Air Conditioning - Replacing window air conditioner thermostat with exterior thermostat

If I replace the interior thermostat with an exterior one for my window air conditioner and set the cooling temperature at 40 degrees Fahrenheit will the air conditioner be capable of cooling as low as 40 degrees?

NOPE!
Refrigerant inside the evaporator coils is about 38F or so. You would need a monstrously oversized window unit (which, by the way, won't fit in your window but will need half of a wall) :D to bring the temperature down to, say, about mid-50's F or so.

And I have not yet seen a cooling thermostat that allows for a setpoint below 50F

NOPE!
Refrigerant inside the evaporator coils is about 38F or so. You would need a monstrously oversized window unit (which, by the way, won't fit in your window but will need half of a wall) :D to bring the temperature down to, say, about mid-50's F or so.

And I have not yet seen a cooling thermostat that allows for a setpoint below 50F

The space I want to cool to 40 degrees is well insulated and only 15' square. I have dormant plants in it that I want to stay dormant through March and April, so the cooling load will be relatively low. If the load is small enough, can the evaporator coils cool the air to 40 degrees? I found a thermostat called the "Dayton heat or vent thermostat" that can be set as low as 30 degrees.

Like said . A window unit cant do what you want. Think more like you want to build a walk in Box there. Then you can buy a condenser unit to sit some place and pipe to a blower coil you hang in the room. That way you can take the room down to what you want. Think more of a unit thats like on a fridge. thats what you need .

NOPE!
Refrigerant inside the evaporator coils is about 38F or so. You would need a monstrously oversized window unit (which, by the way, won't fit in your window but will need half of a wall) :D to bring the temperature down to, say, about mid-50's F or so.

And I have not yet seen a cooling thermostat that allows for a setpoint below 50F

Which coolant is this? Every fluorocarbon-based coolant has a boiling point in the -20C range...

I've actually heard of this being done before.

Check this out: http://www.mikevanelst.com/chiller/chiller.html

I will concur that your project is not workable.

The example above would not be considered reasonable for a diy'er considering that the components used to cobble together are expensive and need a certified tech to install.

An R-22 a/c system is designed for a 34 degF evaporator.
A cooler requires a 20 degF evaporator temperature which an R-22 compressor can do.
However, if you were to lower the evaporator temperature as they did in the example your BTU cooling capacity would drop to next to nothing.
This is why this project is to cool a tiny cpu, not a large area like what you propose.

Not to mention that the evaporator temperature you need will require a significant amount of defrost time which will also be a problem.

Sorry but you need a unit designed for the application.

I still disagree with the proposition that it is IMPOSSIBLE to convert a window AC unit to a refrigeration device.

The biggest problem is when the temperature in the room drops below the dew point. In a room with zero humidity, this is a non-issue, but this is also impossible to achieve. When the temp decreases below the dew point, condensation will freeze in the evaporator, ceasing air flow through the unit.

Regarding the units themselves, the coolant does reach temperatures well below freezing. Once the coolant leaves the compressor, its pressure is much higher (it becomes a very hot, high pressure gas). The back of the AC unit (the fins) act as a heat sink to help dissipate the heat quicker.

Once the coolant leaves the condenser, its temperature is cooler and it's now a liquid at high pressure. This liquid goes into the evaporator and is allowed to convert back to a gas. When this change occurs, heat is lost from the room you're trying to cool.

Remembering that hot air is lighter than cold air, it's ideal to place an AC unit higher up in the room.

Now, why would people say that it's impossible to cool a large area with a simple window AC unit? Because it can be if done wrong. The only reason an AC unit won't cool a room to the temperature of pipes (when steady state is reached) is that the BTUs/hr "lost" (through air leaks and such) is larger than the BTUs/hr "gained" (through the AC unit).

How can we fix this? Insulation. Lots of it. Making the structure airtight is the only way to achieve this. As for cooling plants, this will be difficult: there needs to be air exchange or the plants will die. So now you have two things to be concerned about: enough air exchange to keep the plants alive and the BTUs/hr lost below the BTUs/hr gained.

Regarding proof that this works, I'll even post another link:
http://www.brewrats.org/walkin.cfm

This is actually very do-able. (no really!) I've seen a kit that some company on the net sells that you use to modify your window unit and install the unit in an insulated room/box/area to do this.... lemme see if I actually remembered to book mark this thing...

....sipping coffee...

....dangit! I can't find the link! I remember something about the 'box' kit and having to do a few thing like attach a probe (probably a heater) to the sensing bulb, a temp sensor in the supply and/or return and the like. I think that it was doing this to control the compressor remotely.

Not sure how defrost cycle was handled, just cutting the compressor and running the fan I guess.

...going to look a bit more here... Well I give up, can't think of any good key words on google that will pop it up either :/

Michael

Aha! I found the link! Check it out!

http://www.storeitcold.com/

Michael

You are wanting a mid temp refrigeration system. This can be done with r22 but the design parameters are totally different than found in a window unit.
Check with a restaurant supply stor you should be able to get something pretty cheap.