Replace Dead Cooling Units with Air Conditioners

I have an old Hussmann refrigerator with two sliding glass doors, that was originally used as a refrigerated produce case in a grocery store (I use it to store eggs from my free-range egg farm). This was a nice unit in its day—it’s built like a battleship—but its refrigeration unit is shot, and was an inefficient dinosaur even when it ran properly.

These old refrigerators are good news/bad news. The good news is that their decrepit refrigeration units mean that you can buy them almost nothing. The bad news is that their decrepit refrigeration units mean that they’re worth almost nothing.

New Life for Old Refrigerators

After having mine repaired twice at about $350 each time, I took the advice of the HVAC technician:

“Get a window air conditioner, replace the thermostat so you can turn it down to 40 F, and stick it in the side of the refrigerator.”

So I did just that. For $179, I got a new 7,500 BTU air conditioner. I cut a rectangular hole in the side of the refrigerator with a saber saw and installed it, sealing the edges with aluminum tape.

Window air conditioners are cheap, energy-efficient, quiet, and lightweight. You can install them yourself if you know how to saw a rectangular hole in the side of a big metal box and how to do simple wiring.

Walk-in Coolers, Too

These same techniques work for walk-in coolers, either for existing or DIY walk-in coolers.

Sizing Your Air Conditioner to Your Refrigerator

For my purposes, the smallest unit I could find was actually too big, and I ran it on “low” most of the time. I would have been better off with a little 5,000 BTU unit for my 50 cubic-foot refrigerator. You could probably run a small walk-in cooler on a larger window air conditioner!

Converting the Air Conditioner to Refrigeration Work

The only thing you have to modify is the thermostat. There are several ways to do this.

The main problem to be solved is that your air conditioner doesn’t want to go below 60 °F, but we want it to go below 40 °F.

Best but Hardest: Direct Replacement of the Thermostat

All you do is replace the original mechanical thermostat with one that goes down to 40 °F or below. Simply unplug the original one and replace it with one that will go down to refrigerator temperatures.

This requires that your air conditioner has a mechanical thermostat. Most new ones have electronic thermostats, so may require an older unit.

You might be able to do a direct replacement, mounting the thermostat in place of the old one, and even use the original thermostat’s knob to there are no externally visible changes.

The business end of the thermostat, the capillary tube, needs to be mounted against the fins on the cold-side radiator of the air conditioner. This is the part of the air conditioner that ices up, but if that’s where the thermostat is, setting it above 32°F will limit how much ice can form before the thermostat shuts it off.

Fooling Electronic Thermostats

If you want to do things the easy way, you can fool the electronic thermostat with a CoolBot controller.

We added one to our cooler when our original A/C unit died and we replaced it with a unit with an electronic thermostat. Without an easy one-to-one thermostat replacement, we decided to use a CoolBot controller.

CoolBot has a probe for the cooling fins to prevent icing, plus another one for the interior of the refrigerator, and their secret sauce: a tiny heated cable that you attach to the AC unit’s existing thermostat sensor. This fools the AC into thinking that the room is hot and the A/C needs to be on.

Since the A/C unit’s thermostat sensor is on the front of the fins, all you have to do is pop off the front grille: you don’t have to disassemble the unit.

Here’s one farmer’s walk-through of his installation in his walk-in cooler:

The Night Light Trick

If you warm up the temperature probe on the A/C unit, it turns on.

Instead of buying a CoolBot, the farmer in the video below used an external thermostat to turn on a night light, which warmed up the A/C unit’s temperature probe.

He used a heater thermostat and a reversing relay, but you can also use an inexpensive digital temperature controller for a simpler installation:

• Mount the controller’s temperature probe against the A/C unit’s fins.
• Plug the night light into the temperature controller.
• Mount the A/C unit’s temperature probe near the night light.
• Plug the temperature controller and the A/C unit into the wall.
• Set the temperature controller to the desired temperature.

Other Tips

In general, a refrigerator is just a big insulated box, so you can saw holes in it wherever you want. However, there might be heater cables here and there to prevent the unit from freezing up, and wires to run fluorescent lights and such. These are usually in obvious places, but unplug the unit before using the saw! If you find any severed wires, you have the choice of not using the fancy-pants features like lights and defrosters, or opening up the junction box and disconnecting anything you broke.

Don’t bother removing the old refrigeration unit, fans, or anything else you’re not using. They’re not doing any harm.

The air conditioner will drip on the outside with water it’s condensed out of the cold air. Put a drip pan under it.

Brush or blow the crud out of the air conditioner once in a while.

How Well Does It Work?

My first A/C unit died suddenly after four years of continuous operation, which was about what I expected. The second A/C unit, with CoolBot, has been in place for eight years or so. (The original refrigeration unit broke every year and averaged $350 in repairs.)

The seals on the refrigerator doors aren’t what they used to be, and on a very hot day the air conditioner is on for hours at a time. Gotta fix that someday. But the same techniques (and the same air conditioners) can cool a walk-in cooler with tight-fitting doors.

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Author: Robert Plamondon

Robert Plamondon has written three books, received over 30 U.S. patents, founded several businesses, is an expert on free-range chickens, and is a semi-struggling novelist. His publishing company, Norton Creek Press, is a treasure trove of the best poultry books of the last 100 years. In addition, he holds down a day job doing technical writing at Workspot. View all posts by Robert Plamondon