Walk in cooler and btu losses in A/C unit

[Isaac Carlson]

I built a walk in cooler recently. It is 5x5x8 and uses an 8,000 btu air conditioner. I am using an inkbird temp controller and a johnson freeze sensor. I am trying to eliminate thermal leaks.
The blower housing seems to be a very good heat exchanger, since it is just a thin piece of plastic. The air returning to the room is 2* different than the air going into the unit when the compressor is not running. I measured a 15* difference of the plastic inside the housing and the blower moves 200 cfm.

I punched the numbers into a heat exchanger calculator and it said I am "losing" 10,000 btu/day just through the blower housing. That is a full third of my thermal energy. I am planning on pulling the a/c unit and insulating the blower housing. I never would have thought that an air conditioner would have so much energy loss in one little area. I have thin flexible foam sheets that I will use on the housing. My other option is spray foam.

Does anyone have any better ideas? Are my numbers way off? The rest of the cooler is air tight. R25 walls and doors, r40 ceiling, r15 floor over concrete. 2 interior walls, 2 exterior. One interior door and one exterior, again, both are r25+. All seams are sealed. Doors are gasketed/weather stripped. It is physically impossible to slam a door unless the other door is open to release the air pressure.

i have never heard of anyone insulating an air conditioner blower, but it makes sense that it would be a high loss area with high airflow and temp difference. Please let me know if I am out in left field with this.

 

[Corey]

Interesting to see this coming along. 10K btu/day, about 420 btu/hr, 123 watts/hr... seems to be right where the old 1.08 x CFM x dT would put it with a 2 degree dT. Some minor differences for air density, humidity, etc. But guess that seems ballpark reasonable. Main thing being that it is constant and never-ending. I suspect most small AC's are designed to be cheap, not necessarily uber efficient...plus they typically deal with a smaller temp gradient.. AC room vs a freezer.

Curious why the blower would be running when the compressor isn't? Even perfect insulation on the housing, the fan is still moving air, moving air is energy, energy is heat. Seems like you'd want that fan on a controller too? If the fan needs to run outside the compressor duty cycle, possibly time it for a few extra minutes vs 'continuous'. The ideal solution would be a sealed freezer with only freon lines coming in... fan/evap completely inside (running as needed) and compressor with its fan completely outside. Though that would likely require some dissection of the AC and an extended line set to the remote parts.

 

[Isaac Carlson]

I am using this cooler for meat, so it has to have constant air circulation. It is easier to just let the a/c fan run than it is to find space for another fan and lose that space.

 

[Isaac Carlson]

I removed the electeonics from the a/c and wired the compressor to a temp controller. The fan turns on when the controller gets power. I gigured that was the easiest way to wire it.

 

[fbelec]

one question that is a different problem possibly. where is the compressor mounted inside or outside?

 

[Isaac Carlson]

The compressor and fan motor are outside, so 99+% of that heat is not a problem. I have a small 6 watt led light in the cooler, but that is only on when I am in there.

I got the blower housing insulated, installed a gasket all the way around the blower housing to stop air infiltration, and reinstalled the unit. I have been heating the cooler to help cure the caulking in the walls. The heater used to run 56% of the time. Now it runs 19% of the time. The results make me very happy. I have a few finishing touches left on the cooler and then it is meat hanging time.

I was expecting a 35% reduction in run time, but this is amazing.

 

[Isaac Carlson]

I have been crunching some numbers trying to figure out where the extra gain came from. It could be the cfm in the blower. The air conditioner is rated at 200 cfm with all original equipment, but I have removed the filter screen, which noticeably increased the airflow. I also opened up the intake grill area. The factory could also be off on their cfm estimate. My thermometer could be off a bit, but not that much.

I know how many btu's I needed before the insulation, so the before and after are 'for sure'. I just have to figure out what I had wrong to only calculate a 35% reduction when it was really 65%. I did not think I was going to gain that much.

 

[fbelec]

i asked that question because any compressor that runs in cold weather has a heater. most window units don't have heaters on their compressors. i hope this isn't a problem for you

 

[Isaac Carlson]

No heater. I have a friend who builds and repairs refrigeration systems and we both think this is a very good setup.