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.

Modulating Zoning provides many advantages over traditional zoning that uses dampers that are either open or close.

  • Material cost for an installation using modulating dampers is about half the cost of traditional zoning.

  • Bypass duct and the bypass damper are eliminated because the dampers never close.

  • The DAT sensor is eliminated because Modulated Zoning has minimal affect on discharge temperature.

  • 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.

  • The discharge temperature in cooling and heating was within 2oF of normal discharge temperatures eliminating the need for a DAT sensor.

  • 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.

  • 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.

For information about this test, contact:

Mike Beck

949-916-0945

mikeb@econtrolsusa.com

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