Thermostat Calibration

Thermostats are accurately calibrated at the factory under controlled conditions and should not require recalibration. Sometimes a thermostat will appear to require recalibration when the problem is actually a quite different one. For example, a thermostat that is not level or one subjected to a high degree of radiant heat from the sun, radiators, convectors, or other heat sources often fails to function properly. Before jumping to the conclusion that it needs to be
recalibrated, check out the possibility that some external cause may be the source of the difficulty. If you are certain the problem is in the thermostat, then you should call a trained serviceman to recalibrate it. Do not attempt to recalibrate it yourself unless you have the necessary experience.

A new thermostat is generally shipped with complete installation literature. This literature also usually contains instructions for recalibrating the thermostat. By way of example, the instructions for recalibrating a Honeywell T87 Room Thermostat include the following steps:

1. Remove the thermostat cover ring (see Figure 4-27). The locking cover will require the use of an Allen wrench to loosen the screws securing the cover.

2. Set the thermostat below the room temperature, and allow it to remain in an off position for approximately 10 minutes.
3. Slowly raise the setting until the switch just makes contact. If the thermostat pointer and setting indicator do not read the same the instant the switch makes contact, then the thermostat requires recalibration.
4. Turn the setting dial a few degrees above room temperature.
5. Slip the calibration wrench onto the calibration (hex) nut under the bimetal coil (see Figure 4-28).
6. If the thermostat has a stationary pointer, hold the dial firmly and turn the calibration nut counterclockwise until the mercury breaks contact. If the thermostat has a movable pointer, turn the calibration nut clockwise.
7. Turn the thermostat dial to a low setting, and wait approximately 5 minutes.
8. Slowly turn the dial until the pointers read the same.
9. Hold the dial firmly, and turn the calibration in the opposite direction from the one in step 6 until the mercury switch slips to the heating contact end of the tube.
10. Recheck the calibration, select the desired temperature, and replace the cover.