Infrared Thermometers Can Cause Confusion


There are three reasons why I don't like infrared thermometers for many HVAC tasks.

#1 – The laser can be misleading

The laser dot is just a point of reference, not an exact point where it is reading. Often the thermometer will read lower, higher, or over a MUCH wider area. Unless you are right up on what you are measuring, you can't be sure the result you are getting is correct.

#2 – They only read surfaces

An infrared reads surface temp only, not air temp. This is not necessarily a problem, but “shooting a vent” is not the same as measuring the air temperature coming out of it.

#3 – They can be VERY inaccurate

Basic infrared thermometers are only accurate on a surface that has high “emissivity” of near 1.0. These are usually darker, less reflective, and generally non-metallic surfaces. Metals have a low emissivity (much less than 1, generally; you can read more about emissivity in this article). That means that if you are reading a pipe, an infrared thermometer could read much lower than the correct temperature.

Infrared thermometers can be useful for making comparisons when reading the correct temperature is less important than comparing one spot to another, such as looking for hot spots in a panel or checking a zone to see if a damper is open. You have to know how to use them correctly and understand what you're looking for, but they can be useful. (I actually made a video showing how to use the Refco LP-88 infrared thermometer HERE.)

So long as you use the right tool for the job, you should be fine, but in general…

I don't like techs using infrared thermometers for most tasks.


P.S. – While I don't generally like infrared thermometers, I REALLY like thermal imaging. I used a thermal imaging camera to help diagnose a problem with NBA player Bo Outlaw's HVAC system; you can watch that process HERE. I also recommend checking out these nice products from Trutech tools.


Robert Gore
Robert Gore
2/7/18 at 03:07 PM

THANK YOU! I’ve told two different bosses that swear by IR guns exactly what you’ve stated about emmisivity. They looked at me like I was crazy until I told them shoot the supply air diffuser and I’ll use my Cooper air probe. Eye opener for them!

Bill Spohn
Bill Spohn
2/22/18 at 08:15 AM

Good solid info.

Regarding point #1, better IR thermometers actually paint points on the diameter or a circular ring with the laser pointer, to show you the target area, not just a center dot.
Even the inexpensive testo 805i smart probe IR thermometer paints a target ring.

6/5/19 at 09:10 PM

Most IR thermometers have the emissitivity preset to 0.95.

It is possible to buy IR thermometers that allow the user to input the expected emisitivity factor for the surface whose temperature is being measured.

If you do your research, you can buy such IR thermometers with this feature for under $20!

Indeed, I own 2 of these style of IR thermometers. That said, it required then to access a table of emissitivity factors for given surfaces and to then take the time to set the correct emisitivity factor before taking measurements. This will significantly improve accuracy.

Given that most techs will not bother to lookup emissitivity tables and program their IR gun prior to use each and every time, I look at IR guns as a means of monitoring systems roughly.

For example, I am performing a service and I want to test that the system is working in both heating and cooling. By pointing my IR gun at supply air registers, I can get a quick feel for if the temperature is falling or rising as well as which supply registers are at least working as well as a reasonable comparison of supply air temperatures to the return air temperature.

As long as users understand and work within the limitations of using an IR thermometer (ie.. accuracy), they are still a useful and timesaving tool.

The problem is when techs do no understand the inherent limitations of these tools and wish to rely on their readings as accurate representations of temperature.

Understanding your tools and how they work is part of being a good tech.


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