After running for a few days with the fan off, I’m no longer convinced the fan is the whole problem.  The temperature reading still doesn’t always make sense.  I received the new fan, and I plan to install it tomorrow.  I’m also going to increase the distance between the flower pot sections from 1 inch to 2.  This to allow for more air flow.  I’m thinking in addition to the fan getting hot from the over voltage, that I’m just not getting the air movement through the housing that is required.  The flower pots are a fairly tight fit when they aren’t spaced far enough apart.  Hopefully these two changes will achieve the desired result.  I’ll post an update after I make the changes, and again after I let things settle for a few days.

Update: I installed the new fan this morning and it works fine.  The rating on this fan is 3700 rpm at 24VDC.  The old fan was rated for 12VDC, but the power supply feeding it is rated for 20VDC.  This caused the fan to spin a lot faster than it did at 12VDC, and that was what led me to believe it may have been running hot.  The new fan is actually rated for more voltage than the 20VDC I’m feeding it, so over-voltage shouldn’t be a problem.  The other concern is whether 20VDC is sufficient for the motor to start.  I tested it with a variable power supply on my bench before installing it and it took about 15VDC to start spinning. So 20VDC should be plenty of voltage to run the fan.

I also spaced the sections of the FARS farther apart to help the airflow.  I created some 2 inch spacers to replace the 1 inch ones I had been using.  The extra length required me to remove several sections that weren’t essential for it working correctly.  Then I mounted everything back on the pole outside.

Initial comparisons with my other outside thermometer and nearby stations on Weather Underground look great so far, with little or no solar heating taking place.  I need to let it run for several days to see how things perform over time, as other changes I made in the past appeared to be working at first.  That could have been caused by my opening the FARS up and releasing any warm air that had built up inside of it.  Over time the warm air would build back up again due to poor ventilation.  Maybe I really got it right this time.  I’ll know better after a few days.