Balancing a Zoned System

Balancing a zoned system requires a few more steps than balancing a typical constant volume system. Let’s take a look at a zoned system balancing procedure and investigate the differences.

The first step is to inspect the system and compare the plan to what actually got built. This sounds really basic, but it’s a critical step in balancing a system. Check the usual components; did all the duct runs get installed? Are all the end caps there? Are the manual dampers open? Are the supply registers delivering supply return air?

Next, inspect and check the zoning components. It’s the bypass duct installed? Is the control panel wired correctly, and all the dampers installed and connected? Does each of the dampers operate freely when called to function?

Also, check for pressure imbalances that may be caused in a building when dampers open and close. Each zone should have a return air duct that can handle the airflow under the zone’s highest airflow condition.

To begin balancing, first make the system act like a constant volume system by setting all zoning dampers in the open position. The better zoning systems have a switch in the control panel that allows the balancer to do this, or set each thermostat in cooling mode at 55F with the fan on to open all the zoning dampers 100%. The added work increases the labor about 25% to a balancing project, so be sure adequate time is available.

Then, using manual balancing dampers, balance each grille and register to the amount of airflow that’s required under normal conditions. Balance the system as though it was a constant volume system. For more basic balancing information, see the balancing procedures available at the end of the article.

Add together all the supply register airflows and check for 400 CFM per ton. If airflow is low, measure total external static pressure by measuring pressure before and after the air handler and adding the pressures together.

If needed, measure the pressure drop over the filter and the coil. Increase fan speed as needed. Then adjust the manual balancing dampers so each register gets the required airflow. The accepted rule is plus or minus 10%. Lock down and mark your manual balancing dampers. The system is balanced under constant volume conditions.

Do final testing that includes system temperatures, static pressures, blower motor and draw, and fan RPM. Record these numbers on your balancing report.

Close down the largest zone in the system, and traverse the bypass duct to check the volume of bypass air.

Then, by manipulating the thermostat controls, open and close each of the zones in the system. Test and record airflows and temperatures through the system under different operating conditions. Check for noise caused by excessive velocities.

As the zones open and close, measure the airflow passing through the bypass duct. Adjust it as necessary to assure quiet system operation and adequate temperatures through the heating and cooling equipment. Check for excessive changes in total external static pressures or temperature drop or rise in the system.

Mark your final damper settings. Complete your final equipment testing by measuring and recording full open pressures, temperatures, speed, and electrical values, and complete the job as you would a constant air volume system.

Write an advisory statement to include with your final report describing the operation of the system with the largest zone closed. Include additional test numbers and observations you’ve made of the system. Make any design change or repair recommendations that may improve the quality of the system, if necessary.

Instruct the customer in proper use of the controls and help them understand what they can expect from their new system. Present a final, bound copy of the balancing report with all the required paperwork, warranty information, and service agreements.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company with technical and business level membership organizations. If you're an HVAC contractor or technician interested in balancing procedure for a constant volume or zoned system, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at for free information, articles and downloads.

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