Manufacturing Controls Since 1998
Effect of Modulating Zoning on Discharge Air Temperature, Discharge Plenum Pressure and Airflow through the Equipment
Introduction
A test was performed on an installation to determine the effect of using modulating dampers to change the distribution of heating or cooling to two zones.
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Modulating Zoning provides many advantages over traditional zoning that uses dampers that are either open or close.
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Material cost for an installation using modulating dampers is about half the cost of traditional zoning.
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Bypass duct and the bypass damper are eliminated because the dampers never close.
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The DAT sensor is eliminated because Modulated Zoning has minimal affect on discharge temperature.
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The 24VAC transformer and its electrical box are eliminated because modulating dampers use low power allowing the system to use the equipment 24VAC to power the control.
Contractors were concerned that modulating the airflow might adversely affect discharge air temperatures in heating or cooling, increase the duct pressure or change the airflow through the equipment that could affect efficiency. Here's what we found.
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The discharge temperature in cooling and heating was within 2oF of normal discharge temperatures eliminating the need for a DAT sensor.
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Duct pressure increased from 0.40 to 0.44 inch H2O when airflow to one zone was increased by 30% in cooling. When airflow to one zone was increased by 50%, the pressure increased to 0.46 inch H2O.
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Airflow through the equipment increased from 2276 to 2371 cfm when airflow to one zone was changed by 30% and increased to 2392 cfm when airflow to one zone was increased by 50% and eliminates the need for bypass.
Temperature Differential in Cooling
A test was performed in cooling to measure the temperature differential while the airflow distribution was changed by 50%. The results are shown in Table 1.
Airflow Distribution
Zone1/Zone2
100%/100%
90%/110%
80%/120%
70%/130%
60%/140%
50%/150%
Return/Discharge Temperature Differential
18 Degrees F
17 Degrees F
16 Degrees F
19 Degrees F
20 Degrees F
18 Degrees F
Table 1. Discharge temperature change in cooling.
Temperature Differential in Heating
A similar test was performed in heating and the results are shown in Table 2.
Airflow Distribution
Zone1/Zone2
100%/100%
90%/110%
80%/120%
70%/130%
60%/140%
50%/150%
Return/Discharge Temperature Differential
48 Degrees F
47 Degrees F
47 Degrees F
46 Degrees F
47 Degrees F
47 Degrees F
Table 2 Discharge temperature change in heating.
Discharge Plenum Pressure in Cooling
The Discharge Plenum pressure was measured using a 0 to 1-inch H2O meter while the airflow distribution was changed by 50%.
Airflow Distribution
Zone1/Zone2
100%/100%
90%/110%
80%/120%
70%/130%
60%/140%
50%/150%
Discharge Plenum Pressure
0.40
0.40
0.42
0.44
0.44
0.46
Table 3 Plenum air pressure in cooling.
Discharge Plenum Pressure in Heating
The same test was performed in heating and the results are shown in Table 4.
About the Test
The test was run on an installed 5-ton Lennox system. Airflow was measured using an Alnor hood and averaging 5 readings. Temperatures were digitally read. Pressure was read using a 0 to 1.0 inch H20 analog meter.
Table 4 Plenum air pressure in heating.
Effect of Airflow Adjustment on Zone Airflow and Total Airflow through the Equipment
The Discharge airflow distribution to Zone1 and Zone2 was changed by 50% and the airflow to each zone was measured.
Airflow Distribution
Zone1/Zone2
100%/100%
90%/110%
80%/120%
70%/130%
60%/140%
50%/150%
Zone1
Airflow
CFM
1170
1104
1009
817
695
608
Zone2
Airflow
CFM
1106
1182
1362
1532
1720
1784
Total
Airflow
CFM
2276
2286
2371
2349
2415
2392
Table 5 Zone and total airflow through the equipment.
Airflow Distribution
Zone1/Zone2
100%/100%
90%/110%
80%/120%
70%/130%
60%/140%
50%/150%
Discharge Plenum Pressure
0.38
0.38
0.40
0.41
0.42
0.43