To determine whether there is a leakage in the air handlers, fan coil and terminal units, leak-by test methods in this document can assist in verifying proper system performance in both new and existing applications.
Valves are tested for leak-by to determine if they are leaking water when they have been set to close. There are several possible test methods, and the most appropriate one depends on whether the purpose of the test is to detect small leaks (15% of design flow or less) or large leaks after the system and component configuration is known. Some test procedures are more time consuming and others can be inconclusive.
This article covers the Heating and Cooling Coil Valve Test Procedures. Refer to Valve Leak-By Test Procedures for full testing guidance.
1. Balancing Valve Method
Many air handlers have balancing valves (circuit setters) on the pipes to their heating or cooling coils which can be used to detect valve leakage by measuring the pressure difference across the valve. This method is only valid with 2-way control valves. Another test method must be used if testing a 3-way control valve.
1.1 Test Setup - Turn the water pump ON and ensure it is providing normal flow to the air handler. Carefully mark the balancing valve’s setting, if not already marked. For linear gauge types, mark the gauge. For screw type adjustments, count the turns as you turn the balancing valve fully closed. Open all balancing valves serving the control valve being tested to 10% open.The balancing valve must not be open further than about 10% or a small control valve leak would give too small a pressure drop across the balancing valve to be conclusive.
1.2 Valve Closure Performance - Command the coil control valve closed. Measure the pressure drop across the balancing valve. A pressure drop more than one psi indicates leak-by. If results are inconclusive, close the manual isolation valves and see if the balancing valve differential pressure lowers, indicating leak-by in the first test. When taking pressure measurements, be sure to keep the transducer at the same elevation during each measurement of a given port so that atmospheric pressure doesn’t skew the results.
1.3 Return system to normal - Relocate the balancing valve precisely to the pre-test setting.
2. Water Temperature Across Coil Method
Because of the effort required by this method, it is most suited to larger valves (>2 inches) on air handling units when balancing valves are not available. This method provides very conclusive results and is applicable to both 2-way and 3-way control valves.
2.1 Test Setup - Turn the water pumps and the heating or cooling plant (chiller or boiler) ON, so there is a source of heat exchange. The pump flow rate may be fixed or variable (i.e., leave in normal control). The water supply temperature set point typically can be left in normal mode with any reset sequence in place, since the reset is slow in relation to the test duration.
2.2 Establish Baseline - Determine central air handler or fan coil unit baseline by commanding the supply and return fans OFF and commanding the coil valve being tested to 100% open. For a terminal box, baseline can be established by commanding air handler OFF or commanding the primary terminal box air damper 100% closed and the coil valve being tested commanded 100% open. If a fan-powered terminal box (either series or parallel) is being tested, ensure the terminal box fan is commanded OFF in addition to following the damper/valve commands outlined for a standard terminal box. Command the central hot water or chilled water plants (i.e. boilers, chillers, distribution pumps, etc.) to be operating and delivering water at design temperature through the system. The intent of turning off fans, closing dampers, and opening the control valve is to eliminate heat transfer from the water due to air flow across the coil. Allow the system to operate for approximately five minutes to stabilize the system.
Check the following:
2.2.1 Measure the supply water temperature near the coil or use the building automation system reading from near the plant. Measure the return water temperature between the outlet of the coil and the point the coil leg returns into the main return pipe. The return water measurement should be made at a point one to two feet (or more) away from the coil as well as two or more feet away from where the coil return pipe ties in with the main distribution pipes. The intent is to minimize the influence that the coil or main distribution pipes may have on the temperature readings. The length of time that the measurements will be valid is significantly reduced, the closer the measurements are made to either the coil or distribution lines, due to heat transfer in the pipe and water from the coil.
2.2.2 The supply and return water temperatures should be within 1°F of each other when the supply water is measured near the coil and within 3°F when the temperature is measured near the plant (to account for heat loss along the piping length). Record this water temperature difference (supply – return).
2.3 Valve Closure Performance - Continuing from the last step with the supply and return water temperatures stabilized, command the coil valve closed and command the corresponding supply fan ON and/or damper open (terminal box). The intent is to allow air to flow across the coil and remove heat or “cool” from the water in the coil. Allow the system to operate at this condition for at least 3 to 5 minutes. Figure 1 illustrates the supply and return temperature profile for a sample valve leak-by test for both heating and cooling coils. Figure 2 illustrates how the water temperature will change over time at the return measurement location when there is no leak-by, based on pipe diameter and the differential between the return water in the pipe and ambient air temperatures.
Figure 1 - Coil Valve Leak-by Test Illustration
Figure 2 - No-Leak-by Return Water Temperature Change in First Hour
Check the following:
2.3.1 Measure supply and return water temperature at the same location used to establish baseline temperature difference. If there is leak-by at the control valve, the return water temperature will change rapidly due to the heat transfer of the air stream across the coil. If there is no leak-by at the control valve, the stagnant return water will slowly creep towards the temperature of the surrounding air. After 3 to 5 minutes, if the supply - return water temperature difference has changed by more than 2°F then leak-by is indicated. If the readings are taken longer than 10 minutes after the valve was closed, there may be some detectable change in the return water temperature (and thus the supply - return difference) even when there is no leak-by, as shown in Figure 1 and Figure 2.
2.3.2 If the test data is inconclusive, try executing the entire test again but closing the manual isolation valves along with the control valve. This should provide the temperature profile for a positively closed coil valve. If the test fails, the test must be performed again once the necessary repairs are made by the appropriate party.
2.4 Return System to Normal - Once all tests are complete, return all control parameters back to original set points and conditions per the design sequence of operations.
3. Infrared Thermometer Method
This method is a variation of the Water Temperature Across Coil Method and measures the temperature of the exposed coil or piping from a distance. It most applicable in identifying gross valve malfunctions and is not generally suited to detecting small fractions of leak-by (< 10% of design flow). It is fast and efficient and is recommended for checking large numbers of reheat coils in air terminal boxes or valves serving radiant heating devices. This method is applicable to both 2-way and 3-way control valves.
3.1 Planning - In new construction, inform the construction team that you will need access to all terminal boxes or radiators for this test. For terminal boxes, this will mean keeping the ceiling tiles below the boxes left out until after the test. Make sure infrared thermometer used during the test is suitable. It should be at least accurate to ±3°F and have a large measurement area-to-distance ratio. Among various models, the typical measurement area-to-distance ratios range from 8:1 to 20:1, with a larger ratio being more accurate. For example, a 20:1 ratio means that when taking a measurement 5 feet from the device , the instrument will average the temperature readings over a 0.25 foot (3 inch) diameter spot [1x5/20]. Whereas, an 8:1 ratio instrument will average the same temperature over a 9 inch diameter spot measured at a distance of 5 feet. A larger measurement area-to-distance ratio means that a more accurate reading can be made at a further distance.
3.2 Test Setup - For air terminal boxes, command the central air handler supply fans ON and the respective primary air valves 100% open. Command all the heating coil valves being tested 100% closed. This will typically be by floor or group of floors. Wait at least 30 minutes more before taking any temperature measurements so that any residual heat in the coil has fully dissipated and the coil temperature is near supply air stream temperature for air terminal boxes and near room temperature for radiant coils or radiators.
Make sure heating water is being supplied to all zones to be tested. Command the distribution water pumps and the heating plant ON. The pump flow rate can be left in normal mode, but should be variable if all valves will be shut at once. The hot water supply temperature set point can be left in normal mode with any reset sequence in place.
3.3 Readings - Take a temperature reading using an infrared thermometer on the exposed coil ends near the supply-side of the coil for air terminal units. For radiant coils or fin tubes take a reading directly on the fins on the supply-side of the device. Only take readings near the supply end of the coil, since hot water from a small leak may be totally cooled off by the time it gets to the outlet of the coil. It is recommended that the distance between the infrared thermometer and surface being measured be 5 feet or less to ensure an accurate measurement.
3.4 Interpretation - An exposed coil end fin tube near the entering supply should read within 10°F to 20°F of the supply air temperature for an air terminal unit or ambient air temperature for a radiant/fin tube device if there is no leak-by at the valve and the system passes.
If hot water was flowing through the coil prior to closing the valve and the temperature measurement is made on the supply piping close to the coil because the coil end is not accessible, then the acceptable temperature variation will be larger than the range provided above. Remember that an insulated pipe with stagnant 170 F supply water in a 70 F room will still be about 140 F after one hour (refer to Figure 2). In this case if the temperature measurement is less than 140°F, there is most likely no leak-by at the coil and the system passes. Note: With the infrared thermometer, SAT at the diffuser could also be checked and compared to SAT at the Air handler.
3.5 Return system to normal - Once all tests are complete, return all control parameters back to original set points and conditions per the design sequence of operations.
4. Air Temperature Across Coil Method - Spot Test
This method is fast, but can be inconclusive unless there is a large control valve leak. The results can be inconclusive because in some circumstances. For example, a 20°F differential coil with a valve leakage of 10% will only result in approximately 2°F temperature change in the air stream. Such a small differential may be difficult to detect across the entire air stream via spot (i.e. single point) measurements. This test is applicable to central air handling units and terminal unit coils with either 2-way or 3-way control valves.
4.1 Test Setup - Make sure hot or chilled water is capable of being provided to all zones being tested. Command the distribution water pumps and the central heating and cooling plants OFF.
4.2 Establish Baseline - For central air handling units, command the supply fan ON. If testing air terminal boxes, command the central air handler supply fan ON and the respective primary air valves 100% open. Command all the coil valves being tested to 100% closed. This will typically be by unit for central systems and by floor or group of floors for terminal units. In addition, close the manual isolation valves on the supply and return pipes serving each device to guarantee no water flow through the coil. Wait at least 30 minutes before taking any temperature measurements so that any residual energy in the coil has fully dissipated.
Check the following:
4.2.1 Measure the mixed and supply air temperature upstream and downstream, of the coil. If possible, make the measurements using the same temperature sensor in several locations within the mixed air plenum and supply air plenum to determine an average temperature for both parameters.
4.2.2 Calculate the differential between the average mixed and supply air temperatures. They should be within 2°F of each other. If the temperature difference is greater than 2°F, then there may be sensor error (if using multiple sensors) or not enough time has elapsed to remove all residual energy from the coil. Wait an additional 10 minutes before taking the readings again.
4.3 Valve Closure Performance - Continuing from the last step with the coil valve 100% closed and the mixed–supply air temperature differential stabilized, open the manual isolation valves. Command the distribution water pumps and the central heating and cooling plants ON. The pump flow rate can be left in normal mode, but should be variable, if all valves will be shut at once. The hot and chilled water supply temperature setpoint can be left in normal mode with any reset sequence in place. Wait at least 15 minutes before taking any measurements.
Check the following:
4.3.1 Measure the mixed and supply air temperature upstream and downstream, of the coil. If possible, make the measurements using the same temperature sensor in several locations within the mixed air plenum and supply air plenum to determine an average temperature for both parameters.
4.3.2 Calculate the differential between the average mixed and supply air temperatures. If the temperature difference is greater than 5°F, then there is likely leak-by at the valve. With careful measurements, even a 3°F differential can mean leak-by. Unfortunately, the normal variation of air temperature across the coil and the inaccuracies of sensors don’t allow firm conclusions with differences less than 3°F. If the results are inconclusive, consider performing an alternate test.
4.4 Return all systems to normal - Open manual valves, release fan and pump overrides, etc.
5. Other Coil Control Valve Methods
There are other viable methods for detecting leak-by on coil cooling and heating valves, but they are generally more time consuming or less conclusive. They are briefly mentioned here for reference.
5.1 Air Temperature Across Coil ~ 30 minute Test - This test is viable when the BAS accurately monitors air handler supply and mixed air temperatures or the terminal unit discharge air on a terminal unit valve test (or an array of data loggers could be used). This method only works if the sensors on both sides of the coil are calibrated to be reading within 0.2°F of each other first. The method involves monitoring the air flow for 30 minutes with the control valve commanded closed and then with the all manual isolation valves closed. This method is more conclusive than the Air Temperature Across Coil - Spot Test method. The test and evaluation procedures are the same as for the spot test, except that the mixed and supply air temperatures are trended for at least 30 minutes at 2 minute intervals once the initial 30 minute time delay to remove residual heat from the coil has expired.
5.2 Differential Pressure Across Coil - This method measures the change in differential pressure across the coil. The method is fast, but can only detect larger leakage rates, since a small leak through the valve represents a very small pressure drop across the coil. The procedures are to turn ON the pump and close all manual coil isolation valves on both supply and return to ensure no flow through the coil. Measure the pressure across the coil. The difference should be zero. Then, command the automatic isolation valve closed. Open the manual isolation valves. Measure the differential pressure again. If the readings with the automatic isolation valve commanded closed show a consistent differential pressure over 1 psi, then leak-by is likely.
5.3 Coil Drain Down - This method is absolutely conclusive, but is suited only to small coils or air terminal boxes with 2-way control valves installed on the supply-side of the coil. Location of the control valve is critical since water will continue to flow to the coil if the control valve is not in the supply line. The test is only recommended for smaller coils since the method requires the entire coil to be drained down, and it should only be performed when potential water spillage will not result in damage to the surrounding area. To execute this test, command the coil vale to 100% closed. Manually close the isolation valve on supply side of coil and open the air bleed cap. Open the drain-down cock and drain water from coil. Water should eventually stop draining. If it doesn’t there is a leak past the control valve.