When an architect begins to design a building, their concept art rarely addresses how the building will be heated, ventilated, and cooled. Moreover, those renderings rarely account for the climate where the building will be located.
Instead, a building systems engineer must design an HVAC system that meets occupants' needs while accounting for climate, federal regulations, initial budgetary concerns, and ongoing energy and maintenance costs.
The first and most crucial consideration is the building envelope and design. After all, you can't select systems that you cannot install in a building. Of course, things become more constrained when considering retrofitting an existing building.
HVAC systems engineers can be conservative in their estimates, and with good reason: no one wants to be subject to a call that their design has failed to meet occupancy loads during the hottest, coldest, or most humid days of the year. Unfortunately, this means that some outdated (but widely used) rules of thumb are used in both manual calculations and even in simulations.
The Air Conditioning Contractor Association, in its manual for residential equipment selection, explains that "the practice of manipulating the outdoor design temperature, not taking full credit for efficient construction features, ignoring internal and external window shading devices and then applying an arbitrary 'safety factor' is indefensible." This is unfortunately just as true of commercial buildings, especially those with variable occupant loads.
A report released by the Department of Energy noted that in climates like Orlando, improperly calculating the cooling load based on the insulation and window design can lead to a 40 percent increase in the recommended system guidelines, from three to five tons for a 2,200 square foot home, or 133 percent. A similar case exists for a model home in Chicago for cooling, where adding safety factors added 1.5 tons of cooling needs.
The simple fact is that not-so-simple calculations are required to ensure that the actual heating and cooling loads are correctly ascertained. The same is true for occupant loads, particularly for office buildings or spaces that see frequent tenant changes.
Depending on location and building envelope design, a commercial HVAC system may only operate for a total of a few days per year during the coldest and hottest days. Unfortunately, while the Department of Energy has updated guidelines regarding energy efficiency ratios (EER) to include part-load conditions, the new guidelines do not apply to all HVAC systems. More importantly, even the updated IEER only gives a ratio based on three separate load conditions, which may not match the actual operation intended.
Instead, designers must use complex performance data that often can only come from distributors or directly from the manufacturers themselves. Otherwise, the energy usage models may be completely different from expectations. This is just as true for occupancy load and use types.
In fact, trying to account for different use scenarios is one reason why the above issue with oversizing is so commonplace. Engineers rightly understand that they may be blamed if the HVAC system fails to perform in specific scenarios, even if they were not discussed. However, when the design-bid-build or other construction process allows for it, ongoing communication is crucial for maximizing efficiency through continuously discussing requirements and needs.
There are two factors in selecting systems to fulfill commercial building air conditioning system design specifications:
In most cases, this may seem simple, with the current standard combination being a variable-air volume system that discharges constant-temperature air at various rates throughout a facility matched with either packaged rooftop systems, split systems, centralized chilled water systems or distributed terminal systems. However, climate and other factors may challenge typical systems, which could limit potential energy savings.
The older standard for airflow around a commercial space was constant air volume, and it's similar to that used in modern homes that are predominantly "single-zone": the air is treated to a specific temperature and then moved throughout the facility.
Adjustments can be made for zone differences by using "terminal reheat" coils, where the supply air can be heated to a certain temperature before entering a specific zone. However, this does not provide as much flexibility as the alternative discussed below.
For large spaces, constant air volume systems still provide significant advantages. They are generally less expensive to purchase and quickly respond to large load swings that shift temperatures. However, they are not the right choice for all situations.
For typical commercial buildings, the first choice is likely to be a variable-air volume system with a dedicated mechanical room for heating and cooling operations. In most climates, this provides the flexibility of meeting different temperature requirements throughout a building without requiring additional parts of the heating or cooling system to be operating simultaneously.
Typical applications include office buildings, schools, municipal buildings, shopping centers, and mixed-use facilities.
Rooftop units are flexible designs that put the building’s compressor and heating system in a box above the facility. Commonly used in manufacturing and industrial settings, they are also options for providing temperature, humidity, and ventilation control in lower structures in an adjunct role to the HVAC plant unit. However, there are limits to rooftop units in terms of the distance the air can travel before pressure and energy losses significantly reduce efficiency.
Because it is impossible to move significant amounts of air over long distances, forced-air systems are not feasible in larger buildings. In that case, designers opt for a chilled water system. The solution (actually sometimes brine to prevent freezing) is generated in a chilled water plant and then piped to air handlers throughout the facility to provide cooling by absorbing heat from individual zones.
Chilled water systems do carry one key consideration, however. Because they can not incorporate ventilation air into the water, designers must incorporate ventilation air into the air handling unit to meet ASHRAE's requirements.
However, VRF systems do have some drawbacks. Because they do not bring in any outside air, engineers must ensure that a building is equipped with an outside air ventilation.
As noted above, HVAC engineers can’t always consult frequently with other members of a build team. Additionally, they can be held responsible for system issues even if the use case no longer remains within the design specification. In addition, the simple fact of the matter is that even relatively small projects are dependent on the efforts and quality standards of many other teams.
As just one example, even the improper application of window film can drastically affect calculations. Even smaller shifts can have a large effect—changing how cabling is run, for example, can impact VAV systems originally meant to run through ceilings or below floors.
It's crucial for design engineers to work closely with the construction team.
While commissioning is the final stage and should allow for handover of the equipment and responsibility to the property owner, building engineers should monitor the construction process and work with building managers and owners to identify and address potential problems.
There are very few quick fixes for HVAC design, and front-end engineering is the most crucial part of the process for an HVAC systems designer. A close working relationship with the rest of the build team—as well as ongoing examination and adjustment of the design specifications—is a close second.
Windy City Representatives works closely with leading commercial building air conditioning system manufacturers. We understand how important it is to take multiple considerations like budget and existing systems into account.