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    Retrofit HVAC System Design

    Dec. 13, 2019
    It’s estimated more than 80% of HVAC retrofit projects only replace the system’s equipment. Tradition dictates box swapping is good enough. Let’s take a look at a different retrofit approach that offers more.

    It’s estimated more than 80% of HVAC retrofit projects only replace the system’s equipment - tradition dictates box swapping is good enough. Let’s take a look at a different retrofit approach that offers more. It allows you to assess and redesign the system to deliver a far superior and more valuable installed HVAC product. Are you interested?

    Retrofit Design
    The redesign approach to retrofit systems has been used for years and is increasing in popularity among better residential HVAC and mechanical contractors, engineering firms, and facility personnel. It’s also being specified in federal project evaluations across the country.

    All too often equipment replacement requires little or no design. Many residential and light commercial designers simply use the HVAC equipment model number and call for another unit the same size.

    Installers figure out the job details and fabricate or buy transitions to connect old duct to the new equipment and make electrical and gas connections. Throw the switch and the box is swapped.

    Low-end engineers offer a more expensive but comparative service by adding mechanical plans with an equipment schedule, a few technical illustrations, and a half-page of generic specifications.

    Many of these engineers have limited hands-on involvement with their projects and little interface with their customers. It’s no wonder why they, and their contractor partners, wind up running the race to the lowest price and end up delivering low-quality results.

    Typical HVAC retrofit project design isn’t designed at all; it just looks like a design to facilitate equipment replacement.

    Retrofit System Assessment and Redesign
    When you approach a retrofit HVAC project, you have a unique opportunity that is unavailable to the system designers described above.

    A retrofit project has an existing system. The opportunity is to address the very issues that may have resulted in early equipment failure and solve long-standing comfort, performance, safety, and health issues. You can also test and diagnose the installed system and build many additional solutions into your project designs.

    When you complete these steps, you can prescribe, sell, and install badly needed improvements to the system. Your customers want the improvements but are not aware of them until you identify the defects and describe the benefits.

    Does it take extra effort and creativity on your part? Sure, it does. Does it cost more? Yes. Are customers willing to invest more for a better product? Yes, they are, but you must help them understand the products and services that bring them what they want and need.

    The reason more contractors, designers, and engineers take the time to check and upgrade the entire system is twofold. First, best practices require it. Second, many find a significant new opportunity for themselves and their customers when addressing the entire system. This assures high-efficiency system performance, in addition to high-efficiency equipment.

    Incorrect Design Assumptions
    The next time you approach an equipment replacement project, consider the many assumptions you make when you believe the system will perform according to manufacturer specifications.

    The reality is the system will not perform to equipment specifications because installation conditions often force the equipment to operate outside specifications. The system will not perform the way you promised your customer.

    Common retrofit designer assumptions and corresponding solutions may include:

    1. The replacement equipment matches the building load. It’s well documented many systems operate at half the equipment rated capacity. If the new system is the same size but functions near full capacity, might it be oversized by 50%? Consider downsizing equipment when your design enables the installed system to deliver full capacity.

    2. The ductwork has the capacity to deliver required system airflow. It’s not uncommon for restrictive ductwork to deliver less than 70% of required system airflow into the conditioned space. Many duct systems are undersized and overly restrictive. Your redesign can include increasing duct capacity.

    3. Combustion and refrigerant charge are properly adjusted. It’s often a long trip from the factory to the job site. Installation conditions also vary widely. Airflow readings are required to correctly interpret refrigerant and combustion adjustments. Include these tests, adjustments, and verification steps in your redesign specifications. 

    4. Individual room airflow is satisfactory. This ever-present problem is corrected through duct modifications required in your redesign. You can make the corrections, only if you are able to measure and quantify the problem. Increase duct capacity and remove pinch points in the duct system.

    5. System accessories function well. When system accessories remain, verify the function of each one. Air filters, humidifiers, outside air ducts, heat recovery ventilators, zoning systems, thermostats, or control systems may need upgrade or repair. Address each of these in your system redesign.

    Of course, this list of assumptions and what you can include in your design to correct the associated problems are endless.

    The next time you arrive at a replacement project, budget some additional time and discover additional products and services the system needs and your customers want. Include them in your estimate and redesign.

    If there are adequate responses to this article, the next “Doc” article will describe in detail the inspection, tests, diagnostics, and documentation you can use to discover solutions that increase the value of your HVAC retrofit jobs. 

    Rob “Doc” Falke serves the industry as president of National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free diagnostic procedure to quantify and reduce air filter resistance, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at for www.nationalcomfortinstitute.com free information, articles, and downloads.


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