The whole idea behind zoning HVAC systems is to deliver the correct amount of air to the right place. Balancing a zoned system is even more critical because of design complexity and constantly changing airflows, temperatures and pressures. Balancing is also a very profitable add-on sale to any zoning system installation.

Air is heavy. A five-ton cooling system moves 9,000 pounds of air an hour. That large volume of particles in the air is what absorbs heat and moves it around to cause the desired temperature change in the building. The zoning system directs the airflow to be sure comfort is achieved.

Design is important, but design alone can’t guarantee the system will perform as it should, under constantly changing conditions. Testing and balancing insures the system builder and the designer accomplished what they set out to do.

When balancing a zoning system, there are several steps added to the process compared to a constant volume system.

The first step is to inspect the system. This sounds really basic, but it’s a critical step in balancing a system. Did all the duct runs get installed? Are all the end caps there? Are the supply registers delivering supply or return air? Was the bypass duct installed? Were all the dampers installed and connected? Are the manual and bypass dampers open?

Balancing Procedure
To begin balancing, set all zoning dampers in open position. The better zoning systems have a switch in the control panel that allows the balancer to do this. You could also set each thermostat to cooling mode at 55F with the fan on to open all the zoning dampers 100%. Then, using the manual balancing dampers, balance each grille and register for the amount of airflow that is required of it under normal conditions – as though it was a constant volume system.

Add all the supply register airflows together and make sure it equals 400 CFM/Ton. If airflow is low, measure the total external static pressure by measuring the pressure before and after the air handler and add the pressures together. If needed, measure the pressure drop over the filter and the coil. Increase the 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.

Next, traverse the bypass duct to check the volume of the bypass air. This should be minimal with all the zoning dampers opened.

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

Final Testing
Manipulate the thermostat controls to open and close each of the zones in the system. Test and record airflows and temperatures through the system under its 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 throughout the heating and cooling equipment. Check for excessive changes in total external static pressures or a temperature drop or rise in the system.

Instruct the customers how to use the controls properly 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 specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in zoning system balancing procedure, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at for free information, technical articles and downloads.

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.