My Walk-in Cooler

I needed a larger cold space for lager brewing, grain storage, cold keg storage, etc. I had an old 11K BTU window AC unit given to me, and my 10-6" x 18' garage brewery has a window in the back about 4-4 above the floor, so I figured I'd build a walk-in cooler to use the AC unit. I could keep it at about 48F for lager fermentations, but that is also a good temp for cold keg storage and dispensing of most beers. It is also good to keep grain cold, and I usually have 200+ lbs of grain around so I figured I'd build it big enough for all this.

The back end of the brewery was largely cluttered and unused, except for my grain bins, so I figured I wouldn't lose much space by putting it there. I wanted to build it to be efficient, yet make it somewhat portable by being able to disassemble and transport it, so I decided on making it 8' wide by 6' high by 4' deep, with an opening for the AC unit in the back panel to align with the window. First I calculated the peak heat loss using the following formula:

Heat Loss(BTU/Hr) = surface area X ΔT
R Value

and came up with less than 1000 BTU / hr with a 4' X 8' X 6' size, R10 insulation value, and 40°F temperature differential. The result is that even the smallest AC unit should be more than enough for this size unit.

So, I built panels about 2 3/8" thick of 2" extruded polystyrene insulation board with a pine frame made from 2x3s planed down to 1 1/4 x 2 1/8 (both to straighten and make the same width as the thickness of the foamboard). Both sides were covered with 1/8" tileboard using foamboard adhesive. I built each panel laying on my shop floor, then laying a piece of 3/4" melamine on it and weighing it down with several bags of cement spread out, then letting it set overnight.

The front and back are 8' x 6'; the ends are 3-7 1/4" x 6'; the top is 4' x 8'; and the bottom is 3' 7 1/4" x 7' 7 1/4" but has no facing on the bottom, and has 3/4" melamine on top. The ends set on the floor and butt against the bottom; the front and back set on the floor and butt against the bottom and ends; and the top sets on top of the end, front, and back. I used 1 1/2" wide foam camper tape (used to seal between a pickup bed and camper top) on all these joints and screwed the panels together with 4" deck screws and finish washers. The door edges were beveled at about 15 degrees and again I used foam camper tape covered with metallic tape as the door seal. All interior joints were well sealed with silicone caulk.

Most window AC units have an internal adjustment on their thermostats so that they can be adjusted to achieve temps outside the normal range of an AC unit. Although I was not, you may be able to adjust this and achieve the desired results. You would still need to rewire the unit so the fan runs continuously. CAUTION: DOING THIS WILL PROBABLY VOID YOUR WARRANTY.

I installed the AC unit in the back wall. There is an internal adjustment screw on the AC unit controller, but I could not get it adjusted right to achieve a 48 degree temp, which is what I was looking for. I could get it down to about 55F, but the slightest bit of adjustment down would cause the AC unit to run continuously, dropping the temp at one point to 20F, but freezing up the coils of the AC unit. So I broke down and bought a Johnson controller, and last night I installed it. This morning the temp in the cooler was 48F with no freeze-up of the coils! I had bypassed the internal controller to force it to run continuously, then plugged it into the Johnson. I was worried about the coils freezing up so I was prepared to disconnect the fan from the AC controls and add a power cord to it so that I could make it independent of the Johnson.

Well, after putting some bulk in the cooler, I had some problems with the coil freezing up. Without the fan running continuously, the coils got so cold that when the unit shut off, moisture from the air condensed on the very cold coils and froze them up solid! So I needed to modify the AC unit so that the fan would run continuously, and the compressor would cycle on and off with the Johnson controller. Here is a detail of the wiring modifications:

Whenever working with electrical circuits, be sure that you know what you are doing. Electricity is dangerous and can kill you. I do not advise you to do what I have done - I am only stating what I have done and it works for me. I will not be responsible for any damage or injury caused by attempting to do this on your own.

Wiring Diagram - Original

Wiring Modifications - Fan: I discovered that the fan will not run simply by wiring the hot & common from the fan to a new cord. It needs the additional connection to the start capacitor to get the fan going. I don't know if this is true for all window AC units, but for this 20 yr old unit it was necessary. The capacitor in my AC unit had 3 terminals: the common from the power cord was connected to the center terminal, the fan start wire to one outside terminal, and the compressor start wire to the other outside terminal. Additionally, the fan has 3 speed control wires that came off of 3 separate switches from the switch unit (Low, Med, Hi). I chose to run the fan at the medium speed (may switch to low speed to see if it is adequate - this may extend the life of the fan). I connected a second power cord - common to the center of the capacitor, and hot to the medium speed wire to the fan motor, leaving the start wire connected to one outside terminal of the capacitor. This power cord was plugged directly into the outlet and the fan runs continuously. A better solution might be to wire in a switch so that I could change the fan speed without rewiring.

Wiring Modifications - Compressor: The hot wire on the AC power cord was connected to the hot side of the switch unit. The compressor wire ran from the other side of the switch to the internal controller, then to the compressor. I wired this hot wire directly to the compressor wire, bypassing the switch and internal thermostat. This power cord is plugged into the external controller.

Wiring Diagram - Modified

I built a shelf unit on each end for specific purposes. On the left end it is about 18" deep, with the first shelf high enough so that I can get at least 10-12 cornies underneath for cold storage and/or serving. I've installed my 12.2 gallon SS conical on the shelf, with room beside it for two 6.5 gallon carboys. The top shelf is about 10" high, and I'll probably use it for horizontal storage of corked bottles and other miscellaneous stuff. The one on the right was sized to allow two plastic storage bins that I use for grain under the first shelf. They are about 21" wide by 24" high by 30" deep, and have wheels on them. Each will hold 3 55lb bags. The shelf above will hold other smaller containers of grain, carboys, etc. The top shelf is about 14" high and will hold several cases of beer bottles of all sizes.

I haven't tallied up the total cost yet, but I'd estimate it to be around $550-$600 - certainly much less than anything I could have bought. Though I paid nothing for my AC unit, a new 6K BTU unit runs about $80 and should be considerably more efficient. The total cost would have been a bit less had I built it in using 2x4's, fiberglass insulation, and drywall, but not by much, and it is worth it to me to be able to take it with me if I ever move.

Thanks to Skotrat for info from a previous thread on walk-ins, and answers to some personal emails I sent him. Without that I would probably never have done this.

I was too stupid to remember to take pics during construction. Here are a couple of shots after completion, and a set of plans I designed and used to build it.

Kind of cluttered up, but you can see its relative size.

Here's a shot of the inside showing the AC unit and the left shelf unit with 12.2 gallon conical installed. NOTE: This pic was taken before the wiring modifications specified above.

Construction Details

The following 3 images are drawings of the Front, Back, and End panels, showing the framing layout, overall size, door details, and the opening for the AC unit.

Back Panel.
Modify the dimensions to line up your AC opening with your window (if you are doing it this way).
Overall size is 96" by 72".

Front Panel
Overall size is 96" by 72". Door is 32" x 60" - don't bang your head!

End Panel
Overall size is 43 1/4" x 72" to fit between the front and back panels.

Panel Construction Details: Frame pieces are made from 2x3 pine. I edge and face jointed them for straightness, then planed to 1 1/4" x 2 1/8", using the width as the thickness of the panels. The joints are half-lapped, glued, and screwed. 2" Corning Foamula® insulation board is cut to fit tight and fill in between all frame members. 1/8" tileboard is laminated to the frame using a good gap-filling moisture resistant glue (I used Gorilla Glue), then pressed flat on the floor and weighed down with a sheet of 3/4" MDF with several sand/cement bags spread on top. At the AC opening and where the tileboard hung over the frame edge it was trimmed with a router flush cutter (you could use a laminate trimmer) after curing overnight.

Panel joining details: All panels were joined using 4" deck screws and finish washers, spaced approximately 1 foot apart. Foam camper seal tape was used on all joints. The bottom is approximately 43 1/2" by 91 1/4" with a 3/4" sheet of melamine faced MDF on top (no bottom face). It is sized so that when the end, front, and back panels were joined to it, the overall dimensions would be 48" x 96", the size of the top, which rests on top of the entire assembly and is fastened with 4" deck screws and finish washers.

Door details: Door frame and adjoining jamb members are 2" x 2 1/8" with a 15° angle cut on adjoining surfaces. The door is sized so that there is a 1/8" gap all around, trimming the edges of the door as needed with a hand plane. 3/16" x 1 1/2" camper seal tape is added to the jamb to seal the door, and metallic duct tape covers the foam for protection. 3 spring loaded screen door hinges are used. I mounted a screen door handle, and the seal is a press fit - it seals very tightly.


12/1/04: Due to slight shrinkage of the door frame with time, the tileboard edge began projecting beyond the edge of the door framing. This caused them to stick, and eventually break the adhesive bond to the framing. I removed the door, removed the tileboard, and scraped off the old glue. I reglued them using 100% silicone caulk, then used 1 1/2" drywall screws with finish washers around the edge to fasten in place. Once cured, I used a belt sander to trim the edges of the tileboard so that there was no overhang. I even went a bit further to ensure that any further shrinking of the frame would not cause the tileboard overhang problem again.

1/8/05: I wanted to be able to switch off the power, and switch on an interior light when I went in the cooler. My solution was to put in a 3-way switch (1 terminal is hot, the others are NC & NO). I connected the NO terminal to the outlet that the unit is plugged into, and the NC terminal to a porcelain light fixture with a 50 watt bulb. When the switch is up, power is supplied to the outlet (the AC unit) only; when down, power is supplied to the light fixture only. So I just flip the switch, open the door and go in, and when I come out I flip the switch back on. If I could find a good push-button 3-way switch I could embed it in the door frame so that it would automatically turn off/on when I opened the door. How cool would that be?

I also plan to put in a 3 position switch for the fan so that I can switch between low/med/hi. Or maybe just a 2 way and use low/med. I can't imagine that I would need hi, unless it is during the extreme heat of the summer (it actually hits 90 here a few times a year). Or better yet, I may be able to put some kind of relay in the power line to the fan that is controlled by the compressor power line, but with an additional circuit that would delay the opening of the relay when the compressor shut off. This would allow the fan to run for a period of time after the compressor shuts off, then come on again when the compressor does. It may be as simple as adding some kind of slow discharge capacitor in parallel to the relay - I'll have to research this more fully. Whatever I do I will keep this page updated with the latest modifications.

10/24/05: After forgetting to turn the switch back on a time or two, I added an indicator light to the switchplate a few months ago. It is on when the switch is in the OFF position (meaning the AC unit is off) so if the light catches my eye, I'll go turn the switch ON (up). I haven't left it turned off since, but at least if I do the light will (hopefully) catch my eye.

7/18/07: After hitting my head for the umpteenth time I would suggest that anyone considering copying my plans make a slight modification: Build the front panel in two sections and make the door full height!