Common Pneumatic Thermostat Problems

Someone once said, “There is no such thing as a dumb question.  There are only dumb answers.”  I subscribe to that thinking and with it in mind I have written down some of the more common problems we encounter when rebuilding stats.  For your consideration here are my observances, checks, and weird things that you can try or try to avoid.

Honeywell TP970 series thermostats   Honeywell installs a couple of  protective pieces of foam rubber when shipping their stats.  They are lodged between the bi-metal and scale plate assembly and SHOULD BE REMOVED before installation.  They are practically out of sight and you may get some functionality, albeit reduced, even without removing them.  Honeywell does mention them last in their 10 step installation instructions.  I have seen many a dirt, dust-encrusted older stat come in with many a dirt, dust-encrusted older foam shipping pad.  Sometimes the best hiding places are in plain sight.

Barber Colman (Schneider Electric) TK Series   To achieve a branch reading where none apparently exists, try checking the branch port O ring.  Sometimes, the point of the branch needle valve screw is able to alter (flatten) the center of the O ring to the point of cutting off the branch signal that is supposed to be going to your pressure gauge.  To rectify this, simply ease a very small hex key (1/32) or a pin if you are surgeon-like careful and manipulate the center area of the ring until you hear air.  Make sure the stat is calling for full or mostly full branch-pressure.

Johnson T-4000 Series  These stats have bi-metals that sit on center atop a cam(s).  The bi-metals also have a calibration screw that is locked in with a ¼ lock washer.  Many a stat comes in where the ¼ inch lock washer has been tightened too much.  Too much as in the bi-metal has slipped off the cam completely (and to the right).  It is easily overlooked by even young, healthy eyes resulting in no control.  Lack of control is due to no corresponding scale/cam vs. temperature/bi-metal position.

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