily escape when opened, but after having bought one and used it for a while I found it was a pain, so I wanted to try an upright.

The things that I like about the upright are that they take up less floorspace and its much easier to access / stack goods. There are elements (if that's what they are called) on each shelf, so the air is cooled evenly throughout. The drawers are clear, and hold the cold air, even when the door is open. But they do come with drawbacks: they are less efficient that a chest design, water condenses on the elements and drips into the drawers - more of an issue in the top two shelves, so food should be stored in containers. They don't have a drainage plug, so the probe needs to break the door seal, and water removed manually. The plastic drawers are clunky, could break and be difficult to replace.

I didn't want to spend a lot of money on the freezer, as I wasn't sure that the system would work well. Not long ago Aldi had a 190lt upright freezer on offer for $299 and I jumped at the chance. Getting to the store on opening there was a crowd already lined up. I managed to get one of the four on offer, finding a large trolley in store to take it to the checkout. Thanks to the fella that gave me a hand with it! It's a bit of a mission taking a freezer through the checkout - and then getting it home.

The freezer I got is only a 2.5 star rated one, not great - but I figured that it would be fine for it's purpose. When used as a freezer it's expected to use 334 kWh per year - 915 Wh per day.

The Control Box

NOTE: I can't recommend that you attempt this yourself as there are high voltages involved in the conversion.

I asked a friend of mine, John - an electrical engineer, about the conversion. He had trialed one in the past and made a programmable controller for the purpose, that he kindly loaned to me. He suggested that I buy a digital thermostat controller on eBay, around $15 delivered - much cheaper than he could make one up for - and make the controller up myself (being the DIY kinda guy that I am).

The basic idea of it is that the temperature probe determines the range at which power is supplied to the freezer, so that it turns on at say 7º and turns off at 3º (these are the settings that I am currently using). I did have it set lower, but found that the temperature sensor has a delay, and so the air space continues to get colder after the power is shut off. You can also set a delay, which allows for fluctuations as the door is opened and closed - I set mine at 2 minutes.

I wanted to wire it up to a power point so that I could plug the freezer in, rather than cut the cable on it. It also means that you could use it for other devices easily. I ran my old bar fridge power cable to the controller (see wiring diagram), and then onto the power point (earth running directly). This was all fitted into a plastic box to keep everything together and looking neat. I extended the wire for the probe so that I could run it up the back of the freezer, along the top and into the back of the freezer compartment. I used white electrical tape to fix it, and used tape to reduce the air gap at the entry point where the wire breaks the door seal. I did this at the top of the door to reduce potential losses through the breaking of the seal.

Digital Temperature Controller - programmable and very flexible device.

Wiring diagram - the wire on the right (to #4) is negative, left (to #3) is positive - it wasn't clear here.

Controller fitted into a box with a power point, so not having to modify the freezer wiring.

Wire for probe extended and fed through top of freezer to be fitted at the back - see water condensing on 'elements'.

The Results

We've been running the 'super fridge' for a bit over a month now and here are the results from my testing:
  • 19/3/13 1:30pm began testing
  • first 29 hours 330 Wh = 273 Wh per day
  • 28/3/13 8:30pm (9 days 7 hours / 223 hours) 1.89 kWh = average 203 Wh per day
  • 12/4/13 10:00pm (24 days 8.5 hours / 584.5 hours) 4.39 kWh = average 180 Wh per day
  • 25/4/13 10:30am (approx 37 days) 6.12 kWh = average 165 Wh per day
Interesting to see that it's dropping - perhaps because we are entering a cooler time of year? Looking at the use between periods show more typical consumption once the system has settled in. 

28/3 - 12/4 (approx 15 days) 2.5 kWh = 167 Wh per day
12/4 - 25/4 (approx 12.5 days) 1.73 kWh = 138 Wh per day

More testing with a better quality device would ensure more accurate results - but these results indicate some great savings. It's not uncommon for similar size fridges to use five times more electricity. The 'super fridge' uses around 165Wh per day, as compared to the same unit used as a freezer which was rated at 915 Wh per day, about 80% less power.

As a side note - I think that cool cupboards should be built into every new home, which helps to reduce the size of the fridge needed - but more research really needs to be done on how to design them well. I am looking at fitting a fan inside ours with a digital temperature controller to help it draw air through when needed - to help regulate temperature. More later...

UPDATE: See the 5 year update post to see how it performed over time.


Anonymous said…
Good to see.... we've done this too (twice!), and I'm amazed at how cheap you got your controller for... ours cost $70!

There's no way I would buy a regular fridge after doing this now.

And our cheese fridge which runs at ~ 10C
antonio said…
What a great article, so very helpful. thanks
Anonymous said…
Hi Richard, Just rediscovered your blog after seeing it some time ago and losing the link. Will add it to my reader now.

Great fridge conversion - really amazing results! We bought the most efficient fridge/freezer unit we could find about 3 years ago (which cost us extra to do so) and this uses about an eighth of the power of that! Really impressive, thanks for sharing.
Nate said…
Just wondering - what box did you use for your control box? And how is this running years down the track? As expected?
Anonymous said…
Did you modify the freezer 's thermostat in any way or remove it ? Thanks for reply if you find my line.
Richard said…
The themostat, in the freezer was not modified in any way. The freezer is in it's original stat and still working fine 2 year on.
Anonymous said…
Can you better explain how this process works? Did you only hook-up the positive/negative power wires along with the temperature probe to the new controller? You didn't hook anything up to the cold/hot spots on the controller? I don't understand how this is working for you if that's the case. How is your controller able to kick on the compressor when it needs to? This is something I'm considering doing but I'd really like it if you could give us some more details. Thanks!
Richard said…
Not sure that I can explain this better than I already have - there's lots of photos there. I'm no electronic whiz, so I can't explain this aspect.
The controller is powered by the power point, the controller can be programed to turn on and off using the thermostat readings. The freezer is plugged into the controller and so is turned on and off as determined by the program in the controller.
Hope that helps.
Anonymous said…
Thanks for that clarification. I didn't realize it was as easy as that. I assumed I had to cut into the wires within the freezer as opposed to this. So it's basically an extension cord of sorts with a temperature controller to power the freezer on and off accordingly. This will make things a lot easier. Thanks again for the article.

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