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A Valuable Aid for Duct Balancing

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It’s always good for software developers to have to use their own products and to get out into the field and see what users are actually facing. In the last few years, we’ve worked with customers who were balancing new duct systems designed with Right-Suite Universal (RSU). Of course, system designers have carefully done the load calculation with Right-J, then selected the equipment using the Manual S selection tools in RSU, and finally, done the duct system design with Right-D. On top of that, registers were selected to give proper throw and dispersion using the register portion of RSU. The next question became, “Can we balance this duct system easily and quickly?” It ought to be easy and it ought to work as designed.

As a result of our field work with customers and after years of talking to all of you about commissioning, we realized that we could make a contribution by automating some parts of the duct balancing process, and we decided to add duct balancing to RSU. This month (July 2016), we will release a new module for RSU, Right-Balance, and this article explains how it works and how to use it.

In ACCA’s Quality Installation Specification (QI 2010), Section 5.2 says simply “The contractor shall ensure room airflows meet the design/application requirements.” Furthermore, that section says that for residential buildings, individual room airflows will be within +- 20% or 25 CFM of the design requirements for the supply and return ducts. (Note that commercial limits are +- 10% or 25 CFM.) That same section says that you actually measure the airflow with an approved device or method and that you will leave behind “documented field data”. So Right-Balance is designed to make it easy to use your duct system design CFM as the basis for data collection and to produce the final report for documentation.

In a manner similar to ACCA’s Manual B, Section 6.4, the overview of the balancing process is that Right-Balance provides a form onscreen, listing all the registers and the design CFM for each register. Next to the design CFM is a column for actual (measured) CFM. The idea is simple: after the air handler is set up (more on that later), measure the actual CFM at each supply register, and enter it into the form. Once all registers are measured, the program computes the ratio of actual to design CFM and then suggests the order and the amount to adjust each register. You adjust the dampers for about five (or your choice) branches with the highest actual to design ratio down to about 90% (your choice) of the design CFM and then on the second highest register you increase to 92% of design CFM and so on, increasing each register by 2%, until you’ve adjusted all of the five highest ratio registers. Once you enter the actual CFM’s, Right-Balance does the calculations and suggests which registers to adjust, and the target CFM.

After you’ve adjusted the five (or your choice) registers, the excess air from the highest ratio registers has been re-distributed into the lower ratio registers, and you can re-measure and repeat. With a well-designed duct system, you may not have to make more than two trips through all the registers. The program indicates which registers are within their allowable tolerance by a light green background, and out-of-range registers by a red background. When all the fields are green, you’re done.

The setup for this effort is the same as mentioned in any balancing text or, for example, ACCA’s Manual B. Essentially, in a multi-zone damper system for a single piece of equipment, the air handler should be set to maximum airflow setting (preferably 110% of the design CFM), the individual zone dampers should be fully open, and if there is a bypass damper it should be closed. In this mode, you are going to balance all registers in all zones for that one piece of equipment. Each room should be tested with doors closed, and then re-measured with doors open.

In situations where there are several secondary trunks (i.e., basement, first floor, second floor), we have added a small worksheet to make it easy to adjust the pressure in each secondary trunk by measuring actual pressure and CFM and computing the target pressure. Using this method, which is similar to Manual B, section 6.5, you will be able to get the trunk CFM into line before attempting to balance the registers for each secondary trunk. By using this trunk damper pressure worksheet first, balancing a multi-floor system will be much easier.
Right-Balance isn’t complicated, and it’s really easy to just measure and enter actual values and then re-adjust iteratively. Of course, good duct design is the first step in easy balancing.

Bill Wright

Posted In: ACCA Now, Technology

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