HVAC Procedure and Performance Guide

TASK: Install defrost thermostat

Training for this procedure can be found in these courses:

                         131 – Oil Heat l  and  161 – Boilers l


Defrost thermostat must be installed according to manufacturer’s specifications.  Thermostat must be securely mounted, wiring connections must be mechanically secure and electrical continuity must be established.  Thermostat must override defrost cycle at specified temperature settings.  Applicable safety procedures must be followed.


  1. Review job order and manufacturer’s specifications.
  2. Determine voltage, temperature adjustment range and type of defrost thermostat required.
  3. Collect tools, equipment and materials.
  4. Put on safety and protective equipment and apparel.
  5. Disconnect electrical power to equipment.
  6. Remove old thermostat.
  7. Verify that new thermostat is a suitable replacement.
  8. Mount new thermostat and connect wiring according to specifications.
  9. Position thermostat bulb on sensing element according to specifications and connect sensing element wires to thermostat element as specified.
  10. Set thermostat to manufacturer’s recommended setting.
  11. Reconnect power and verify that voltage is correct.
  12. Mount temperature-measuring device adjacent to thermostat temperature sensor and insulate according to manufacturer’s specifications.
  13. Start equipment, monitor temperature during defrost cycle, compare to thermostat setting and readjust thermostat as required.
  14. If equipment does not operate to specifications, troubleshoot and make required adjustments and repairs.
  15. Leave system in required operating status.
  16. Remove safety and protective equipment and apparel.
  17. Gather tools, equipment and leftover materials and leave work site clean.