Why resist teaching resistors?

Presented by:   Jack Bartell   

Wednesday, March 23, 2022 until Friday, April 22, 2022, Sonoma C

Why resist teaching resistors? I can't answer the question in the title because I believe every student in an HVAC training course should be taught the benefits of using resistors to replace failed thermistors in any product large or small. Almost every HVAC manufacturer uses 10k negative coefficient thermistors to measure temperature for any number of reasons. The resistance of the thermistor is passed along to the control board as a resistance value which the board then converts to a temperature to use in its decision-making firmware. If the value measured is outside a predetermined set of parameters the board might shut the unit down or limit its ability to perform to protect the unit aa well as the building occupants. Example: A service technician is sent to a home due to a complaint from the homeowner that the cooling is not working. The technician finds an open outdoor air temperature sensor (thermistor). At this point the open sensor is telling the board the outdoor air temperature is below -40ºF but the unit has a safety built in that prevents cooling below 60ºF, meaning no cooling can be energized. By replacing the sensor with a 10k resistor the unit now thinks the outdoor air temperature is 77ºF allowing the cooling to operate. At this point the technician MUST return to the job to remove the resistor, replacing it with the correct thermistor.