I'm looking to purchase some thermistor-based probes to
monitor the performance of the HVAC system (return and
supply duct temperatures along with liquid and suction
lines. Maybe the temperature of the compressor, etc.)

As part of my research, I look at the problems people
(consumers) report with the units used in their systems.

I would have thought NONE of these would fail: they are
out of the way so not likely to be physically disturbed;
often encased in a protective sheath (e.g., probes);
shouldn't be operating beyond their design constraints
(unless poorly designed); etc.

Even the pigtails from such devices should be reasonably
safe from disturbance!

Yet, I see folks replacing them and proud that they
did so without calling in the HVAC contractor!

Are there other "wear" factors that come into play?
What sort of service life (in a conservative design)
should I expect?

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On 10/07/2025 4:20 pm, Don Y wrote:

I'm looking to purchase some thermistor-based probes to
monitor the performance of the HVAC system (return and
supply duct temperatures along with liquid and suction
lines.  Maybe the temperature of the compressor, etc.)

As part of my research, I look at the problems people
(consumers) report with the units used in their systems.

I would have thought NONE of these would fail:  they are
out of the way so not likely to be physically disturbed;
often encased in a protective sheath (e.g., probes);
shouldn't be operating beyond their design constraints
(unless poorly designed); etc.

Even the pigtails from such devices should be reasonably
safe from disturbance!

Yet, I see folks replacing them and proud that they
did so without calling in the HVAC contractor!

Are there other "wear" factors that come into play?
What sort of service life (in a conservative design)
should I expect?

About the only failure mode that I'm aware of is drift - thermistors are sintered metal oxides, so there are gaps in the material and stuff could diffuse in.

The good ones are enclosed in glass beads - Yellow Springs were the
first to introduce them - and those drift a lot less than their
predecessors. I had my nose rubbed in this back when I was working at
Kent Instruments in England in the early 1970's, and I've used them from
time to time since then.

--
Bill Sloman, Sydney

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On Wed, 9 Jul 2025 23:20:43 -0700, Don Y <blockedofcourse@foo.invalid>
wrote:

I'm looking to purchase some thermistor-based probes to
monitor the performance of the HVAC system (return and
supply duct temperatures along with liquid and suction
lines. Maybe the temperature of the compressor, etc.)

As part of my research, I look at the problems people
(consumers) report with the units used in their systems.

I would have thought NONE of these would fail: they are
out of the way so not likely to be physically disturbed;
often encased in a protective sheath (e.g., probes);
shouldn't be operating beyond their design constraints
(unless poorly designed); etc.

Even the pigtails from such devices should be reasonably
safe from disturbance!

Yet, I see folks replacing them and proud that they
did so without calling in the HVAC contractor!

Are there other "wear" factors that come into play?
What sort of service life (in a conservative design)
should I expect?

Thermistors should last forever.

Probably bad solder joints inside. Cheap junk from you-know-where.

--- SoupGate-Win32 v1.05
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On 7/10/25 10:47, john larkin wrote:

On Wed, 9 Jul 2025 23:20:43 -0700, Don Y <blockedofcourse@foo.invalid>
wrote:

I'm looking to purchase some thermistor-based probes to
monitor the performance of the HVAC system (return and
supply duct temperatures along with liquid and suction
lines. Maybe the temperature of the compressor, etc.)

As part of my research, I look at the problems people
(consumers) report with the units used in their systems.

I would have thought NONE of these would fail: they are
out of the way so not likely to be physically disturbed;
often encased in a protective sheath (e.g., probes);
shouldn't be operating beyond their design constraints
(unless poorly designed); etc.

Even the pigtails from such devices should be reasonably
safe from disturbance!

Yet, I see folks replacing them and proud that they
did so without calling in the HVAC contractor!

Are there other "wear" factors that come into play?
What sort of service life (in a conservative design)
should I expect?

Thermistors should last forever.

Probably bad solder joints inside. Cheap junk from you-know-where.

Or fatigue failure from the chronic vibration.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

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On 7/9/2025 11:20 PM, Don Y wrote:

I would have thought NONE of these would fail:  they are
out of the way so not likely to be physically disturbed;
often encased in a protective sheath (e.g., probes);
shouldn't be operating beyond their design constraints
(unless poorly designed); etc.

Even the pigtails from such devices should be reasonably
safe from disturbance!

Yet, I see folks replacing them and proud that they
did so without calling in the HVAC contractor!

Are there other "wear" factors that come into play?
What sort of service life (in a conservative design)
should I expect?

This:

<https://product.tdk.com/en/techlibrary/applicationnote/failure-modes-countermeasures-of-ntc.html>

enumerates the obvious. But, none seem particularly likely.
The folks designing these controls are either manufacturers
of the equipment to be instrumented or (should be) intimately
familiar with those environments. I.e., it's not likely
that some installations will see radically different operating
conditions than others.

Sadly, one can't research the devices involved after the fact
as there is no guarantee that the manufacturing technology
is unchanged from the unit(s) experiencing the failure(s).

But, it may be a worthwhile exercise to buy a sampling of different
devices and examine their construction.

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