Fewer Shades of Gray: Reducing Confusion Regarding HVAC Design in Energy Star Homes
Life is rarely black and white. Just go to your local paint store and you’ll find many shades of gray – Monorail Silver, Morning Fog, Snowbound, Tinsmith. The differences are subtle and too many choices can be paralyzing.
How does this relate to HVAC design? In 2012, Version 3 of the Energy Star Certified Homes program significantly expanded its HVAC design requirements. While the prior version relegated design to a single footnote, Version 3 required extensive design documentation for every home. Despite the best of intentions, Version 3 nearly ground the Energy Star New Homes program to a halt. It turned out that there were too many shades of gray.
This article discusses what the Energy Star program has learned, what it has done in response, and how it’s bringing better design, and fewer shades of gray, to new homes everywhere.
‘Tinsmith’ Gray Clouds on the Horizon
“As you can see, it is a slippery slope trying to meet Energy Star while making the customer happy and comfortable.” This is what we heard from one HVAC designer who was eager to please his client and also support the home’s certification. Yet, he had the impression that these two goals were incompatible.
This was not the first time that I had heard this refrain. However, when Version 3 was first launched, this feedback puzzled me and the rest of the Energy Star team. The new design requirements were certainly expansive. By and large, though, they simply mirrored what was already required by code. That is to say, they were not the “best practices” known to the industry. Quite to the contrary, they represented the bare minimum required by law. So, why were so many designers encountering difficulties?
The primary answer was that this was the first time that any national program had enforced HVAC design requirements in a systematic way. While these requirements were written into code, jurisdictions rarely tasked inspectors with enforcing them. On top of that, Home Energy Raters had never been asked to review designs as part of their scope. And, understandably, most builders weren’t familiar enough with the details of proper design to demand it.
And what of the designers? The best of them were offering quality designs, despite the lack of guidance and oversight. However, each designer had to interpret how best to translate industry standards into everyday practice. Ultimately, this resulted in small variations from one designer to the next. Compounding the challenge was that each designer often interacted with multiple Home Energy Raters, each of whom also interpreted the rules slightly differently.
The result? Conflicts and confusion that were both avoidable and unnecessary. In short, too many shades of gray.
Burning Off the Gray Morning Fog
Understanding that our partners were encountering unexpected challenges, we knew we needed to make improvements. With the release of Revision 08 of the Energy Star Version 3 program requirements, we worked hard to clarify our design requirements. And a key part of that effort was the Energy Star HVAC Design Report, consisting of five simple sections–the design overview, the whole-house mechanical ventilation design, the room-by-room heating and cooling loads, the heating and cooling equipment selection, and the duct design. This one-page document consolidated and standardized all of the design information needed for certification. By doing so, it completely eliminated the need for designers to supply pages of supplemental documentation to Home Energy Raters.
Not only did the HVAC Design Report consolidate information from more than a dozen previously sprawling pages, it also clarified key design policies. Of particular importance were ones related to the whole-house mechanical ventilation system design, outdoor design temperature selection, and equipment sizing limits.
Whole-House Mechanical Ventilation System Design
Prior to Revision 08, only some of the requirements for the whole-house mechanical ventilation system were included directly on the design documents. The rest of the requirements resided in the checklists for the Home Energy Rater, who was ultimately responsible for verifying them. However, whole-house mechanical ventilation systems were so new when Version 3 was released that many designers were not familiar with the details needed to ensure their proper and effective operation. As a result, some requirements were easily overlooked at the design stage and only caught once the home was constructed.
To mitigate these challenges, Revision 08 consolidated all of the requirements into a single section of the HVAC Design Report (see Figure 1). In addition to addressing the design airflow of the system, it covered lesser-known, but equally important, areas such as system controls, sound levels, efficiency, and air inlet location. By complying with each of the items in this section, designers are able to satisfy nearly every major requirement of ASHRAE 62.2, the industry standard that underpins the Energy Star program requirements. While this might seem like a simple change, it set much clearer expectations for designers unfamiliar with this emerging area of design.
HVAC Design Report
Outdoor Design Temperature Selection
Prior to Revision 08, many designers found it difficult to determine the right outdoor design temperatures for their load calculations. This happened for a variety of reasons. There were two different sources of design temperatures–ACCA’s Manual J and ASHRAE’s Handbook of Fundamentals. Finding the closest geographic weather station could be subjective. And the reality was that some designers used a single set of design temperatures for a whole metropolitan area to streamline their work. Sometimes this didn’t have much impact on the design at all, while others times it did.
To make the process more clear, the Energy Star program introduced county-level design temperature limits. To accomplish this, we mapped every weather station in the country from ACCA and ASHRAE, using their exact GPS location (see Figure 2). Then, we looked at the weather stations in each county and essentially picked the weather station with the most lenient design temperatures. From this analysis, a table was created with every state and county in the United States. For each county, the table providers the designer with the cooling temperature limit and heating temperature limit, along with a ratio that’s helpful for equipment sizing, and the precise weather station selected for that county. See Table 1.
Map of Weather Stations
Table 1. Design Temperature Reference Guide
This table gives designers a quick and easy way to determine the design temperature limits for any county they are designing for. And Home Energy Raters can use the same table during the verification process, further minimizing conflicts and confusion.
Equipment Sizing Limits
Prior to Revision 08, three little words generated more questions than almost anything else. Those words were next nominal size. In concept, selecting the next nominal size of equipment was easy to explain. Equipment is typically sold in nominal half-ton increments–2 tons, 2.5 tons, 3 tons, and so on. Under this policy, designers first used the cooling load and a sizing limit to calculate how big the equipment was permitted to be. Designers were then permitted to select equipment that was the next nominal size above that limit. For example, if the sizing limit indicated that 2.3 tons was the maximum allowed capacity, the designer was permitted to round up to the next nominal size of 2.5 tons. While easy to explain, in practice, it was quite difficult to enforce. That’s because the next nominal size of a piece of equipment is specific to each manufacturer and model line. Without manufacturer-specific product catalogs, there was no way to determine what the next nominal size actually was. That put both designers and Home Energy Raters in a bad spot.
With Revision 08, the phrase next nominal size was struck from the record. In its place, the program defined clear numeric limits (see Table 2). In addition, we provided higher limits for certain equipment types, which better reflected the latest edition of the industry standard for equipment selection, ACCA Manual S.
Table 2. Equipment Sizing
With purely numeric limits, designers and Home Energy Raters can quickly assess whether they’re compliant, without the need for manufacturer-specific catalogs.
A Monorail Silver Glimmer of Hope
Revision 08 brought a glimmer of hope to Energy Star’s partners. It was released in July 2015 and a full year was provided to transition to the new process. We used that period to pursue another key improvement–automation. Now that the HVAC Design Report consisted of a short and standardized form, we engaged HVAC software developers to see if they’d be willing to automate the completion of the report. Wrightsoft was the first to leap at the opportunity. With the HVAC Design Report integrated into their software, designers can now simply click a few buttons to print a fully completed report once they have finished their design. In addition to preventing inadvertent transcription errors, this saves a great deal of time and effort. Some designers have reported migrating to Wrightsoft just to use this feature. We hope to see other software developers add a similar feature to their programs in the future.
A year and a half later, how is the HVAC Design Report performing? The number of design-related partner questions has slowed to a trickle. The number of homes certified in 2016 increased for the first time since the release of Version 3. And, most importantly, designers and Home Energy Raters have greater confidence that they are meeting the intent of the program, and doing so with much less effort.
“As you can see, it’s a slippery slope trying to meet Energy Star while making the customer happy and comfortable.” Remember the designer who said that? We checked his calculations and discovered that one key input was incorrect, resulting in loads that were too small. When he corrected this input, his preferred size of equipment fell right in line with the Energy Star program requirements.
Learn more by visiting www.energystar.gov/homes.
It goes to show that the two goals of meeting Energy Star and keeping customers satisfied are definitely not incompatible. It also shows that while there are fewer shades of gray today, there are even more ways that we can make the Energy Star design process clearer. That’s why we’re continuing to engage designers about how to use the simplified process, and collaborating with RESNET and other stakeholders to promote even greater standardization. In the end, a straightforward design process is one that can benefit every new home, not just those that earn the Energy Star label.
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