Sunday, April 26, 2015

Modifying a window air conditioner for precise temp control

When we put in the new kitchen,  we choose a single window air conditioner for climate control.  This particular model had both heading and cooling capabilities,  it seemed perfect...  Except for one minor flaw.  It will do hearing or cooling, but only one at a time.  It must be set to heat mode or cool mode,  but it won't automatically switch.  This presents a challenge to maintain a constant temp in a mostly unattended room intended for fermenting,  cheese aging ,  etc.  So finally I decided is time to change it.

What prompted this decision was a nice find..  We also need to expand our cheese capabilities and that requires more automatic cheese vats.  In finalizing plans to build these,  I ran across some really interesting digital thermostat controllers for incredibly cheap!  For about 15 bucks you can get a precise temp controller that not only does heat and cool,  but auto switches between those!
So...  I decided to add one to the window air unit in the kitchen /fermenting room /cheese cave.  Simple right?  Well...  Sort of.

My plan was to simply cut the wires to the heater and compressor,  to write then direct through the new controller.  I wanted to leave original functionality in place and have this as an add on.  So here we go!

First problem...  Find a wiring diagram.  Not easy!  Looking up the model number leaf to the sears parts website where there were no useful manuals,  no diagram.  But...  On one page I found a fuzzy tiny diagram that you could click on to order parts.  I downloaded it,  enlarged it with Windows paint,  and printed.  The keys were barely legible but with some effort I worked it out.  On.  Job one.

Next I tried to slide the air unit it to dismantle it.  Problem...  It was glued!  The installer had used foam insulation around the cracks and effectively glued it to the case.  Wouldn't budge and by this time I was running out of project time.  So..  Pause till I could get help from Bradley.

A few hours latter he had it lose,  not an easy task,  but ready to modify.  Ok.  Back to the project!
Interesting find..  Inside the cabinet is a very nice wiring diagram!  Ok..  Time wasted but no harm done.

Problem...  Checking the plate...  The compressor was within the limits of the new controller of 10 amp at 230 volts...  But the heater was twice that.  Past my comfort zone.! But at this point I had the unit dismantled already and unusable.  What to do...

I realized that I could just hook up the cooling through the new controller and user the existing control panel for heat!  Ok. That could work.   I worked out the wiring for that method.
After some effort wiring this all together...  Time to test.  Exciting!

Well...  Ok.  Problem.  It technically worked but....  The unit controller wad smart enough to turn off the fan when the heating element was off.  So when the extra controller turns on the compressor it runs but no air flow.  Hmmmm. 

Bottom Line is.. I ended up bypassing both coolling and heating relays on the control board, and running the line out to the new thermostat. That way, the air conditioner unit can be left on "fann only" and let the outside temp controller  control the heater and cooler as it needs to

That setup worked great! finally, the kitchen and fermenting room is held precisely at temperature, adjustable on one simple panel, and auto switching from heat to cool as necessary, and hte fan running constantly.

The only downside to this setup is that the outside ambient temp sensor is also bypassed, so the compressor could be engaged to cool when it is too cold outside to run properly.

Oh well, thats as close as I can get for now.

Here is a pic of the finished product:


  1. That's a pretty genius setup. Bet you guys could get a lot of efficiency out of this compared with other people's window units.

  2. I would say this is a job well done! Thank you for being so intricate with the details on how to get this done. I was thinking along the same lines of the end result but was not sure the steps I would need to take to get there. This posting has made it so much easier to finish.

    Jodi Bennett @ Marsh Heating