Where’s The Heat?
By IAN MCTEERHVAC Systems Test & Measurements
Lower cost thermal imaging cameras and their role in the HVAC industry.
In the daily struggle to keep buildings healthy and happy, troubleshooting HVAC equipment still combines science and art. That intuition we attribute to talented HVAC technicians is secondary to science, which is the only real answer for today’s technicians who spend considerable lengths of time collecting and analyzing data.
The tools required for collecting said data are numerous and can be expensive. I have said before that the three most important tools are: knowledge, skill and a bright flashlight. On any particular service call a technician might also take to the jobsite a variety of hand tools: a clamp-on digital multimeter, manometer, gauges, a bunch of wire nuts, jumpers, a thermal imaging camera – wait a minute, a thermal imaging camera? Wow, another expensive tool, is what I am hearing from the field already.
THERMAL IMAGING IS EVERYWHERE
If you have ever watched Mike Holmes’ television show, you know that he uses a thermal imaging camera (TIC) regularly to find gaps in insulation and evidence of water leaks. In that case you are right: his camera is too expensive for most service technicians to even consider adding that model to their tool chests. However, costs have come down considerably and it is now possible to purchase a TIC with adequate imaging technology for less than $1,000. This is still a considerable monetary investment, but I think the benefits outweigh the costs.
I recently had the opportunity to “test drive” a new, less expensive TIC unit. I can report that thermal imaging is, at the same time, falling off a log easy to fairly complex when an accurate image is what is needed.
Falling off a log easy: turn the TIC on, point it at your Tim’s coffee and you will notice it is a bright yellow compared to the cooler table and other surroundings. But you already know your Tim’s is hot. Determining how hot is where accuracy comes into play; understanding how to take and to interpret accurate images takes some time and education.
THERMAL IMAGING CONCEPTS
In our world of HVAC/R, we are always moving heat either into or out of buildings, foodstuffs, or process applications. We deal with particular amounts of heat; too much or too little heat added or removed, or moved to the wrong places presents problems for customers and damages machinery. Arguably, technicians have a plethora of tools right now designed to test and to diagnose system failures. Thermal imaging will not override the need for conventional HVAC tools or skillsets. A TIC, in my view, is a complementary technology that helps technicians savetime and make more accurate decisions.
Even though the camera is easy to operate, the quality of the data coming from any image is dependent on the following concepts:
Blackbody: a totally non-reflective object. All of its radiation is a result of its own temperature.
Emissivity: the amount of radiation coming from an object as opposed to that of a blackbody. It is usually a numerical value between zero and one, although highly polished or shiny metal has an emissivity of 0.07. Flat black oil-based paint is 0.94. The camera requires the operator to choose an appropriate setting based on the material to be sampled.
Reflected apparent temperature: the camera requires an input for this parameter if temperature accuracy is the goal. It is used “to compensate for the radiation reflected in the object. If the emissivity is low and the object temperature is relatively far from that of the reflected temperature it will be important to set and compensate for the reflected apparent temperature correctly.”1
Distance: “is the distance between the object and the front lens of the camera. This parameter is used to compensate for the following two facts:
• That radiation from the target is absorbed by the atmosphere between the object and the camera.
• That radiation from the atmosphere itself is detected by the camera”.2
Spot meter: uses the value from one pixel in the centre of the spot marker to determine the temperature of the target. However, to determine temperature accurately, the spot marker must cover the complete target. Thus, the camera cannot accurately measure the temperature of small tubes or pipes.
BECOME A PRO
To become proficient in the use of TICs, students of thermography must take formal training and achieve American Society for Nondestructive Testing (ASNT) certification. Thermographers start with Level 1 certification and can continue to Level 3 where they learn how to establish thermography programs in various industries. Visit the Infrared Training Center for more information on courses: www.infraredtraining.com. Achieving Level 1 is a highly recommended goal for any HVAC technician wanting to provide customers with detailed thermography reports.
Because I have not taken a thermography course, my images are mostly uncorrected for emissivity and reflected temperature. I do believe the images have some value, so let’s take a look at my findings. My TIC was capable of taking a thermal image and a .jpg at the same time. Using the camera manufacturer’s free software, I could have created a professional report to show to a customer. Both images on the report provide a level of credibility a customer would need in making decisions that involve large expenditures of money or perhaps an extended shutdown of crucial equipment needing repairs.
After operating my two-stage heat pump in second stage for 10 minutes, I wanted to find out if the duct temperature and air inside the duct temperature would be the same. I did correct for this image by painting a small portion of the end of the trunk duct with flat black paint (emissivity 0.94, reflected temperature 20C). I used a distance setting of 0.5M. I placed the spot meter on the duct roughly where I thought the end of the type K thermocouple temperature probe might be. As shown in Figure 1, the outside of the duct reads 32.9C and the air temperature inside the duct is 35.8C, a considerable difference. Nevertheless, a quick application of black paint and one thermal image proved this duct is working. I am thinking of several jobs I had in the past where I spent hours probing ductwork with thermometers looking for restrictions. This might have been made easier by creating a roadmap of thermal images beforehand.
In Figure 2, the thermal image of a three-phase fuse block clearly shows one set of fuses running warmer than the others. This circuit must be examined for potential failures. Expect the same results from a circuit breaker panel. An overheating breaker will show up like an east coast lighthouse beam. Once you are familiar with thermal imaging, you will never want to pass-by an electrical panel or transformer again without imaging it for posterity.
Even though the indicated temperatures in Figure 3 are incorrect, it is easy to see the temperature transition as the mostly liquid refrigerant introduced into the bottom of the coil gradually boils off and warms to superheated vapour at the top of the coil. Any restriction in a particular refrigeration circuit would show-up as an easy to spot colour difference.
The thermal image in Figure 4 appears to show the TXV is feeding properly, but it is not possible for the spot meter to gather enough temperature data from the small capillary tubes. Each tube would have to be temperature-tested using conventional methods.
I attempted to create a blackbody on the outdoor coil of an air-to-air heat pump in heating mode. I did not attach a gauge set to monitor the coil condition, however, the thermal image in Figure 5 shows the temperature at the centre of the blackbody to be -3.4C. This coil has five parallel circuits so it is possible to see some superheating in each circuit. Notice the exaggerated temperature difference where the damaged fins are located.
Again, as shown in Figure 6, temperatures are off but it is clear the valve is operating correctly. Suction gas is coming into the valve on the right side then directed to the compressor through the centre pipe. Hot discharge gas is routed to the condenser through the pipe on the left side of the valve. Reversing valve failures are sometimes misdiagnosed. In addition, replacement of this part is time consuming and expensive. A thermal image could help the technician to eliminate the valve as an issue in some cases.
A TIC cannot “see” the air, thus conventional methods must be used to measure air temperature directly. In Figure 7, observe the pattern that cooling mode airflow from the register has inscribed on the wall behind. The image indicates the register has an effective throw, although in a detailed study the technician would still require conventional HVAC methods to determine if the airflow volume and velocity are reasonable.
SIMPLIFY YOUR LIFE
I have seen several thermal images of condensing units taken by a thermographer that revealed two common HVAC problems: one unit was clearly overcharged, the thermographic image showed the outdoor coil was almost half full of liquid. The second image clearly indicated a refrigerant leak location on the outdoor coil – as the liquid leaked from a pinhole, it vaporized thus lowering the temperature in the vicinity of the leak. A dark blue dot on the coil image could not have been caused by anything other than a refrigerant leak.
Considering the time I have spent using conventional methods searching for leaks, I was stunned at how easily this particular coil leak was detected. Every year, I would image every condenser coil on every job going forward.
Every HVAC/R service organization should consider purchasing a TIC and send a senior/lead technician to thermography school. Use the TIC manufacturer’s software to create thermography reports so customers will have detailed information about necessary repairs. The new, lower cost cameras are, in my view, ideal for our industry because, we simply need to know, “where’s the heat?”
Ian McTeer is an HVAC consultant with 35 years experience in the industry. He was most recently a field rep for Trane Canada DSO. McTeer is a refrigeration mechanic and Class 1 Gas technician.
1,2 Flir Systems Inc. User’s manual, Flir Ex series (2013), 19,35