warming the temp probe on an A/C unit

[limulus]

Hi Lads,
I have been using a fermentation cabinet that I built several months ago. It cools with an A/C unit and an STC-1000. The temp probe from the A/C is outside the cabinet. In summer, it stays warm enough in the garage to fool the A/C compressor into coming on when the STC-1000 applies power. Now that it is not 80F+ in my garage, I have noticed it runs a lot longer and I think that is because the compressor is not running much. I was going to buy some pipe heating tape to wrap around the probe, but I think those kits have a thermostat that cycle on and off below freezing. My garage is fully insulated and even has insulated doors, so it does not get frigid inside. But it is definitely not hot inside now. Has anyone come up with a way to keep their probes warm enough other than buying a Coolbot? Other than the tape, I thought about wiring a 100w light into the cold side of the STC-1000 so it would come on when the A/C is called on to chill the cabinet.

 

[Junkster]

Can't you just electrically bypass the factory control so that it runs all of the time when the STC-1000 supplies power the the unit?

 

[mcl]

It is very easy to bypass. Find the thermostat and cut the wire going in and cut the wire going out and connect them together.

 

[limulus]

It is mounted and running. I can crawl inside the cabinet and if I can get to it that way, I may give it at try.

 

[makokiller]

How about using a coolbot? http://www.storeitcold.com/

 

[limulus]

How about using a coolbot? http://www.storeitcold.com/

Thanks for the reply, but if you'll read my entire first post, you'll see I did not want to buy a coolbot. It is not out of the question, but for now, I'm going to try something different. My cooler is only 4x4 and the coolbot cost as much as I have in the whole thing! Soon, it won't matter and the STC-1000 will only need to apply a little heat to the cabinet instead of cool air. The first really cold air of the season is coming on Tuesday and the temps are dropping into the 20s.

 

[Bobby_M]

How about letting the controller switch a 12v DC source to a resistor packed tight against the ac sensor? Generating something like 5 watts should be plenty if you insulate.

 

[limulus]

How about letting the controller switch a 12v DC source to a resistor packed tight against the ac sensor? Generating something like 5 watts should be plenty if you insulate.

Now that is something I can definitely do! I have a lot of extra resistors from my old speaker crossover building projects. I assume a little 12v power adapter from an old phone or something will also work.

 

[Bobby_M]

Well, let's figure it out. 12 VDC into 30 ohms pulls .4 amps and generates 4.8 watts. As long as the wall adapter is rated for at least 400 mA you're good to go. Warning though, make sure the resistor is at least 5 watt capable. That would be something the like the diameter of a pencil. You can also just use a 12vdc light bulb, a couple watts...

 

[limulus]

It is very easy to bypass. Find the thermostat and cut the wire going in and cut the wire going out and connect them together.

Either I've just killed the compressor on this A/C or the weather is having some effect on it. I have a a halogen shop light, which is hotter than the surface of the sun, right next to the probe of the A/C unit and it still runs for way too long now. It used to take 2min or less to get them temp back down. It was only $99 but I've only had it a few months and it never had to run very much. I wonder if using the STC-1000 and not allowing it to properly cycle on/off has killed it. I think I'm going to look for the wires you speak of and give it a try. I may even use my small brain and unplug the damn thing first:smack:

 

[mcl]

I wonder if using the STC-1000 and not allowing it to properly cycle on/off has killed it.

The STC-1000 has an anti-shortcycle setting, I believe mine is 3 min. If you have trouble post some pictures. I have done this on 3 AC units.

 

[limulus]

Short cycling is not the problem at all. It holds the temp for an hour and I do have a delay set in the STC-1000. When it does power on, it is now running a really long time and I don't know why. Before, it would run <2min and stop and that is when it was in the 80s and humid. The garage is 73F inside right now and I let a halogen shop light shine on the probe all night. I have the STC-1000 set for 18.5C. The A/C was new and only $99 but I would think it would last longer than one season. We have central air and don't use window units in our home. But, I think under normal operating conditions, the window unit will run a few minutes after the compressor turns off, correct? Using the STC-1000, the A/C and compressor turn on and then off with no in between. Could that be bad for the compressor?

I'll need to get inside my fermentation cabinet try to find the thermostat. The A/C is sealed in place with Great Stuff. It is a GE with a single analog control. The probe was right in front and I simply bent it around the side of the A/C and out of the cabinet. Then sealed around the edges of the A/C with Great Stuff.

 

[Junkster]

Can you post a pic and the model number of the A/C unit? What is your desired temperature inside the cabinet? If you're shooting for lager temp & it's relatively cool in the garage, it's possible that the refrigerant pressures / temps are are way below what it's designed for and not moving heat the way you want - do you know if the evaporator is icing over? If the temperature control bulb is a cap tube, be careful that it doesn't get kinked if you've relocated it. Also, does this unit have the option to run the fan continuously and a fresh air option?

 

[limulus]

I have no worries now. The garage is at 18.5C and the inside of the fermentation cabinet was 19.0. The STC-1000 is set to 18.5 with a 1C temp differential. When it warms up again, I hope the A/C works as it did back when it was much warmer.

 

[Junkster]

Hopefully, you're all set then!