AHR Expo 2023 presenter Matt Matheny, Engineering Director at HVI, shares the key takeaways of this project.

Can you briefly summarize the DTF research project that HVI and AHAM conducted?

First, I’d like to recognize Randy Cooper of AHAM, Mike Moore of Stator, LLC and Dr. James Sweeney of the Texas A&M Riverside Energy Efficiency Laboratory. They were integral members of the project team, and this would not have been possible without them.

The “Range Hood Rating Metric” working group of the ASHRAE 62.2 Standing Standards Project Committee (SSPC), chaired by Randy Cooper, was established in 2019 to develop a new KRH rating metric, called “nominal installed airflow” (NIA), that better represents real-world, installed performance. The NIA is the intersection of the KRH airflow curve, and a nominal duct system curve characterized by 10’ of straight rigid duct, two 90-degree elbows, and one DTF, all the same size duct size as the KRH. The nominal duct system curve is developed from its assumed duct size and length, friction factors, and component loss coefficients. Loss coefficients for straight rigid duct and elbows are well-established, but not for DTFs commonly used with KRHs, hence the need for the project. The project consisted of sourcing, testing, and analyzing results for 36 unique DTFs sized appropriately for the application (i.e., no less than 6” round and 3-1/4 x 10” rectangular) to establish accurate DTF loss coefficients for determining the NIA. 

Could you describe some of the Kitchen Range Hood airflow rating metrics that contractors use on a day-to-day basis?

KRHs are rated for airflow in cubic feet per minute (cfm), sound in sones, and energy (or input power) in watts, at a specified static pressure. End-users including consumers, contractors, and design engineers use these ratings primarily for product selection and comparison. The application usually dictates the minimum KRH performance requirements. For example, a code-minimum application might simply require an economical KRH that moves 100 cfm, while an energy efficiency-focused application might require an ENERGY STAR qualified KRH, and a custom home application might require a high-end 1,200 cfm KRH to match the 6-burner commercial gas range required by the client. Most of the time, contractors are responsible for installing the specified equipment according to manufacturer installation instructions and design and code requirements. The design engineer is usually responsible for designing duct systems and specifying equipment that meets the project and local code requirements, so they routinely refer to KRH airflow, sound, and energy ratings listed in the HVI and AHAM product performance directories. This isn’t always the case though. For example, in high-end custom homes, contractors can be responsible for designing the system and specifying and installing the equipment per the client’s desires (while meeting code of course). The entire duct system needs to be considered to ensure the installed performance of the KRH meets expectations.

Could you describe how the new range hood airflow data you’ve produced better represents real-world, installed performance on these systems? 

0.1” of water column (in. w.c.) of static pressure is the current rating point for KRHs and was established decades ago, when code requirements were simpler and rating points were primarily used to compare products A, B, C, etc. This static pressure under laboratory conditions is useful for considering the fan only and does not account for the duct termination or system resistance of a typical installation. It is useful for estimating the performance of a KRH that tops out at about 100 cfm when installed on a 6" duct, but it can significantly overestimate airflow at higher speed settings. Here’s a hypothetical example of a typical installation: a design engineer designs a KRH system for a house that requires 200 cfm. At 200 cfm: 10’ of rigid straight 6” diameter duct = 0.03” w.c. + two 90 degree 6” diameter elbows = 0.24” w.c. + one 6” diameter wall cap DTF = 0.19” w.c. for a grand total of 0.46” w.c. of system static pressure. This is much higher than what is typically found in the 3^rd party certified product performance directories. Once NIA is established, end-users will be able to select a KRH based on a 3^rd party-certified NIA that is much closer to the design conditions and will better represent how the product can be expected to perform once installed. As codes and industry have evolved, so has the need for rating points that better reflect how products perform in the real-world when installed, hence NIA.

In what ways do the different termination fittings available for installation affect airflow?

DTF design characteristics can really impact the system airflow performance, and a few general takeaways are:

• DTFs with more net free area, smooth transitions, radiused hoods, and low-resistance backdraft dampers tend to perform better.
• DTFs with spring loaded dampers typically have higher pressure drops than DTFs with gravity dampers, especially when the airflow rate is not high enough to “pin” the damper completely open.
• In general, roof caps have higher loss coefficients than wall caps, likely because of gravity pushing down on the damper due to its installation orientation.

Could you describe existing standards for wall and roof cap termination fittings that typically accompany these systems? What kind of data have you generated and made available to the industry in this area?

Very little currently exists in terms of standard requirements for DTFs. Because of their simplicity, designers might underestimate their impact on system performance. HVI Publications 916 and 920 describe how to test and certify DTFs, but there are very few models currently certified due to a lack of market demand for certified performance for these devices. We’re thinking this might change as the industry begins to understand the impact that DTFs have on installed performance of KRHs as a result of the NIA rating metric. In turn, industry is likely to begin requiring listed DTFs in their designs. Currently, the only data we’ve made available to industry is what we presented at the 2023 AHR Expo.

It’s important to keep duct sized appropriately for the air volume of a given system, referred to as Cubic Feet per Minute (CFM). How does your work impact the kitchen ventilation system’s resistance and leakage characteristics more generally?

This project led to the completion of the NIA equations that will provide a rating metric that better represents in-situ airflow performance of KRHs. By rating KRHs at a static pressure produced by a typical duct system rather than 0.1” w.c. of static pressure, industry will have a better understanding of how the products perform when installed and how different duct sizes and DTFs impact the overall system resistance. This will lead to better designed systems.

What’s the next step for establishing a “nominal installed airflow” for the industry?

Currently, the updated NIA requirements are working their way through HVI committee approvals for publication in HVI Publication 920 HVI Product Performance Certification Procedure Including Verification And Challenge© later this year. From there, it will likely be referenced in the ASHRAE Standard 62.2 Ventilation and Acceptable Indoor Air Quality in Residential Buildings and building or energy codes.