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Roger Siling

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What Is a Thermistor & How Does It Work?

Modern HVAC systems today rely on thermistors for proper sensing of air and liquid temperatures to be used by system control boards for adjustments to operational speeds. As temperature conditions change, the thermistor reports these temperatures to the control boards which then adjust the compressor and blower speeds to compensate for these changing conditions.

  • Thermistor type selection varies according to the use needed or application required.
  • HVAC thermistors resistance ranges used by CBP are the 10k-ohm & 50k-ohm varieties.
  • These are called NTC Thermistors, as they respond inversely to the temperature chance.
  • If the measured temperature rises, then the thermistor resistance value drops.
  • Likewise, if the measured temperature drops, then the thermistor resistance value rises.
thermistor HVAC Supply House

Glass Encapsulated NTC Thermistors

These are NTC temperature sensors sealed in an airtight glass bubble. They are designed for use with temperatures above 150°C, or for printed circuit board mounting, where ruggedness is a must. Encapsulating a thermistor in glass improves the stability of the sensor as well as protecting the sensor from the environment. They are made by hermetically sealing bead type NTC resistors into a glass container. The typical sizes range from 0.4 – 10mm in diameter.

thermistor HVAC Supply House

NTC Thermistors

An NTC thermistor is a thermally sensitive resistor whose resistance exhibits a large, precise and predictable decrease in resistance, as the core temperature of the element increases over the operating temperature range.


Characteristics of NTC Thermistors

Unlike RTDs (Resistance Temperature Detectors), which are made from metals, NTC thermistors are generally made of ceramics or polymers. Difference materials used result in different temperature responses, as well as other characteristics.


Temperature Response

While most NTC thermistors are typically suitable for use within a temperature range between -55°C (-67°F) and 200°C (392°F), where they give their most precise readings, there are special families of NTC thermistors that can be used at temperatures approaching absolute zero (-273.15°C (-459.67°F) as well as those specifically designed for use above 150° C (302°F).


Thermistors are of two types, NTC (negative temperature coefficient) and PTC (positive temperature coefficient types). As their name indicates, the resistance of an NTC thermistor will decrease with temperature rise and the resistance of a PTC thermistor will increase with temperature rise.

Both PTC as well as NTC thermistors can be checked by using an analogue or digital multimeter. Keep the analogue or digital multimeter in resistance mode. Connect the multimeter leads to the thermistor leads. Polarity is not an issue here. Now heat the thermistor by exposing to a known temperature source. You can see the multimeter reading smoothly increases or decreases depending on whether the thermistor under test is PTC or NTC. This will happen only for a healthy thermistor.

For a faulty thermistor, following observations are possible: The change in reading will not be smooth or there will not be any change. A shorted thermistor will always read zero while an open thermistor will always read infinity.


Using an ohmmeter to test the thermocouple will determine if the thermocouple sensor is defective or not.

If a thermistor goes out of calibration, shorts, or is open, then the HVAC system cannot adjust operational speed according to the changing load condition. A simple test meter can be used for testing of the thermistor.

Using the following 10K-ohms chart, and based on the thermistors associated location temperature, then ohm out the thermistor quickly and compare the ohms value from the chart to the actual ohms reading on the meter.

Smoke Detectors HVAC Supply House

That is all there is to testing the thermistor, reading should be with-in 5-10 percent depending on the thermistors tolerance value.

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Alabama Locations

Anniston (256) 241-3026 105 E D St, Anniston, AL 36201
Auburn (334) 734-8011 2295 1st Ave, Opelika, AL 36801
Birmingham (205) 250-7714 516 31st St N, Birmingham, AL 35203
Dothan (334) 836-1035 205 Wedgewood Dr, Dothan, AL 36303
Gulf Shores (251) 968-2660 820 Commerce Dr, Gulf Shores, AL 36542
Huntsville (256) 859-2691 391 Nick Fitcheard Rd NW, Huntsville, AL 35806
Loxley (251) 964-2707 30020 Co Rd 49, Loxley, AL 36551
Mobile (251) 476-2220 5522 W Commerce Blvd West, Mobile, AL 36619
Montgomery (334) 262-4855 861 Plantation Way, Montgomery, AL 36117
Muscle Shoals (256) 383-6904 107 Commerce St, Muscle Shoals, AL 35661

Georgia Locations

Albany(229) 432-5254205 Baldwin Dr, Albany, GA 31707
Athens(706) 548-2563105 Whitetail Way, Bogart, GA 30622
Atlanta(404) 875-77551290 Tacoma Dr NW, Atlanta, GA 30318
Augusta(706) 651-19503618 Wrightsboro Rd, Augusta, GA 30909
Brunswick(912) 265-5193275 Rose Dr, Brunswick, GA 31520
Columbus(706) 322-38706201 W Hamilton Park Rd, Columbus, GA 31909
Conyers(770) 918-9132335 Gees Mill Business Pkwy NE, Conyers, GA 30013
Forest Park(404) 361-24745158-C Kennedy Rd, Forest Park, GA 30297
Gainesville(770) 297-93352472 Hilton Dr, Gainesville, GA 30501
Lawrenceville(678) 442-10602111 Cedars Rd SE, Lawrenceville, GA 30043
Lithia Springs(770) 948-3451320 Thornton Rd #106, Lithia Springs, GA 30122
Macon(478) 474-85445527 Thomaston Rd, Macon, GA 31220
Marietta(770) 426-05511060 Triad Ct, Marietta, GA 30062
Newnan(770) 239-210570 Enterprise Ct, Newnan, GA 30265
Peachtree Corners(770) 239-21006675 Jones Mill Ct, Peachtree Corners, GA 30092
Savannah(912) 232-1228103 Central Jct Blvd, Savannah, GA 31405
Statesboro(912) 764-8841126 W Parrish St, Statesboro, GA 30458
Valdosta(229) 245-8200363 Connell Rd, Valdosta, GA 31602
Warner Robins(478) 953-3727110 Byrd Way, Warner Robins, GA 31088

South Carolina Locations

Hilton Head(843) 689-34043715, 21 Hunter Rd, Hilton Head Island, SC 29926

Florida Locations

Fort Walton Beach(850) 244-201776 Beal Pkwy SW, Fort Walton Beach, FL 32548
Panama City(850) 913-05853012 Lisenby Ave, Panama City, FL 32405
Pensacola(850) 438-10054405 N. Palafox Street, Pensacola FL 32505

Mississippi Locations

Hattiesburg(601) 579-96701851 Lincoln Rd, Hattiesburg, MS 39402
Ocean Springs(228) 497-5090Sunplex Industrial Park, 5717 Suntec Drive, Ocean Springs, MS 39564

Commercial Sales

Norcross – North Georgia Hub
(800) 282-4911

Birmingham – North Alabama Hub
(205) 250-7714

Macon – Southwest Hub
(800) 342-9250

Loxley – Gulf Coast Hub
(800) 239-7419

Savannah – Southeast Hub
(800) 822-7617

Commercial Parts

[email protected]
(844) 99-CHILL

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Paynewww.payne.comResidential Heating & A/C Products
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Renew Airewww.renewaire.comFixed-Plate ERVs
Totalinewww.totaline.comCompressors, Parts, and Supplies
United CoolAirwww.unitedcoolair.comModular All-Indoor HVAC Systems
 Warren Technologywww.warrenhvac.comElectric Heaters
 Wilo Pumpswww.wilo-usa.comPump Systems for HVAC and Water Supply and Sewage