LEXINGTON, MA — “We’re going to make some changes to our office space and we’d like you to take a look at the hvac system to see if it needs any modifications. Could you drop by and give us an estimate?”

Commercial hvac contractors get this request all the time; sometimes it’s offices, and sometimes it’s a library or retail space. Sometimes there is an expansion planned, and sometimes the space stays the same — the only change is how the space is used.

“The objective is always to figure out whether the existing hvac system can serve the new usage, or how to rework the system to save as much of the existing system as possible,” said Bill Wright, president of WrightSoft Corp., a provider of residential and commercial hvac design software based in Lexington, MA.

He said the building owner frequently views the existing system as a valuable asset that only needs a little modifying. The owner’s question usually is, “Can we just change a few ducts and keep the rest?”

The answer can vary from a complete replacement of the entire system, to replacing a single duct run, to moving diffusers in the drop ceiling. Performing retrofits and modifying systems is a complex task; the hvac contractor needs a simple procedure and an uncomplicated tool that does not require much design effort.

To evaluate possible modifications, the hvac designer can use software tools that will answer several design questions and then quickly produce an estimate to convert the existing system into a redesigned system.

Given a list of parts to be removed, and a list of new parts to be installed, the estimate can be produced with standard takeoff and estimating techniques. The hardest part is playing “what if” with the existing system in the new building situation.

“In other words, isn’t there some way to easily figure out the minimum changes to the existing unit and ductwork so that they will work well in a new building configuration, and to get a list of those changes so that the estimate is easily calculated?” asked Wright.


Wright used his own software program, Right-Suite Commercial (RSC), as a model for duct design and evaluation:

Design Mode: The program uses load-based calculations to specify the number of registers and the airflow required for each register. Whether the duct system is automatically generated, or manually drawn by the user, RSC computes duct sizes and the required fan pressure for any of three calculation methods: equal friction, modified equal friction, and static regain. The design mode can also design duct system sizes to match a specified airflow and/or pressure.

Existing System Mode: This mode assumes that the duct sizes and fan pressure are known, and uses these data to compute the actual cfm for each register. The existing system mode is valuable for determining how out of balance an existing system may be, and for playing “What if?” to re-design poorly performing systems.

“Neither of these modes will allow us to keep part of any existing system fixed while changing and evaluating the rest of the system,” said Wright. “We realized that if we could use the design mode, with the ability to designate existing duct sizes as fixed (unchanging) sizes, we could evaluate proposed modifications while keeping parts of the existing system.”

Wright pointed out that the next version of RSC will be released with some additional description.

“Any duct or component in the design can be marked ‘existing,’ and RSC will then modify the rest of the duct system as needed to supply the required airflow,” he said.

RSC was developed specifically for retrofitting jobs and, according to Wright, the software now has the ability to evaluate airflow and duct size modifications to a system while keeping portions of the ductwork fixed.

Wright used RSC as a model to show how software can be used to modify the design of an existing system to meet a new building usage situation, and how the software is used to produce the estimate.


The first question that comes to mind is, “Will the existing equipment meet the new load?” If the existing equipment is too small for the new system, the equipment can be changed or additional changes to the ductwork can be made.

“We believe that there is logic in asking, ‘Does the existing equipment meet the existing load?’” said Wright. “Phrased another way, ‘Is the existing system sized correctly as installed?’

“If the existing system is incorrectly sized, the owner should know about it. Most owners will appreciate that piece of data for establishing credibility, and the contractor can use the data to help make other decisions.”

Question 1. Does the current system meet the current loads?

“To determine this, we describe a building by using as-built drawings or sketches, and by drawing in the existing duct system,” said Wright. “Using RSC’s drag-and-drop tools, we draw the floor plan for the existing space [not necessarily the entire building] and then draw the duct system.

“RSC automatically calculates the loads and sizes the ductwork accordingly. We have a clear idea of how well the installed system was designed, and we immediately identify potential trouble. For example, if the main trunks are oversized for the existing building usage, then it is possible to keep that ductwork for expanded use. If the main trunks are undersized, the system is already inefficient and could be a limiting factor when designing the new system.

“Again, checking the existing system can help identify specific ‘trouble’ fittings that can drastically affect system performance.”

Question 2. If a duct system is redesigned to meet the new building’s usage and layout, will the existing unit have the capacity and pressure to meet the load and new duct layout?

“To determine if the existing unit will be sufficient for the new building design, we make a copy of the project file and modify it to show the new floor plan,” said Wright. “Then we modify the layout of the existing duct system, attempting to meet the calculated loads. We might remove some parts of the system and add new trunks and branches. Using the one-line manual-input mode of RSC, we can delete or add ducts and registers as needed.”

Figure 2 shows the new floor plan with an additional trunk to supply the new room. The load in the main office area will be reduced due to the loss of the east-facing windows and east exterior wall. RSC has removed one register; it is not necessary due to the reduced load.

“At this point we have a proposed modified duct system, which RSC has sized to meet the new loads,” Wright added. “RSC has computed and sized the system and calculated the pressure required for that system using the new layout.

“We then decide what existing ductwork we can keep with the new layout,” said Wright. “We know the pressure requirements for the unit and duct sizes for a completely new system, calculated by RSC [i.e., with none of the duct sizes from the existing system] with the same layout as the existing system.”

Question 3. What minimum changes need to be made to the existing duct system to meet the new loads?

“To complete this step, we use RSC’s new ability — we mark any duct as ‘existing’ and input its current size,” said Wright. “RSC will recalculate the pressure required to deliver airflows that meet the load, using that fixed duct size. We have input the original duct dimensions as the existing sizes.” (See Figure 1.)

Figure 3 shows the resulting system with the existing ductwork, he explained.

“We replaced the missing register and reduced air delivery in the main room, both of which help to relieve the effect of making the right-hand supply trunk 11 in., rather than the 14 in. requested by RSC,” he said. “With this configuration, the fan pressure required increased to 0.49 in. wc, which is well within the existing unit supply capability. To double-check our design, we can switch to the Existing System mode to look for balancing problems.”

Figure 4 shows RSC’s prediction for the installed system before balancing, he explained.

“Clearly this system will require some work to balance it, since the new office area is undersupplied,” he said, noting this is partially due to keeping the undersized 11-in. trunk.

Question 4. How can the list of ductwork be added with other changes to produce the estimate?

“RSC’s normal mode of operation is to combine its automated list of duct parts with any other parts or labor you may have added to the project. In the new design mode, any parts that you have marked as existing are omitted from the list,” said Wright. “Your estimate automatically includes only the new parts. Figure 5 shows the on-screen, automatically compiled parts list.

“This simple example has illustrated the steps to evaluate a retrofitted system. We made this one simple so that you might say you could have predicted the result by looking at it.”

John Frugard, C.E.M., vice president and general manager of Capitol Engineering, Newton, MA, contributed to this article. For more information on RSC, visit www.wrightsoft.com.

Publication date: 05/13/2002