When it comes to an integrated-design approach to HVAC, there’s no one size fits all. Instead, integrated design is truly a game of reconciling competing demands and developing creative design. On the architectural side, there are requirements to achieve the right aesthetics, materials, and flexibility, while the engineering side has a different set of requirements to achieve the right indoor air quality, meet necessary codes, and specify equipment that meets the building’s unique needs.
Together, these demands compete for the same finite resources — space and capital. But neither can exist without the other. The question is, how can competing demands be intricately woven together to enable form to follow function? What needs to be done? What are the chief considerations?
The answer is an integrated design approach to the HVAC system. Integrated design of an HVAC system starts from the outside and works in. The process should give careful consideration to exterior climate, building envelope, and materials inside, as well as codes the building was designed to, HVAC equipment, and necessary flexibility.
To achieve a truly integrated HVAC design, the designer should take into account the environment, context, and even the actual site of the building.
Generally, the design of any HVAC system is first and foremost about dehumidification or control of humidity in the facility — not just for comfort, but also to keep building materials and occupants healthy.
In humid climates, moisture is a constant threat to the air-conditioned environment, as air-conditioned buildings in humid climates are filled with surfaces colder than the dew point of the air surrounding them. It’s not enough to simply insulate cold surfaces. In this case, an integrated design needs to limit the amount of moisture penetrating the envelope assemblies using the proper vapor retarders, vapor barriers, or air barriers. When moisture does get in, the HVAC system must be designed to dehumidify it.
Moreover, the HVAC system should use air distribution that prevents stagnant pockets of moist room air from developing; the key is proper return air distribution. Most importantly, and contrary to some new sustainable design practices, HVAC systems in very humid climates should avoid distributed cold surfaces (chilled beams, chilled ceilings, distributed fan coils, etc.) or apply them with extreme care.
Cold and dry climates are typically more forgiving with respect to the control of humidity indoors. However, the effects of moisture can wreak havoc on the building envelope materials due to the freeze-thaw phenomenon.
The right materials
Exterior materials can have a profound impact on building performance parameters often associated with the HVAC system, like energy efficiency and humidity control.
Consider glass curtain wall. This is likely the most common system used in commercial construction, but that doesn’t make it the best solution. Glass curtain walls have the advantage of providing occupants with a connection to the outdoors, but they often leak water, air, and heat, and expose interiors to solar energy.
Historically, building envelopes were made of heavier mass assemblies using brick, stone, concrete, block, steel, and wood. While these systems were heavy and field-labor-intensive, they had the distinct advantage of being able to repel, absorb and drain, and manage water more effectively.
With the introduction of air conditioning and energy-efficient buildings came a variety of envelope and HVAC systems that don’t work together to manage water properly. The result: water causes either physical or environmental damage.
For example, air tightness is generally a good thing. Air tightness can save energy and help keep the building healthy. But exterior air tightness requires good interior ventilation systems. The ventilation systems are needed to manage the indoor humidity, which can be elevated in a relatively air tight building. For instance, in a cold climate, buildings with newly required air barriers will retain moisture that would have been “mitigated” by dry cold air entering through a leaky envelope. Condensation can now form on the cold surfaces exposed to humid indoor air. Proper ventilation, dehumidification, and air distribution are needed to mitigate these newer occurrences.
To avoid problems, contemplate the intended use of the building when selecting exterior materials. Look for potential roof and envelope assemblies that are ideal for the local climate. Some wall system configurations don’t work well in cold, mixed, or humid climates. It’s a complex matter — two systems made of the same materials, but with different configurations, can fail in the same climate — so it’s difficult to give rules of thumb. Where a glass curtain wall is desired, use it judiciously and make sure installation is tight. The designer should also specify other envelope materials to work hand-in-hand with the glass that provide other necessary insulating properties.
HVAC Systems: To Centralize or Decentralize?
One key question when integrating HVAC systems is whether to centralize or decentralize mechanical equipment. While there is no right answer to this question, a number of considerations can help make the decision.
The centralization of the HVAC system can be attractive in that it sends the otherwise non-desirable mechanical equipment to the mechanical room or penthouse, away from the adoring public. The downside of doing this is that buildings that have overly large central air handling units (AHU) systems, monolithic boilers, and large single-compressor chillers have a tendency to perform poorly during partial load conditions and off hours. As a reminder, a typical building will operate at part load more than 99 percent of the year.
Instead, the more modular, or decentralized, systems perform better in varied use cases. Decentralization provides the flexibility to operate various portions of the building at different times.
Another benefit to an appropriate amount of modularity — or decentralization — is the level of redundancy it naturally provides. A system that has the ability to take the building down in a single failure is a huge risk to uptime.
Decentralization is easy to overdo as well. For instance, large buildings with many distributed compressors can be a maintenance nightmare. When attempting to reap some of the benefits of modularity, it’s best to consolidate the things that are likely to break, leak, condense, or are expensive to replace, and distribute the other things like AHUs, fans, and terminal devices to give end users control.
Unexpected impacts of codes
While it sounds antithetical, one of the biggest challenges to integrated building design today is the application of current energy codes. As the codes, and therefore the insulating properties of the building envelope, become more and more stringent with each iteration, more moisture problems surface in and around a building. That’s because if the building envelope can’t breathe, moisture that’s on its way out will be trapped inside, creating a ripe opportunity for freeze-thaw damage or mold growth, depending on the climate.
Truly integrated design comes from understanding all the properties of the facility and working together across the entire building team to create the right solution for the facility.
A key aspect to a truly integrated design is facility context. Understanding the programmatic functions of each facility will be important to integrating its design. For example, is the owner of the facility going to be operating the building? Are they planning to sell it in three years, at which time the function may change? Is it a public building? A private building? Does it need a specific level of flexibility?
This is important to know during design because as facility and tenant needs change, HVAC equipment will need to respond accordingly. For example, a new tenant could need more access to outdoor air to exhaust for lab equipment; another may need better filtration, supplemental cooling, etc. — all important considerations when designing an integrated HVAC system.
Truly integrated HVAC system design will have a symbiotic relationship with the building’s other systems, where they complement each other and maybe even become more integrated over time with the addition of new technology and equipment. That’s where design innovation comes into play.
In today’s busy world, some designers are just copying what they did yesterday. It’s easier. And there’s less perceived risk. But there’s also no opportunity for a true symbiotic relationship. To quote the late Sital Daryanani, a trailblazer of integrated design before anyone was doing it, “If I give you a dollar and you give me a dollar, we’ve both gained nothing. But, if I give you an idea and you give me an idea, we’ve both gained everything.”
Here’s to solving the puzzle of HVAC integrated design by applying the right solutions to each unique situation — not just the ones that are the easiest, or simply the most energy efficient.
The Right Way To Tackle HVAC Renovation Challenges
One turn-of-the-century historical complex was originally designed with natural ventilation from operable windows and an uninsulated building envelope comprised of stone veneer, masonry backing, and plaster. When air conditioning was added over the years, windows were replaced by air intakes and insulation was misapplied in certain areas. Now that the building is being renovated, three challenges — and potential trade-offs — to an integrated design exist:
1. Moving daylight into the central spaces of a multi-hundred-thousand-square-foot floor plate with 70 percent interior space.
2. Designing an air-conditioning system for a facility with an envelope that wasn’t meant for air-conditioned interiors.
3. With all of the legacy issues that exist in the facility, designing HVAC system to be as energy efficient as possible.
Addressing those challenges involved tradeoffs. To enable access to daylight and views, the design maximizes daylight penetration but gives up an opportunity for an airside economizer (free wintertime cooling) that would have substantially reduced precious window area. Using a dedicated outdoor air system on the roof provides the necessary ventilation, while relying on a river water cooling system to provide nearly identical free cooling potential.
Exterior wall systems will require new interior layers for thermal insulation, air barrier, and vapor retarder to be compatible with an air-conditioned building.
Don’t Fail These HVAC Design Tests
Two seemingly successful sustainable designs turned into major HVAC failures because they failed to consider basics.
Failure to Understand the Local Climate. The designers of set out to design a truly integrated, LEED certified facility. Because an indoor swimming pool complex was built in a mixed climate, designers chose to try to reduce the amount of energy required to heat, cool, dehumidify, and re-humidity the building’s indoor air by specifying an enthalpy — or heat recovery — wheel. But during the winter, the wheel constantly recycled moisture from the swimming pool throughout the facility. The result was a complete inability to control moisture in the building.
Failure to Design for Future Flexibility. The chief priority for developers of a multitenant building was to build an efficient, integrated HVAC system that, although initially cost prohibitive, would provide an ROI over years of operation. The HVAC system was built with hundreds of small valves that distributed heating/cooling throughout the facility. While this would have worked great for a building that didn’t require much flexibility, the facility had high tenant turnover, and the efficient — but highly inflexible — HVAC systems components couldn’t be easily changed out as new tenants moved in and re-configured their spaces. Instead, developers had to invest in re-configuration of HVAC piping — an additional expense and a significant business interruption for tenants.