IR thermometry boosts safety and accuracy of HVAC maintenance

Determining the efficiency of an HVAC system is often tedious and time consuming. It can lead to injuries when a technician on a ladder must overreach to attach a thermocouple or accidentally brushes against a hot surface.

By Sarah Evans February 1, 2000

Determining the efficiency of an HVAC system is often tedious and time consuming. It can lead to injuries when a technician on a ladder must overreach to attach a thermocouple or accidentally brushes against a hot surface. Today, infrared thermometers offer a faster, more effective way to diagnose duct leaks, blocked steam traps, wet insulation, malfunctioning coils, and other heat-related problems common to HVAC systems.

Although the technique has been around for many years, advances in IR measurement technology have only recently allowed the development of lightweight, low-cost tools for monitoring processes and speeding up plant maintenance. Modern IR thermometers are now used in scores of industries.

How does an IR thermometer work?

Infrared (IR) thermometers operate on the principle that all objects naturally emit invisible IR energy. IR thermometers measure the surface temperature of objects without touching them by measuring the IR energy emitted from the target in the 1­14-micron region of the electromagnetic spectrum. (The electromagnetic spectrum is illustrated is Fig. 1.)

As energy passes through the atmosphere, the IR thermometer captures it, directs it through the optical system of the instrument, and converts it to an electrical signal at the detector. The signal is displayed in degrees Fahrenheit or Celsius on the LED readout.

IR energy can also be transmitted through an object from another source or be reflected off the surface of an object. Therefore, an important objective in IR temperature measurement is to isolate the emitted energy as much as possible because only that energy is indicative of the temperature of the object.

Emissivity. The term used to quantify the energy-emitting characteristics of a material is emissivity. When the precise (emitted) temperature of various materials must be known, the technician can use the emissivity setting present on most IR thermometers to compensate for the ways the various materials transmit, reflect, and emit energy. Some newer high-performance thermometers have on-board tables of preset material emissivity values.

Speed. IR thermometers designed for plant use are noninvasive and simple to use. They are typically pistol shaped, easily holstered, and can weigh less than 10 oz. A technician simply aims, pulls the trigger, and reads the temperature in less than half a second. In the time required to attach a thermocouple to one duct joint, a worker can determine the temperature of all the joints of the same duct with an IR device.

Distance-to-spot ratio. The target area being measured is defined by the distance-to-spot ratio . The closer a unit is to an object, the smaller the measurement area is (a flashlight beam is a good example). As the unit is moved away from the target, the measurement area expands and the surface temperature within that area is averaged. A distance-to-spot ratio of 6:1 means that from a distance of 6 ft, a 1-ft area is measured. The laser sight present on most instruments helps the user aim at the target area more accurately. Some IR thermometers include a laser sighting circle that shows the spot size being measured at any distance. This state-of-the-art technology helps take the guesswork out of determining the target area.

Safety . Noncontact temperature measurements can be taken in areas that are inaccessible or when inadvertent contact with a hot surface make it unsafe to take a contact instrument reading. (In ceiling areas and around steam traps are two examples.)

Using IR devices during installation and maintenance

Many HVAC system tests can be performed during installation and maintenance by conducting simple temperature measurements. These tests include:

– Temperature drop for cooling

– Temperature rise for gas heating

– Air temperature

– Thermostat calibration

– Airflow

– Duct leaks

– Thermal gain through ductwork.

In many cases, an IR instrument saves substantial time. During air balancing, for example, thermometers are placed on walls, floors, and ceilings to determine the temperature at various strata. Technicians typically must wait 15­20 min for each thermometer to stabilize. In large facilities, such methods are inefficient, tying up maintenance personnel for long periods.

Infrared thermometry allows air stratification measurements to be taken from floor to ceiling on an interior wall in a few seconds. A technician takes readings in small increments starting at the base of the wall and moving upward, recording the distance from the floor each time the temperature rises a degree.

Supply and return duct temperatures can also be measured using IR technology, allowing a technician to establish if vents are properly located, if an HVAC unit is adequate for the space being heated or cooled, whether more or more efficient ducts should be installed, and make similar determinations.

Troubleshooting a system

Finding the problem can be a long and arduous task, especially in large facilities where ductwork may be difficult to access. Infrared thermometry frees a technician from ladders and scaffolding. Using point, shoot, and read techniques, he can perform most key maintenance diagnostics for an HVAC system. Here are a few typical applications.

Determining inefficient steam distribution . One way to determine the condition of a steam trap is to observe the outlet flow. The procedure may require removing the condensate return line or opening a valve and observing the operation for several minutes before reconnecting the return line or closing the valve. The activity can be time consuming and sometimes hazardous, especially when traps are difficult to access.

A portable noncontact thermometer can help diagnose blown or plugged traps by allowing the maintenance worker to check the devices from a distance in about a minute per trap.

Checking registers. If an HVAC system is not cooling or heating properly, a technician can first check the return and supply registers to see if the temperature difference is correct for the installed system. In a cooling unit, for example, the temperature difference between the supply and return air should be 18­22 deg F. In a unit that is heating, the temperature difference should range from 30­70 deg F. A greater variance could indicate a blocked coil or a leaking supply duct.

By reading the temperature of each grill, a technician can determine if the temperature differential is acceptable (Fig. 2).

Identifying leaky ductwork. Breaks in ducts can cause a variety of problems. They can shorten the operating life of an HVAC unit by making it work too hard. Supply and return ducts with loose or failed joints can direct hot or cold air to the wrong areas, as well as draw air from surrounding areas.

If a break or hole in a duct joint is not visible, a portable thermometer can be used to measure the temperature along the length of the duct. Any sudden drop or rise is a good indication of a break, hole, or place where insulation has come apart.

Other troubleshooting problems. Other investigations for which IR thermometers are well suited include:

– Measuring for heat/cold transference on an outside wall

– Checking rooftop HVAC units

– Evaluating return and supply temperatures on blower/filter units and ducts

– Monitoring supply and return pipes from the HVAC compressor

– Checking door and window insulation integrity.

Selecting an IR device

Important considerations when selecting an instrument include temperature range, distance to spot (D:S) ratio, operating features, and cost.

Temperature range. Select an instrument with a temperature range that most closely matches the application range. Available instruments measure temperatures as low as -25 F for sub-zero and refrigeration applications; others measure temperatures as high as 5400 F for specialized applications involving metal or glass.

D:S ratio. All IR thermometers have optical resolutions expressed as a distance-to-spot size ratio. The D:S ratio is determined by dividing the distance from the object or target to the thermometer by the size of the spot being measured. The target should fill the field-of-view of the instrument during the measuring period. If the target is smaller than the spot size, the sensor may be affected by objects in the background (Fig. 3).

When an instrument is chosen, the user should determine which one is right for the application on the basis of how far away the user will be from the target and the size of the target. For example, some instruments can measure a spot size as small as 0.3 in. in dia at a distance of 1 in. (low optical resolution). Such a feature is useful for measuring the temperature of refrigeration coils.

Devices are also available for long-distance readings and can measure a 1-ft dia target from a distance of 180 ft (high optical resolution). A number of IR thermometers today have D:S ratios with a range of optical resolutions. For instance, typical optics might be 8:1, 30:1, and 60:1 (Fig. 4).

Features . Units are available with a variety of features. These include:

– Laser sighting for pinpoint accuracy

– Maximum, minimum, differential, and average temperature calculations

– Audible high and low alarms

– Built-in datalogger for generating custom graphs and tables

– Adjustable emissivity to compensate for reflective surfaces

– Simultaneous IR and contact temperature measurement

– Windows-compatible software for data management.

Most users prefer laser sighting. The latest models offer a 16-point laser-sighting circle with a center aiming point to delineate the area being measured. For applications that require the monitoring of temperatures over time, onboard software lets the device store a series of readings. Data may be subsequently downloaded to a computer where it can be organized, displayed, and printed.

Cost. Basic models can be purchased for under $200. Units with more advanced features typically range from $350­$2000.

Noncontact infrared thermometry is among the technological innovations available today whose cost savings are apparent and efficiency improvements can be seen immediately with initial use. It has proven itself in scores of industries from high-tech manufacturing and food processing to automotive diagnostics. For the maintenance employee, non-contact temperature measurement means getting more done in less time under safer conditions.

Ease and speed of IR measurement lead to more frequent monitoring of HVAC systems and significant savings in all phases of system installation, maintenance, and troubleshooting. At a time when virtually every company is under pressure to reduce expenses, IR thermometry has become a timely addition to the maintenance technician’s toolbox. — Edited by Jeanine Katzel, Senior Editor, 630-320-7142,

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