Skip to content
Thermostat Calibration

Thermostat Calibration

Thermostat calibration should only be done by an experienced service technician, as there is no "universal" way to calibrate a thermostat. A BJ thermostat, KX Thermostat, and FD thermostat can all be calibrated, but are drastically different thermostats. This is not meant to a specific guide for your application, rather a general help.

If you are in need of a calibration screwdriver, please see our TOOL032

Fenwal operating thermostats:

VERY IMPORTANT, DO NOT SPIN THE SHAFT! This includes before or after installation. It may be tempting, but don't do it! These are calibrated from the factory, and once you start spinning it, there is no way to get back to that calibration. These thermostats have a dial plate with a stopper pin and a pointer knob that stops by hitting said pin. This is there to prevent overturning of the thermostat, taking it out of calibration. Once the spinning begins, there is no way of knowing which direction or how many times it was spun.

If the thermostat needs minor calibration because it is off a few degrees take the following steps:

Bring the unit up to temperature and make sure it has been running approximately 15 minutes. Put a thermometer in the unit to get an accurate reading of the temperature. Remove the knob (it most likely has 1 or 2 set screws holding it in place). Set the pointer to the temp indicated on the thermometer and retighten the set screws. 


BJ Thermostats:

Set the temperature to 350F and let the unit run for a minimum of 15 minutes. Check the temperature of the unit using a thermometer. Wait 5 minutes and check a second time. If the readings are more than 5 degrees different, repeat the steps to ensure accurate settings. If the reading is more than 10 degrees off from the dial setting (being at 350F), recalibration should be needed. 

Hold the dial firmly, insert a calibration screwdriver through the center of the dial (after removing the center insert) and push the calibration stem inward. Slowly turn the screwdriver until it is set at the actual temp shown by the test equipment. Release the pressure on the calibration stem and replace insert in the dial. 

Set the dial to 400F and recheck the temperature the same manner as before to ensure proper calibration was completed. 

KX style thermostats:

Place a reliable test probe or thermometer in the center of the area being controlled by the thermostat. Turn the dial to the midpoint of its adjustable range and wait for unit to come to temp (time may vary as these thermostats have a wide range of uses and temp ranges). Let the unit cycle at least twice to ensure proper temp has been achieved. Compare dial to test equipment reading. If calibration is determined to be needed, use a calibration screwdriver to accomplish the following. Hold the thermostat stem in place and slide the screwdriver down the middle of the stem. Turn clockwise if the temp of the device is over the dial setting and counter clockwise if it is below the dial setting. See below for aprox change in temp based on quarter turns. Recheck calibration using previous steps and repeat if needed. 


Temp indicated on dial 60-95F 60-250F 100-220F 130-180F 140-550F 200-400F
Aprox change in degrees per 1/4 turn 6 17 12 14 35 18
Previous article What should an aftermarket warranty look like?