By Andrew P. Nolfo, on July 21, 2018, The NEBB Professional – Q3 2018 Edition
Temperature measurements have always been a part of testing, adjusting, and balancing (TAB) work. However, the value of those measurements is subject to interpretation.
Contractual Issues
Measuring and reporting temperatures in mechanical systems are required by most of the standard specification sections prepared by the specifying engineer. While the requirements for temperature measurements are defined in the NEBB Procedural Standard, the engineer delineates the contract scope of work for the TAB services in the project specifications which takes precedence over the requirements in the NEBB Procedural Standard. If the design professional requires a project to be NEBB Certified, he is free to specify all requirements that may be in complete agreement with the minimum NEBB requirements, or he may completely define his own project requirements, including temperature measurements.
Capacity Testing Requirements
Aside from the historical and contractual status of the issue, the accuracy of field temperature measurements and the ability to utilize those measurements as an indication of heat transfer capacity in the field (when comparing field capacity to factory or rated capacity performance) must be examined.
Many designers, commissioning providers, owners and operational personnel are under the impression that normal TAB data, such as fluid flows and fluid temperatures which are identified in a final certified TAB report, can be used to easily equate the performance of a heat transfer device in the field to the manufacturer’s rated capacity performance. Nothing could be further from the truth.
Most manufacturers rate the capacity performance of almost all HVAC equipment through standardized testing procedures. While many manufacturers utilize independent or third-party testing agencies, some perform capacity testing in their own factory. Under either scenario, the testing procedure is usually performed in accordance with an industry standard such as ASHRAE, ARI, AMCA, Hydronic Boiler Institute, Cooling Tower Institute, etc. Many of the testing protocols are jointly sponsored or co-authored, and many testing protocols are also American National Standards Institute (ANSI) standards.
While the exacting requirements of capacity performance testing vary with the type, size and features of the various pieces of HVAC equipment, there are some imperative common threads:
- All capacity performance testing is performed in idealized laboratory conditions.
- Testing is performed in strict accordance with either industry accepted protocols or manufacturer’s standard practices.
- Instrumentation requirements necessary to measure and report data such as temperature, fluid flow, pressures, and electrical characteristics are industrial-grade, laboratory quality digital instrumentation that typically involve data logging of measurements over an extended period.
- Capacity performance of heat transfer equipment is determined under steady-state, or quasi steadystate, heat transfer conditions that must exist over an extended period.
- Capacity performance testing protocols are used to guarantee consistency in the performance of the equipment in the laboratory to control manufacturing tolerances. These ‘controlled testing conditions’ also allow engineers to compare performance of similar equipment from various manufacturers.
- These testing protocols are NOT meant to replicate the performance of the equipment in the field.
While the last bullet is probably the most important, the first four conditions do NOT exist in the real-world conditions in the field. For example, fan measurements in the field cannot be reliably compared to a manufacturer’s fan performance curve and they were never meant to. In the same way, capacity performance ratings of heat transfer equipment are difficult at best to replicate in the field.
Accuracy of Temperature Measurements
The NEBB TAB committee has developed and regularly maintains minimum standards for quality, accuracy and repeatability for instrumentation for various purposes. The requirements identify measurement type, appropriate instrumentation, accuracy, range, resolution and the minimum calibration requirements for instrumentation to be used in the field when performing TAB work. TAB instrumentation requirements is also addressed in ANSI/ASHRAE Standard 111-2008, Measurement, Testing, Adjusting, and Balancing of Building HVAC Systems.
One of the unique features of the ASHRAE document is the method the standard uses to describe instrumentation requirements. For many of the instruments, the standard discusses various standard TAB instruments, uses, limitations, calibration requirements and accuracy of field measurements. In other words, the title of the paragraph for each instrument is: Accuracy of Field Measurements. Yet, instrumentation accuracy is NOT the accuracy of the measured quantity in the field.
A thermometer, a manometer or electrical test meter may be rated by the manufacturer as having a stated accuracy, meaning the accuracy of the instrument. Most analog instruments are rated as a percent of full scale or percent of reading, while digital instrumentation is normally rated by the components of absolute accuracy and range accuracy.
A typical digital micromanometer used for measuring airflow velocity pressures may be rated as ±3 percent of reading plus ±5 feet per minute (fpm), meaning absolute accuracy will be within 3 percent of the measured value and the range accuracy will be within an additional 5 pm. As an example, a measurement of 824 fpm for a duct traverse, using this instrument, means the actual velocity could be within the range of 795 to 853 fpm. That would equate to an overall accuracy of ±3.5 percent, which is excellent.
Now, let’s take a digital thermal anemometer with the same rated accuracies measuring airflow velocity at the face of a fume hood. For this example, our measurement is 96 fpm. Using the same values as the previous example, the actual velocity could be within the range of 88 to 104 fpm. That would equate to ±8.3 percent accuracy, which may be acceptable. But, in either example, the values identified are instrument accuracy, not field accuracy.
The ASHRAE Standard discusses the accuracy of the instrument to measure a correct value when used by a technician in field conditions. This terminology of Accuracy of Field Measurements means that an average technician, standing on a 10-foot ladder performing a duct traverse will NOT be the same as the instrument accuracy. While any field measurement will be no greater than the stated accuracy of an instrument, the accuracy of the field measurement will almost always be worse than this stated instrument accuracy.
As a rule, the average accuracy of any field measurement will be ±10 percent. That is why the ASHRAE Standard, the NEBB Procedural Standards, the AABC National Standards and TABB Standards all reference ±10 percent as the acceptable tolerance for field measurements. It’s the standard. Yet we see designers specifying unobtainable tolerances on TAB work such as ±5 percent. If a facility design needs tighter tolerances, design criteria should change rather than the standard.
Finally, when taking temperature readings, accuracy is affected – sometimes drastically – by temperature uniformity of the fluid flow. For water systems, the temperature of the fluid is usually thoroughly mixed and of the same uniform temperature as it enters or exits the heat transfer device. Air temperatures are a different story.
The most common misnomer in the HVAC industry is the term “mixing box” as a standard part of an air handling unit. If outdoor air enters the “mixing box” at 20°F and return air is 75°F, it would not be unusual to measure mixed air temperatures anywhere between 20 to 75°F within the mixing box. Mixing boxes are poor “mixers” and other items within the air handling unit like filters, coils and fans offer little assistance in producing a homogeneous airstream with uniform temperature. Under these conditions, a temperature traverse and a weighted airflow volume/temperature calculation may be required, though performing an airflow traverse inside a mixing box is generally unreliable due to both turbulence in the airstream and the impact of the measurement technique required to get a traverse across a coil. Together, these items compound the issue of determining actual entering and leaving coil temperatures.
Accuracy of Capacity Calculations from Field Measured Data
Let’s examine an issue that would appear to be relatively simple: determining capacity performance from field measurements for a hydronic heating coil at a VAV air terminal unit. The following measurements must be made to determine capacity:
- Volumetric airflow
- Entering and leaving air temperatures (dry-bulb)
- Hydronic flow
- Entering and leaving water temperatures
While the above listing appears to be pretty simplistic in nature, let’s examine each component:
Volumetric Airflow
With several methods to determine airflow at a VAV box, the most accurate would be a traverse. performed at the inlet or outlet. In order to obtain what ASHRAE and AMCA define as an ideal velocity pressure profile, the duct traverse should focus on a straight section of duct of at least 2.5 inches in diameter. For a 10-inch VAV box, the traverse should be performed on a straight, hard section of duct at least 25 inches upstream of the inlet. Let’s assume the outlet duct is 12 by 10 inches, which is equivalent to 12.4 inches. A downstream duct traverse should be performed in a straight section of duct that is 31 inches long before any fittings, takeoffs, etc. While either of these conditions may be attainable, the accuracy of a traverse at either location would be within ±10 percent of the actual airflow.
In addition to a traverse, the inlet static pressure could be used to determine the airflow. The manufacturer of the VAV box provides a pressure/airflow chart directly on the side of the VAV box, which is dependent upon the inlet and outlet condition of the actual ducts. Remember, this capacity chart is produced under the same rating conditions as all other HVAC equipment, and not representative of field conditions. To equate field performance with factory ratings, the exact same testing parameters must apply, assuming the VAV box does not leak.
The outlets could be measured, and the values added together to determine the airflow. Of course, each outlet measurement has an accuracy of ±10 percent and we now must take into account any downstream leakage in the low velocity duct, spin-ins, flexible duct and connections to the air devices themselves. Again, the accuracy of the field measurement would be less than the traverse.
Entering and Leaving Airflow Temperatures (Dry-Bulb)
As the supply air enters the VAV box, it is safe to assume that temperature uniformity has been attained. We can now take a single temperature of the air. The field accuracy of that temperature using a digital thermometer should be within ±2 percent. Now, we need to take the temperature of the air after the heating coil. For the sake of discussion, let’s assume the temperature of the air leaving the coil is uniform and a single measurement can be used. Again, the field accuracy of this measurement will be within ±2 percent.
Hydronic Flow
The hydronic flow can be measured in by the following methods, ordered from greatest to lowers level of accuracy:
- A flow meter (venturi, orifice plate, magnetic flow meter)
- A calibrated balancing valve
- An ultrasonic flowmeter
- Using rated equipment pressure drop
- Using a pump curve
- Performing an energy balance
Because we are dealing with a hot water heating coil at a VAV box, the most logical would be a calibrated balance valve, and the coil piping may also be provided with pressure and temperature ports at the coil. Since we are focusing on accuracy, we will assume that piping for this heating coil has a calibrated balancing valve on the return line. The type of calibrated balancing valve does not impact accuracy; adjustable orifice, fixed orifice or self-adjusting should all have the same relative degree of accuracy. The field accuracy of a hydronic flow measurement will be within ±10 percent.
Entering and Leaving Water Temperatures
Again, there are several methods to determine the water temperatures. If the coil is provided with test ports, they could be used to measure the entering and leaving temperature. The ports should be located immediately adjacent to the coil connections. This is how the coil rating was determined by utilizing temperature and pressure measurements directly at the connections. Again, the field must replicate the laboratory if we are to utilize performance data that was obtained in the laboratory. If the piping material was metallic (copper or steel), we could simply measure the surface temperature of both the supply pipe and the return pipe and subtract the values to obtain the differential temperature. While this method may not be as accurate as actual temperatures measurements using test ports, this method is employed by TAB firms on a regular basis. Taking temperature measurements with the appropriate thermocouple and digital electronic temperature instrument should have a field accuracy of ±2 percent.
Heat Transfer Capacity – Airside
Now we can calculate the amount of sensible heat being transferred to the airstream by using Equation 1.
Equation 1: Sensible Heat – Air
Q = Cp • d • 60 • CFM • ΔT,
Where:
Q = Heat Transfer (Btu/hr)
Cp = Specific Heat of the air (Std Air = 0.24 btu/lb-°F)
d = Density of the air (Std Air = 0.075 lb/ft3)
60 = Constant (60 minutes/hour)
cfm = Airflow volume (ft3/min)
ΔT = Temperature difference between the leaving air and the entering air (°F)
Note: At standard conditions: Cp • d • 60 = 1.08
Heat Transfer Capacity – Waterside
Now we can calculate the amount of sensible heat being transferred by the heating hot water by using Equation 2.
Equation 2: Sensible Heat – Hydronic
Q = Cp • d • 60 • GPM • ΔT,
Where:
Q = Heat Transfer (Btu/hr)
Cp = Specific Heat of the water (Std Water = 1.00 Btu/lb-°F)
d = Density of the water (gal) (Std Water = 8.33 lb/gal)
60 = Constant (60 minutes/hour)
gpm = Water flow volume (gal/min or gpm)
ΔT = Temperature difference between the entering water and the leaving water (°F)
Note: At standard conditions: Cp • d • 60 = 500
Example #1:
We will use the following example to summarize all the above issues. A 10″ VAV terminal unit with a hydronic heating coil has been designed with the following requirements: 650 cfm (max), 200 cfm (min), 55°F entering air temperature, 100°F leaving air temperature. The heating coil is designed to heat the air using 0.972 gpm of heating hot water entering at 180°F and exiting at 160°F. The air terminal unit is installed at sea level. The sequence of operation identifies that the heating coil is not energized unless the VAV box is at minimum position.
Field measurements indicate the minimum airflow is 200 cfm, the entering air temperature is 56°F and the leaving air temperature is 98°F. The measured water flow is 1.00 gpm, the entering water temperature is 179ºF and the leaving water temperature is 161°F. Determine if the heating coil is providing its rated capacity.
Solution:
The design heating capacity is: Air:
Q = 1.08 • 200 • (100 – 55) = 9720 Btuh
The design heating capacity is: Water:
Q = 500 • 0.972 • (180 – 160) = 9720 Btuh
Thus, we have an energy balance.
Measured Air Heat Transfer
Based on our data, the measured heat transfer (air) is: Q = 1.08 • 210 • (98 – 56) = 9526 Btuh or 98 percent of design. However, each of our measurements contains a degree of error, i.e.: the field accuracy of the measurements. As previously stated, the airflow measurement and each of the temperature measurements contain an accuracy of field measurements. So, the individual values could be within the following ranges:
Airflow: 210 cfm, ±10% = 231 cfm – 189 cfm
E.A.T.: 56°F, ±2% = 57.1ºF – 54.8°F
L.A.T: 98 F, ±2% = 100.0°F – 96.0°F
Thus, the actual heat transfer could be as low as:
Q = 1.08 • 189 • (96.0 – 57.1) = 7,940 Btuh
Or, the actual heat transfer could be as high as:
Q = 1.08 • 231 • (100.0 – 54.8) = 11,276 Btuh
Measured Water Heat Transfer
Based on our data, the measured heat transfer (water) is: Q = 500 • 1 • (179 – 161) = 9000 Btuh or 93 percent of design. However, each of our measurements contains a degree of error, i.e.: the field accuracy of the measurements. As previously stated, the water flow measurement and each of the temperature measurements contain an accuracy of field measurements. So, the individual values could be within the following ranges:
Water Flow: 1 gpm, ±10% = 0.9 gpm – 1.1 gpm
E.A.T.: 179°F, ±2% = 182.6°F – 175.4°F
L.A.T.: 161°F, ±2% = 164.2°F – 157.8°F
Thus, the actual heat transfer could be as low as:
Q = 500 • 0.9 • (175.4 – 164.2) = 5,040 Btuh
Or, the actual heat transfer could be as high as:
Q = 500 • 1.1 • (182.6 – 157.8) = 13,640 Btuh.
So, let’s put all this data into perspective and put all of the design, measured and calculated data into tabular form to make some sense of it all.
| Heating Capacity | Btuh | % of Design | % of Measured |
|---|---|---|---|
| Design Value (Air) | 9,720 | – | 102% |
| Measured Value (Air) | 9,526 | 98% | – |
| Low Value of Field Accuracy (Air) | 7,940 | 82% | 83% |
| High Value of Field Accuracy (Air) | 11,276 | 116% | 118% |
| Design Value (Water) | 9,720 | – | 108% |
| Measured Value (Water) | 9,000 | 93% | – |
| Low Value of Field Accuracy (Water) | 5,040 | 52% | 56% |
| High Value of Field Accuracy (Water) | 13,640 | 140% | 152% |
As shown by the data in Table #1, the actual sensible heat transfer to the airstream using the measured data can be accurately stated as being somewhere between 82 to 118 percent of design. The actual heat transfer of the water using the measured data can be accurately stated as being somewhere between 52 to 152 percent of design. Not very impressive, is it? It’s also not very accurate. Remember, this is a simple VAV box with a single row hot water coil. If this would have been an 8-row chilled water coil with wet-bulb temperatures, the results would have been even worse.
In conclusion, taking and reporting temperature measurements associated with TAB work is not a valid method of comparing field performance to rated capacity. The only value in temperature measurements is to approximate the instantaneous heat transfer at that single point in time. And it has absolutely nothing to do with capacity. Utilizing field data to compare rated equipment capacity performance should not be done due to the inaccuracy of field measurements. Standards and protocols used in rating capacity performance are not meant to be performed in the field.
And finally, remember designing a project’s HVAC systems is not an exact science. Performing TAB is not an exact science. That’s why they are both identified as engineering.
About the Author
Andy Nolfo, NEBB TAB CP and BSC CP, is a former NEBB Technical Director and TAB Seminar Instructor. He has served on numerous NEBB committees, currently as a member of the TAB Committee.