Performance-based design: HVAC systems

At its essence, all engineering design is performance-based. If we don’t have a performance goal, what are we aiming for? Measuring our success against? How do we even start without such a goal?

What are we getting at within the topic of performance-based heating, ventilation and air conditioning design? In current parlance, performance-based HVAC design covers three broad spectrums:

1. Design solutions that do not meet prescriptive code/standard approaches or requirements; e.g., energy cost budget design under ASHRAE 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings. 
2. Design to specific metrics that may be viewed as nonstandard; e.g., designing to meet specific indoor air quality criteria. 
3. Operational performance targets; e.g., designing to a specific energy use intensity target. 

Prescriptively noncompliant design

The first spectrum is likely the one most engineers are familiar with — at one time or another most mechanical, electrical or plumbing engineers have either done this or run into it as a potential hurdle in design aspirations. Energy code prescriptive requirements are generally the most common aspect that warrants exploration of performance-based alternatives.  

While each locale, climate and building typology each have its own drivers and common exceptions, these two are fairly common code variances: 

• Economizer requirements for small cooling systems that impose significant infrastructure costs and constraints if implemented in most interior locations. 
• Energy recovery requirements for air handling units that mandate air-to-air energy recovery systems. 

When we look at alternative pathways for noncompliant MEP components, the engineer must find either an alternative solution that has equal performance or look to superlative performance in another area to offset the desired deficiency. The big non-MEP driver of performance-based energy code compliance is noncompliant building envelope performance; this is perhaps one of the biggest drivers of performance-based code compliance analysis. When the envelope is deficient, the MEP and lighting systems typically are called to offset the envelope’s shortcomings, eroding overall project energy performance.  

Regardless of the root cause, the solution for most of these analyses is to run a whole building energy model that demonstrates that the proposed design meets or exceeds the performance of the code or ASHRAE standard reference design. How we as engineers come up with these solutions is where our creativity gets called into play — the solutions are as varied as the projects we undertake.  

A common solution is simply specifying equipment with higher than code performance, e.g., condensing gas boilers, higher efficiency chillers, etc. Alternative solutions include different solution approaches than code, such as the use of water-to-water heat pumps and exhaust air energy recovery coils for heat recovery in lieu of direct air-to-air recovery approaches. 

However, one of the big challenges is that the codes and standards are catching up to our favorite solutions and making them mandatory. Performance targets are ever-evolving and require constant re-evaluation of previous solutions and approaches. Engineers have a societal duty toward sustainability — which generally means doing better than code minimum. Engineers should be always leading the code and doing better; the legal minimum should not be our legacy.  

A possible future is to incorporate those items resisted in the past and allow the systems that offset those past losses to be tomorrow’s gains. How might that be done? Let’s take the first item on the list — small cooling systems. For the most part, those systems end up serving data closets — 24-hour loads that require constant cooling. In many climate zones an economizer makes a lot of sense for such a load — and is required by code. So how does one get that done right?  

Perhaps the easiest way is to work with the architect and the client to get the initial programing locating those rooms along or near the exterior of the building. With nice, short duct runs the cost and spatial impedance arguments really fall away and we are left with a better design. The old solution (higher efficiency cooling systems) can then be harnessed to make our overall building better than code, not just break–even.  

HVAC system design to specific metrics and ASHRAE standards

The specific design metrics seen most frequently are those around indoor environmental quality. IAQ, thermal comfort and acoustics are areas where designers are often asked to meet specific performance-based targets. Of these, acoustic design is probably the most common of the IEQ design requirements and HVAC engineers often rely on acoustical consultants for input and analysis of our designs to meet the project performance targets — interior noise criteria and exterior noise ordinance requirements.  

Performance-based IAQ design typically references the IAQ Procedure of ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality. In this method, the engineer designs the system to meet specific pollutant concentration criteria rather than using more generically derived ventilation rates. While not typically used, this procedure gives the designer the flexibility to determine the pollutants of concern and design to specific concentration levels. Some of the drivers for this approach include: 

• Indoor pollutant sources that are atypical for the space type (e.g., if furnishings are known to contain excessive volatile organic compound content). 
• Space or use types not covered under the prescriptive ventilation rate procedure. 
• Owner or process requirements. 
• Outdoor air conditions that require more detailed analysis. 

Regardless of the driver, the analysis approach is consistent — a mass balance analysis examining internal source emissions, outdoor concentrations and filtration effectiveness. The solutions can include modifying the amount of outside air brought in, additional source control measures, providing improved filtration within normal HVAC equipment or using recirculating air cleaners. One caution that is warranted in this approach is to always keep in mind pollutant sources that are not part of the specific analysis. In general, it is not recommend lowering base ventilation rates below the ASHRAE 62.1 prescriptive rates unless it is well-known that indoor pollutant sources are, and will remain, as analyzed or any specialized filtration covers the full range of indoor pollutants.  

Thermal comfort is perhaps a bit unusual to list as an atypical performance-based design metric. However, the industry tends to design to air temperature conditions, not comprehensive thermal comfort for the occupants as defined by ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy. A comprehensive design for thermal comfort involves analysis of not just the air temperature, but also humidity, average air speed, the mean radiant temperature and direct solar radiation incident on the occupants. The code typically does not mandate true thermal comfort design; in general, the code uses only air temperature and most designers follow suit with a set of design criteria that at best look at air temperature and humidity. 

Compliance with ASHRAE 55 is more complicated than just air temperature at a thermostat and requires a more detailed evaluation of the HVAC system performance to consider average air speed and air temperature across the space, not just a room average condition. Compliance also requires evaluation of the building envelope performance — surface temperatures (for mean radiant temperature calculation) as well as direct solar radiation penetration and incidence on occupants.  

Envelope performance for comfort is different from energy evaluation — the spatial specifics and solar geometry come into play with potentially greater impact to comfort than overall energy use. ASHRAE 55-2017 includes new requirements for accounting for the direct solar radiation impacts on comfort that require much more considered attention to glass orientation and shading device performance. While the ASHRAE standard has some prescriptive tabular options for glass and interior shading device performance, many conditions will require more detailed analysis. This analysis requires more specialized tools and consideration of different sun angles that may not coincide with peak HVAC load conditions (e.g., winter low sun angle conditions). 

Operational performance targets for HVAC systems

Designing to operational performance targets is relatively uncommon but increasing in popularity, whether for net zero energy projects or simply owners who wish to have an operational performance guarantee. Some codes also are allowing an outcome-based code compliance path as well (which in some jurisdictions comes with financial performance bonds for the owner). This is a topic and task complex enough to warrant its own article (and more). The contractual requirements typically focus around energy performance, though water performance also is occasionally required.  

Unlike code compliance or U.S. Green Building Council LEED energy modeling, getting the right prediction is fundamental to the team’s contractual and operational success. Uncertainty analysis is one of the foundational approaches to having confidence in the design solution and compliance with the performance target. There is not just one answer coming out of one energy model, like for code compliance. The engineer’s role is to work with the team to evaluate many different possible energy model predictions to determine whether the design meets the performance threshold and the level of contractual risk is acceptable. 

For most buildings, the uncertainty should consider the following aspects, at a minimum: 

• Climate variability. 
• Occupancy patterns and usage.
• Internal load variations.
• Equipment and envelope performance variations/degradation. 

The importance of uncertainty analysis cannot be overstressed when committing to an outcome-based performance target. There are so many variables that affect overall building energy consumption and so many of them are out of the design team’s control that they need to be assessed for sensitivity. Those aspects that have a high sensitivity need to be controlled, either from a design perspective or contractually. Uncertainty analyses can help inform the contractual language around the performance guarantee by identifying those highly sensitive aspects of performance and regulate their risk. 

Regardless of the type of performance-based design or its drivers, as HVAC engineers we need to be cognizant of the increased risk and responsibility that accompanies such design goals. We are at risk of code compliance, LEED certification status, IAQ compliance or even whole building energy performance and base most of that risk on analytical solutions. In many cases the analysis itself needs to be designed to ensure it is robust enough — a robust workplan that tests for uncertainty, sensitivity and risk and that includes rigorous checking. Most certainly it is more effort, but has the potential to bring about greater design freedom, success and higher overall performance.

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