Modifying room air conditioner to cool below 60 deg. F

Thread Starter

PeteHL

Joined Dec 17, 2014
340
Not sure that 60 deg. Fahrenheit is the lowest temperature that room air conditioners are normally designed to cool to, but assume that that is correct. What I want to do is, in a simple way, to make the conditioner's compressor not stop running until after a "room" temperature of about 40 deg. Fahrenheit has been reached.

In other words, the room (window) air conditioner will be used to make a walk-in refrigerator.

Assume that the air conditioner has a thermistor-type room air temperature sensor. A thermistor can be either postive or negative temperature coefficient (ptc or ntc). Another given is that the temperature setting of the air conditioner (the desired room temp) is 60 deg. F.

If the temperature sensor is a ptc, then I would connect a rheostat in series with the thermistor. In that way, assuming a correct resistance value of the rheostat, resistance of the thermistor is less than normal when the compressor is shut off as room temperature reaches 40 deg. F.

If the temperature sensor is ntc, then I would connect a rheostat in parallel with the thermistor. Where the resistance of the rheostat is correct, then resistance of the thermistor must be greater than normal before the compressor is shut off and room temp. again equals 40 deg. F. Given a 40 deg F. room temperature, the total resistance of the thermistor in parallel with the resistor is equal to the resistance of the thermistor with respect to room temp. equal to 60 deg. F lacking the rheostat connected in parallel with it.

These two different possible circuits are shown in the drawing below.

Over the past two days I have been trying to understand the workings of air conditioners, but I have previously done very little with them. So I'm looking for advice from anyone who is experienced with them as to whether or not what I have in mind is practical and not cause damage to an A.C. .

thermistor.jpg
 

Dodgydave

Joined Jun 22, 2012
9,169
If your sensor is a NTC type the resistance increases with low temperature, so shorting it out makes the AC unit think it's at hot temperature.


A PTC type is the oppsite.
 

wayneh

Joined Sep 9, 2010
16,390
What I want to do is, in a simple way, to make the conditioner's compressor not stop running until after a "room" temperature of about 40 deg. Fahrenheit has been reached.
They vary. The one I was just working on uses a thermistor in front of the evaporator (indoor) coil. The controller is programmed to shut down if the sensor sees ambient 62°F and that doesn't work for me because I'm using the A/C to dehumidify my basement. In the spring it's easily below 62° down there. So I removed the thermistor from the coil to outside the innards, and mounted a small lightbulb next to it to keep it warm. The A/C now runs constantly.

An older A/C I had previously used a completely manual, old fashioned dial with no fancy electronic controller. When cranked all the way down, it just ran all the time. I have no idea what temperature could have shut it off.

It's obviously a little trickier if you want it to shut off at some temperature and not just keep going. That's not an issue in my basement - there's simply no way it could be that cold. Rather than alter the A/C, I'd be tempted to use an external thermostat controller to switch power to the unit. Of course you'd still have to fool the thermistor.

Have you examined your A/C to see how it controls? Do you have some idea of what capacity you'll need?
 

ebeowulf17

Joined Aug 12, 2014
3,274
Is there a risk that as you approach freezing, the condensation forming on the fins freezes solid, and then you get no more airflow? I seem to remember that a coffee shop I worked at had an AC unit the froze up periodically. Presumably there's a greater risk of that as you get colder.
 

BobaMosfet

Joined Jul 1, 2009
1,108
@PeteHL - Your A/C for a room is NOT the same as that used in a refrigerator or freezer. It may freeze (the coils) if below 68F. It's not really designed to cool a large area below that. It was engineered to 'cool' rooms for humans, not for refrigeration. Freezing the coils can cause damage, work the compressor harder, and/or cause unit failure or shorten it's lifespan.
 

wayneh

Joined Sep 9, 2010
16,390
I've never seen one of these small refrigeration units (A/C or dehumidifier) without a sensor to shut it down if ice is detected in the coils. The sensor is usually near the bottom in the first 25% or so, where the coil will freeze first. This sensor is not part of the thermostat system.
 

Thread Starter

PeteHL

Joined Dec 17, 2014
340
Have you examined your A/C to see how it controls? Do you have some idea of what capacity you'll need?
Actually I am inquiring for a neighbor of mine who needs an inexpensive walk-in refrigerator for his bread baking work. I don't know whether or not he already has the window A/C that he would plan to use for his walk-in. Now that I think that I might have a basically cost-less method to make a window A/C do what he wants, I'm going to find out about that.

Regards,
Pete
 

Thread Starter

PeteHL

Joined Dec 17, 2014
340
Is there a risk that as you approach freezing, the condensation forming on the fins freezes solid, and then you get no more airflow? I seem to remember that a coffee shop I worked at had an AC unit the froze up periodically. Presumably there's a greater risk of that as you get colder.
Here in the states using a room (window) A/C to cool a walk-in is currently being put to practice, although I haven't specifically run across anyone else using the technique that I'm describing here. Ice on the fins I agree I think would become a greater possibility if the A/C is cooling close to freezing temp.
 

MisterBill2

Joined Jan 23, 2018
6,087
Is there a risk that as you approach freezing, the condensation forming on the fins freezes solid, and then you get no more airflow? I seem to remember that a coffee shop I worked at had an AC unit the froze up periodically. Presumably there's a greater risk of that as you get colder.
This is a very important consideration indeed. I had exactly that problem with a double-door cooler. The evaporator kept the thermostat at 38 degrees, as set, but the solid block of ice stopped the air flow. So you will need to do a defrost every couple of days. And I like those mechanical thermostats. The same cooler has a computer controlled, totally goofed up, microprocessor controller that does not control right. So the control is now a mechanical thermostat and the electronics just runs the temperature display. And we manually defrost it every 4 days.
 

BR-549

Joined Sep 22, 2013
4,938
I think it's a terrible idea. AC is not designed to operate in the temp range needed for refrigeration.

Walk in coolers are even more designed than just the temp range. They have to be designed for fast and often recovery. They are workhorses.

Just a passing thought.
 

wayneh

Joined Sep 9, 2010
16,390
The u-pick farm I go to uses a window A/C to chill their small walk-in cooler. I think it just runs non-stop but I’m not sure.
 

MisterBill2

Joined Jan 23, 2018
6,087
You will need to modify the air flow so that it does not constantly bring in fresh air. That will be important. The present thinking among those who believe that they are much smarter than everybody else, is that all cooling systems must constantly have an air exchange. So you will need to completely block that function. The constant air change will bring in the humidity to frost up the evaporator more quickly, and also reduce the efficiency of the cooling process.
Any window unit should be OK to run constantly unless it is total junk, which unfortunately some of them are.
 

wayneh

Joined Sep 9, 2010
16,390
Any window unit should be OK to run constantly unless it is total junk, which unfortunately some of them are.
On that note, my research found that only Frigidaire offers a 5-yr warranty on the sealed parts. Most are only one year. My last Frigidaire A/C ran for two seasons and they replaced it without much hassle. So I don't know if they're built any better or worse, but their warranty is better. The last half dozen such appliances I've dealt with (A/C and dehumidifiers) have all failed on lost freon. The rest lasts forever but the freon is gone after one season or two.
 

Thread Starter

PeteHL

Joined Dec 17, 2014
340
You will need to modify the air flow so that it does not constantly bring in fresh air. That will be important. The present thinking among those who believe that they are much smarter than everybody else, is that all cooling systems must constantly have an air exchange. So you will need to completely block that function. The constant air change will bring in the humidity to frost up the evaporator more quickly, and also reduce the efficiency of the cooling process.
Any window unit should be OK to run constantly unless it is total junk, which unfortunately some of them are.
My thought was that sizing the air conditioner to the size and insulation of the room would be important. You would want to have the A/ C cooling the room fairly slowly so that when the air temperature is approaching 10 degrees above freezing, most of the humidity in the room has been removed. That would help to prevent the fins icing up. But then if as you say fresh air is being constantly brought in, then that would defeat achieving low humidity.
 
Last edited:

wayneh

Joined Sep 9, 2010
16,390
In the 10,000BTU A/C I just installed (as a dehumidifier in my basement), the hole that lets fresh air in is less than the size of a quarter. There'a little plug connected to a rod that goes to the front control panel, allowing you to plug the hole or not by pulling the rod out or pushing it in.
 

Ioannis66

Joined Nov 7, 2012
31
The refrigerator has fins with bigger distance so ice is more difficult to prevent air flow.
I think room A/C is not suitable for what you are after.

Ioannis
 

MisterBill2

Joined Jan 23, 2018
6,087
The simple way to learn if the window unit in question will work as desired is to first disconnect the mains power and then bypass the temperature control device, and then put it all back together and try it. With constant running it will eventually cool a room to some temperature, which will either be cool enough, or not. Depending on the humidity frost buildup may be a big problem or just something to watch out for. Not knowing the exact situation everything is best guesses, some based on experience and some not that well based. And a temporary bypass is something that can be undone if it does not work out.
 
You will need to modify the air flow so that it does not constantly bring in fresh air. That will be important. The present thinking among those who believe that they are much smarter than everybody else, is that all cooling systems must constantly have an air exchange. So you will need to completely block that function. The constant air change will bring in the humidity to frost up the evaporator more quickly, and also reduce the efficiency of the cooling process.
Any window unit should be OK to run constantly unless it is total junk, which unfortunately some of them are.
Yes, you need to keep the unit from bringing in outside air. But obviously, you can’t block the condenser exhaust air. It’s the reason you can’t open your refrigerator door to cool your kitchen.
 

Ylli

Joined Nov 13, 2015
949
A properly designed window air conditioning unit should not ice up. Currently, R410A refrigerant is used, and typical low side pressure for a window air conditioner is 120 PSIg. Checking the temperature/pressure charts, vapor pressure of R410A of 120 PSIg translates to an evaporator temperature of about 40°F. That is as cool as it gets.
 

BR-549

Joined Sep 22, 2013
4,938
Have you ever studied evaporators? If you have ever studied boilers, then you should understand evaporators.

An evap is just like a boiler, only it has a reverse heat flow. And just like a boiler, the temp of the evap, is controlled by the evap pressure.

An AC evap is designed for a AC temp range. The temp of that evap, must be lower than that range.

So, if you want refrigeration, that evap temp, needs to be lower than the refrigeration range.

Which is lower than AC range. AC with a 40 degree evap, the air never reaches 40 degrees.

Using an AC for refrigeration, is like using a boiler, to melt steel. An AC just doesn't get cold enough, like the boiler never gets hot enough.

Unless, >40 degree cold source, is the coldest temp you need. Like an AC.

The FDA recommends keeping your refrigerator at or below 40° F (4° C). AC can not do that.

And there are other things that must be taken into account, like the super heating needed at the end of evap......before going into compressor.

Lots of engineering goes into controlled cooling.
 
Top