Troubleshooting Thermostats

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Thermostat problems are sometimes the result of poor wiring connections. Check the wiring first. You should also make certain that the fan and system switches and the temperature setpoint are properly set.

The troubleshooting chart in Table 4-3 includes many of the more common symptoms and possible causes of operating problems associated with thermostats.

Table 4-3 Troubleshooting Thermostats

Symptom and Possible Cause Possible Remedy
Room temperature overshoots thermostat setting (too cold).
(a) Thermostat not mounted level (mercury-switch types). (a) Remount thermostat in level position.
(b) Thermostat not properly calibrated. (b) Recalibrate or replace.
(c) Thermostat exposed to heat source. (c) Move thermostat to better location.
(d) Thermostat setpoint too low. (d) Reset.
(e) System sized improperly. (e) Determine correct sizing and make system adjustments.
Room thermostat does not reach setting (too warm).
(a) Thermostat subject to draft. (a) Wiring hole may not be plugged. Move thermostat to better position.
(b) Thermostat not mounted level (mercury-switch types). (b) Remount thermostat in level position.
(c) Thermostat not properly calibrated. (c) Recalibrate or replace.
(d) Thermostat setpoint too high. (d) Reset.
(e) System sized improperly. (e) Determine correct sizing and make system adjustments.
(f) Thermostat damaged. (f) Replace thermostat.
System cycles too often.
(a) Thermostat exposed to heat source. (a) Relocate thermostat.
(b) Thermostat differential too small. (b) Reset or replace thermostat.
(c) Thermostat heating element improperly set. (c) Reset or replace thermostat.
(d) Thermostat subject to vibrations. (d) Remount thermostat in location free from vibrations.
(e) Thermostat exposed to cold draft. (e) Remount in better location.
System does not cycle often enough (burner operates too long).
(a) Thermostat not exposed to circulating air. (a) Remount in better location.
(b) Contacts dirty. (b) Clean or replace.
(c) System sized improperly. (c) Determine correct sizing and make system adjustments.
(d) Thermostat differential too great. (d) Reset or replace thermostat.
(e) Thermostat heating element improperly set. (e) Reset or replace thermostat.
(f) Thermostat set too high. (f) Reset.
Room temperature swings excessively.
(a) Thermostat not exposed to circulating air. (a) Remount in better position.
(b) Thermostat exposed to heat source. (b) Remount in better position.
(c) System sized improperly. (c) Determine correct sizing and make system adjustments.
Thermostat jumpered (system works).
(a) Thermostat contacts dirty. (a) Clean or replace.
(b) Thermostat setpoint too high. (b) Reset or replace thermostat.
(c) Thermostat damaged. (c) Replace thermostat.
(d) Break in thermostat circuit. (d) Locate and correct.
Burner fails to stop.
(a) Thermostat in cold location. (a) Relocate in better location.
(b) Thermostat set too high. (b) Reset.
(c) Defective thermostat. (c) Replace thermostat.
d) Thermostat out of adjustment. (d) Recalibrate or replace thermostat.
(e) Thermostat contacts stuck. (e) Correct.

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