Mechanical and plumbing systems in mixed-use buildings

Engineering mixed-use buildings is a fine art—specifiers must combine multiple engineered systems for several business and residence types into one structure. Mechanical systems (HVAC) and plumbing systems must be carefully designed and aligned.

By Consulting-Specifying Engineer September 16, 2013


Robbie Chung, PE, LEED AP, Senior associate, Environmental Systems Design Inc., Chicago

Raymond Holdener, PE, Senior associate, Dewberry, Fairfax, Va.

Andrew Lasse, PE, LEED AP, Associate principal/senior mechanical engineer, Interface Engineering, Portland, Ore.

Gary Pomerantz, PE, LEED AP, Executive vice president, building systems, WSP, New York City

John Sauer, PE, LEED AP, Senior director, BSA LifeStructures, Indianapolis

LeJay Slocum, Assistant director, Atlanta regional office, Aon Fire Protection Engineering Corp., Suwanee, Ga.

CSE: What unique requirements do HVAC systems have, and how have they changed in the past 1 to 2 years?

Sauer: The rapid evolution of technology is changing faster than the level of maintenance and operations in these facilities. The technology behind these high-performance systems is very complex and requires a sophisticated staff to ensure that the high performance is maintained during occupation. In some instances, we have seen the performance in buildings gradually taper off because the staff wasn’t trained to maximize the efficiency of the complex engineering systems.

Pomerantz: It’s difficult to say what the unique requirements are. We work in an industry where each building is unique and each building requires unique solutions. While all buildings require unique solutions, mixed-use buildings solutions are more custom and more unique due to the nature of the project. There are different possibilities for energy recovery from high heat producing spaces (commercial and retail) to spaces that require heat (residential). There are opportunities for capacity reduction associated with nonsimultaneous peaks of the different use groups, which results in reduced capital cost and possibly operating cost. And at times a system that is required for a use group can also service or provide backup for an area in the project that does not require a service.

Holdener: The HVAC systems are expected to function properly, maintain space conditions, and not be heard, felt, or otherwise noticed. While this has been the case for a long time, more focus on energy efficiency and indoor air quality have increased the challenges to the HVAC systems designers in the past few years. Physical equipment sizes increase as they become more energy efficient and accommodate higher efficiency air filters, and the HVAC designer needs to “coordinate” the corresponding increased equipment space with the architect where there just doesn’t seem to be a lot of extra space for it. Mixed-use projects also have been trending toward more flexibility for the retail tenant spaces because the marketing team wants to accommodate multiple types of tenant now and in the future. For example, a particular retail space may initially be proposed to be a clothing store, but when that lease expires, perhaps a restaurant could take its place. This means that provisions for increased makeup air, commercial exhaust systems, and higher cooling capacities need to be addressed in the original design and construction to accommodate that future what-if scenario.

CSE: What systems or best practices do you use to test the building envelope?

Chung: eQuest and IES (Integrated Environmental Solutions) are the typical software that we use to model the building envelope and evaluate building energy consumption. We also use Project Vasari to get a better understanding of building wind effects and the incident solar radiation of building envelopes.

Holdener: It is important that the outside air and exhaust systems be balanced on each floor and within each space to maintain a negative pressure in residential units in comparison to the adjacent common corridor(s), and an overall positive pressure on each floor and the building as a whole. Keeping the building at a positive pressure will reduce air infiltration into the building and thus reduce the amount of unconditioned outside air introduced into the building. Unconditioned outside air introduced into the building via openings like windows and doors, cracks, and other areas of the building envelope can cause occupant discomfort and condensation (exposed or concealed), which can lead to damage, biological growth, and even health concerns. Ideally, outside air is pre-conditioned (filtered, cooled/dehumidified, and heated) via a centralized system and then is positively distributed to each individual air handling unit and space.

Pomerantz: The building is modeled in an energy simulation program. Parametric tests are performed using different envelope coefficients (solar heat gain coefficient or SHGC, U, and shade devices) to determine their effect on load and energy consumption. The cost of the thermally improved envelopes is compared with the capital cost savings and the energy savings for the building. Combining the financial analysis with architectural considerations, marketing, and other factors, an educated decision can be made by the developer. 

CSE: What type of test-and-balance or air balancing issues have you resolved?

Lasse: Rated shafts and sub-duct systems can pose some of the larger challenges when it comes to testing and balancing. The first rule of thumb is to always line the rated shafts with sheet metal ductwork. No matter how much a gypsum shaft can be theoretically “sealed” to limit infiltration and leakage, it will not work in the field, and the specified rooftop fans will struggle to maintain the required exhaust rates at air devices in restrooms and kitchens. Changing impellers and fan motors is the last thing anyone wants to do at the tail end of construction, but this can be inevitable when rated shafts are installed without sheet metal ductwork. 

CSE: What types of water conservation or reclamation systems have you specified into these buildings?

Holdener: Just about all projects, regardless of type or whether the project is pursuing LEED or not, are using water-saving fixtures and appliances. Most projects include some type of stormwater and cooling coil condensate reclamation, treatment, and reuse for cooling tower makeup water and/or irrigation. Most projects also include low-flow plumbing fixtures for water conservation, although residential and retail projects tend to not go as low-flow as office buildings.

Pomerantz: Water conservation has varied depending on the intensity of the water scarcity. We have collected rainwater—easily and often done. We have collected air conditioning condensate and reused it in cooling tower makeup (easily done). We have installed sewage treatment plants and used the treated effluent for flushing, irrigation, and cooling tower makeup. (Care must be used in cooling tower makeup to prevent the buildup of dissolved solids in the condenser water system.) And in India, we have collected and stored all of the rainwater from the monsoon season in underground wells and have pumped this water during the balance of the year for many uses.

Chung: We have specified stormwater reclamation systems for re-use with the irrigation system, which resulted in achieved points in the LEED Sustainable Sites and Water Efficiency categories. There are a number of design considerations for these systems. First off is to get approval from the local AHJ. In addition, the engineer must consider filtration, water treatment, booster systems, overflow, and associated drainage for successful installation. A cost analysis may also be required by the client to determine whether the water usage cost savings is worth the first cost of the system.

CSE: What unique restaurant kitchen ventilation issues have you encountered, and how have you solved the issues?

Lasse: I have seen some very creative approaches to kitchen ventilation on mixed-use projects. Usually these approaches are coined together after the project has been constructed and a kitchen tenant is moving into a retail space that does not have provisions for grease exhaust within the building core. No matter how attractive sidewall exhaust of Type 1 kitchen hoods appears to be, don’t do it! Code requires a system approach of this type to not be a “nuisance,” which is almost impossible without very expensive scrubbing equipment. Kitchen exhaust really needs to exit through the roof, and it is imperative that provisions be made in the original building design to be able to accommodate this if the opportunity exists for a kitchen tenant to occupy the space in the future.

Holdener: We had a mixed-use project a few years back that included a condominium atop a full-service hotel. The restaurant and kitchen were located on the ground floor, and the commercial kitchen hood grease exhaust duct had to extend horizontally for almost 150 ft before it could turn and extend upward to a fan on the roof. The duct had to slope to a localized “high point” about midway in the horizontal section, which allowed it to slope toward low points at the hood and at the base of the vertical riser section. A means of collecting and recovering “grease” was provided at each of these low points as required by code. In addition, the entire length of the grease duct was enclosed with 2-hour, high-temperature-rated, fire-rated insulation for both fire-rating and “temperature maintenance” purposes—both required by the local AHJ (the AHJ directed that the exhaust temperature in the duct not drop too low in the ductwork, which would result in coagulation of the grease in the airstream). Engineering analysis included computation of exhaust air temperature in the ductwork to demonstrate compliance with the AHJ’s requirement. The design and supporting documentation were approved by the AHJ and, as far as I know, the hood exhaust system has operated as intended and without issues. 

Chung: The designer should have a thorough understanding of the detail of the kitchen hoods if specified by a separate food consultant. Not only are the airflows important to appropriately design the HVAC infrastructure to maintain desired pressurization levels, but also understanding the kitchen hood configuration to make sure the appropriate makeup air temperatures are provided. For example, if the makeup air is directly ducted to the exhaust hood, the cooling coil of the associated air handling unit could be sized to a reduced load, leading to increased energy savings.