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HVAC Thermistor Training: Testing Methods, Common Failures & Splicing

By Roman Baugh Jeff Crable

February 1, 2026

Learn everything about HVAC thermistor training including proper testing methods, common failures, and splicing techniques in this comprehensive guide. This HVAC thermistor training covers NTC (Negative Temperature Coefficient) sensors used in refrigeration and HVAC systems.

What You’ll Learn:

HVAC thermistor testing methods using resistance measurements
Common thermistor failures: moisture infiltration, overheating, wire rubout, and corrosion
Why thermistors typically fail HIGH in HVAC applications
Understanding 10K, 20K, and 200K thermistor types
The 77°F (25°C) reference point for all thermistors
Proper splicing techniques and connection methods for thermistor replacement
How to identify thermistor types without charts
Applications in walk-in coolers, mini-splits, and temperature control systems

Key Topics Covered:

HVAC thermistor testing with multimeters
Resistance-to-temperature relationships
Common failure modes and diagnostics
Proper wire connections (solder vs crimp vs wire nuts)
Moisture-related failures and prevention
Using resistors as temporary troubleshooting tools
Service applications and real-world scenarios

#hvac #resistance #splicing #thermistor

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Don’t Die! w/ Louise from UEi

byBryan Orr

In this episode, recorded live at AHR Expo 2026, Bryan sits down with Louise from UEi (United Electronics Inc.) for an in-depth conversation about one of the most underestimated dangers in the HVAC trade: carbon monoxide. Rather than treating CO as just another checkmark on a safety list, Bryan and Louise dive into the full history of the gas — all the way back to early publications from 1922 and 1923 that were already studying the effects of low-level CO exposure. One of the most fascinating takeaways from their discussion is just how long the industry has known about the dangers of carbon monoxide, yet how frequently it remains misunderstood, misdiagnosed, and overlooked on the job site.

Louise breaks down the science behind CO poisoning in a way that is both accessible and eye-opening. Unlike high-level acute poisoning — where symptoms are immediately obvious — low-level, chronic carbon monoxide exposure is an entirely different beast. It can mimic the flu, chronic headaches, early-onset Alzheimer’s, and even heart attack symptoms. Because CO is colorless, odorless, and tasteless, it is notoriously difficult to diagnose, and victims often have no idea what is causing their illness. Compounding the problem is the cumulative nature of CO toxicity: the gas builds up in the bloodstream over time and can linger in the body for days, months, or even years after exposure ends. Bryan draws an entertaining but sobering historical connection between the prevalence of carbon monoxide poisoning during the Victorian era — when gas lamps were common inside homes — and the widespread reports of ghost sightings, fugue states, fainting spells, and the spiritualist movement of the time. It was almost certainly CO poisoning, not the paranormal.

The conversation then shifts to practical, on-the-job guidance for technicians and contractors. Bryan and Louise walk through the proper process for combustion analysis: starting your analyzer outside in fresh air, identifying test ports on furnaces, boilers, and water heaters, and understanding what readings at steady state actually mean. They emphasize that CO production is not limited to furnaces alone — ranges, ovens, dryers, fireplaces, water heaters, and even propane-powered forklifts are all legitimate sources. One of the most critical points they make is that carbon monoxide hazards are often intermittent and condition-dependent. A system can appear to be working perfectly during one visit and be producing dangerous levels the next time a door is closed or a vent is accidentally blocked. Visual inspection, awareness of combustion air sourcing, and thoughtful analysis are just as important as having the right instruments.

On the product side, Louise introduces two exciting new offerings from UEI. The first is the UEI Clip, a compact personal CO detector that clips onto a bag, belt, or lanyard, activates automatically at 20 ppm, and is designed as a set-it-and-forget-it safety device with a two-year sealed battery lifespan. Priced under $100, it is an affordable way to outfit an entire team with a baseline layer of personal protection. The second highlight is the new High Accuracy Clamp Meter (HAC), a commemorative product celebrating the United States’ 250th anniversary. This meter excels at wattage measurement — particularly with low-draw ECM blower motors where most meters fall short — and Bluetooth integration with MeasureQuick is on the horizon. Louise also walks through UEI’s revamped combustion analyzer recertification program, UEI Service Plus, which offers same-day turnaround on standard recertifications at their Indianapolis facility, extended warranties with each annual service, and transparent, upfront pricing directly on their website.

Topics Covered

The history of carbon monoxide research dating back to publications from 1922 and 1923, and a 1923 patent for a hot water heater that sparked a friendly debate between Bryan and Louise about terminology.
How low-level, chronic CO exposure is underdiagnosed and can mimic conditions like the flu, chronic headaches, early-onset Alzheimer’s, and heart attack symptoms — and why CO builds up cumulatively in the bloodstream.
The fascinating historical theory connecting widespread Victorian-era reports of ghosts, fugue states, and fainting spells to chronic carbon monoxide poisoning from indoor gas lamps.
CO safety thresholds: residential standard (9 ppm), World Health Organization guideline (4 ppm), first responder evacuation threshold (35 ppm), and how UL-rated home alarms may not trigger until 70 ppm sustained for 1–4 hours.
Sources of CO that technicians often overlook, including ovens, ranges, dryers, fireplaces, water heaters, propane forklifts, generators, and vehicles idling near garages.
Proper combustion analysis procedure: starting the analyzer outside in ambient air, identifying or installing test ports, running tests at steady state, and interpreting O2, CO, and stack temperature readings.
Industry CO standards for furnaces: official standard (400 ppm), HRI recommendation (200 ppm), industry best practice (100 ppm), and modern high-efficiency benchmarks (50 ppm or less).
How combustion air sourcing — and changes to it (new roofs, blocked vents, swapped doors) — is one of the most commonly missed risk factors for CO production.
Why even “heat pump markets” like Florida still need combustion analysis, since over 90% of homes have at least one fuel-fired appliance.
Introduction of the UEi Clip personal CO detector: set-and-forget, activates at 20 ppm, sealed 2-year battery, priced under $100, and its unexpected popularity among refrigeration technicians.
Introduction of the UEi High Accuracy Clamp Meter (HAC) commemorative edition: exceptional accuracy at low current draws (ideal for ECM blower motors), full accessory kit included, and upcoming measureQuick Bluetooth integration.
UEi’s Service+ combustion analyzer recertification program: same-day turnaround at their Indianapolis lab, extended warranty with each annual service, prepaid UPS shipping labels, and transparent pricing on their website.
A brief spotlight on UEi as a family-owned business founded by Michael Kane’s parents, with their own manufacturing operations in the UK and Korea.

Learn more about UEi’s new and longstanding products, as well as the Service+ guarantee, at https://ueitest.com/.

Have a question that you want us to answer on the podcast? Submit your questions at https://www.speakpipe.com/hvacschool.

Purchase your tickets or learn more about the 7th Annual HVACR Training Symposium at https://hvacrschool.com/symposium.

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