Window AC only blowing cool air

[markstache]

I use a mechanical 5k btu window air conditioner for my fermentation cabinet. The cabinet is about 40 cubic feet. I've bypassed the thermostat by wiring the inputs together with a wire nut so that the compressor will always run when the unit kicks on via the STC 1000.

My problem is that it can't seem to get my carboys down to the proper temp. They are hovering around 21.5C, whereas I'd like them around 19C or 20C. The air coming out of the blower vent is cool, but I wouldn't call it cold. It may be no cooler than 21C, but I haven't figured out a way to tell directly.

The unit had been running for a long time yesterday. So I decided to open it up to see if anything was obviously amiss. Other than the compressor being extremely hot, everything seemed in order from a visual inspection.

Any suggestions on diagnosing the problem?

 

[Hello]

Unplug the STC-1000 and see if the unit can cool down past 21C?

 

[markstache]

Unplug the STC-1000 and see if the unit can cool down past 21C?

I gave it a try, but no improvement.

Sticking my digital thermometer in the cool vent showed 22.6, which was less than ambient, but not much.

 

[Spellman]

My un-expert opinion suggests that the unit is low on coolant. Is it an old unit or newer? Something else to try, maybe plug into wall and run the thermostat wires to the stc. That would allow it to run the warm up cycle and cool down cycles to help prolong the life (not sure if that makes much difference as mine is wired like yours for my glycol chiller and has been running fine for a year now). When I was building one for a friend his seemed to have an extra sensor on it. I'm guessing one was know the temp and kick the unit on, and I suspect the other was to know if the coil was getting too cold to prevent freezing up, and or keep you from running it like you intend. The extra sensor was in an open copper tube attached to the cooling coil. His was a digital unit though. Just something to look for, hope it helps.

 

[markstache]

Yeah. I'm coming to the conclusion that the most likely solution for me is complete replacement. Perhaps I can sell the old units for parts. So it goes...

 

[DrunkleJon]

One of the ways people get around the minimum temperature thing in window units is with a Coolbot or something similar. Is it perhaps freezing up?

 

[flars]

Try running the air conditioner with the fermentation chamber open to see if the temp of the air coming from the conditioner lowers. May need a vent on the chamber to allow air to flow through, eliminating back pressure.

 

[markstache]

One of the ways people get around the minimum temperature thing in window units is with a Coolbot or something similar. Is it perhaps freezing up?

I've bypassed the thermostat and rely on the STC exclusively. Call it a poor man's coolbot.

I can't rule out the freezing theory, but I've let the unit sit overnight, and when I start it in the morning the air coming out isn't any colder. I think freezing would cause cold air early, and room temp air later.

 

[markstache]

Try running the air conditioner with the fermentation chamber open to see if the temp of the air coming from the conditioner lowers. May need a vent on the chamber to allow air to flow through, eliminating back pressure.

I've done that for short periods (say 10 minutes) with no change.

 

[markstache]

I should add: I appreciate the suggestions, even if they haven't worked out.

 

[DrunkleJon]

One more question. I ask the obvious questions because I work in IT and, well, people like to skip the simpler things. You havent blocked airflow to the outdoors part of the window unit, right?

I have always found that window units take a little while to get cold, but you should get cold air from it. What temp is the outdoor side of the unit at? I have read that window unite tend to give you 15-20 F lower output temperature than the outdoor, so that should convert to maybe 8-11 degrees C.

 

[markstache]

One more question. I ask the obvious questions because I work in IT and, well, people like to skip the simpler things. You havent blocked airflow to the outdoors part of the window unit, right?

Legitimate question. The compressor side (which has the fewest vents) is about 8" from the wall. All other sides are open to the room.

I have always found that window units take a little while to get cold, but you should get cold air from it. What temp is the outdoor side of the unit at? I have read that window unite tend to give you 15-20 F lower output temperature than the outdoor, so that should convert to maybe 8-11 degrees C.

The room ambient is about 80F when I did my measurements. I've read that 15-20F is the best to be expected, BUT I also know people have used window ACs to get 30-40F degree lower output (at the risk of icing the coils, which I don't think is happening in my case).

In the later case, the trick I've seen has been to use a coolbot or bypass the internal thermostat to get the unit to run longer than it otherwise would. The compressor will run as long as the unit is on, but I'm just not getting much in the way of cooling.

 

[markstache]

To wrap this up, I replaced the whole unit. Instead of by passing the thermostat electrically, I wrapped the thermostat tube in some electrical tape. Seems to be working well.