When building owners want their building systems to be designed for resiliency, the discussion between the owner and the engineers typically includes redundancy and temporary HVAC systems that can support all building HVAC systems or portions thereof. This discussion is most important, as it will be used by the mechanical engineer to establish the design criteria and associated mode of operation of the building HVAC systems during an emergency. What exactly constitutes an emergency? To find the answer to this question, one first needs to have a good understanding of the relative code requirements. However, since code requirements represent a minimum set of performance criteria, it is not unusual for the building owner to ask for increased resiliency. In this scenario, determining what needs to stay operational beyond what is required by code is a balance between cost and the owner’s risk tolerance. Hospitals, BSL-4 research laboratories, and data centers are some of the facility types that typically require redundancy and resiliency levels beyond code requirements.

Figure 1 shows a riser diagram of a large building (i.e., greater than 200,000-square-foot) that is being served by a water-cooled chilled water plant and a heating plant with high-efficiency condensing boilers. The chilled water plant serves the air-handling units (AHUs), various IT rooms, and a data center on the first floor. The heating plant serves the AHUs and any terminal units that are provided with a hot water heating coil. The terminal units and other HVAC system components are not shown in the figure nor described in this article.

The chilled water plant is arranged in a variable primary configuration and is designed with N+1 redundancy for chillers and primary pumps; chilled water piping connections are extended to the perimeter of the building for connection to a temporary trailer provided with an air-cooled chiller. 

As shown in Figure 1, an emergency mode of operation may be as follows: Upon the loss of two of the three water-cooled chillers and after the temporary trailer has been connected to the building chilled water system, the chilled water plant will operate in an emergency mode of operation. Based on the design criteria established at the beginning of the design process and as agreed upon with the building owner, the AHUs on floors 3 through 5 are to be considered noncritical and will not operate during the emergency mode of operation. All other HVAC systems connected to the chilled water plant shall remain fully operational.

In addition to having robust sequences of operation and controls hardware, to successfully implement the scenario described above, at minimum, the following design elements need to be carefully analyzed and implemented by the engineer: 1) The chilled water control valves that serve the noncritical cooling loads are to be provided with actuators that fail closed; 2) analysis of the water volume in the chilled water system; 3) chilled water plant and heating hot water plan bypass valves; 4) system fill pressure; and 5) the entire building automation system (BAS) needs to be connected to the optional standby branch of the emergency power system, and all control panels need to be provided with an uninterruptible power supply (UPS).

Since the chilled water plant is in a temporary mode of operation, having fail closed actuators on chilled water control valves ensures that pumping and cooling energy is not wasted on noncritical loads.

The onboard controls of the chiller often require a certain system volume (i.e., loop volume) to maintain good chilled water temperature control and ensure compressors are not cycled unnecessarily. Just like the building’s water-cooled chillers require a minimum amount of water volume in the chilled water system, the same applies to temporary chillers — particularly if the temporary chillers use scroll compressors. Based on the chiller type and unit control capabilities, each chiller manufacturer has specific loop volume requirements. To make matters worse, the system is now “deprived” of piping length due to the isolation of the noncritical loads. Typically, the loop volume is calculated by multiplying the design chilled water flow with the loop time. For example, the building chiller may require a loop time of 2 minutes, while the temporary chiller requires a 3-minute loop time; if the chilled water flow in the temporary mode of operation is 1,000 gpm, then the minimum loop volume is 2,000 gallons for the permanent chiller and 3,000 gallons for a temporary chiller. As such, additional chilled water buffer tanks may need to be added on the chilled water system to meet the more stringent loop volume requirements, (i.e., 3,000 gallons).

Just like any other chiller, temporary chillers also have minimum flow requirements. In order to maintain the minimum flow through both the permanent chiller and the temporary chiller, a bypass valve must be installed in the system. Further, and as shown in Figure 2, it is imperative the piping connections for the temporary chiller are provided upstream of the main chilled water bypass valve. This is done to ensure that minimum flow through the chiller is maintained regardless of the flow required by the building loads. A bypass valve is also required on heating hot water plants provided with a high-efficiency condensing boiler. Figure 3 shows a layout of a heating hot water plant provided with three condensing boilers arranged in a parallel configuration. The temporary boiler is designed to operate in parallel with the permanent boilers and its piping connections to the system are located upstream of the main bypass valve.

Standard temporary air-cooled chillers are typically rated for an  American Society of Mechanical Engineers (ASME) evaporator working pressure of 150 psig. In most cases, this rating will suffice to safely operate the air-cooled chiller. However, in the case of relatively tall buildings, as shown in Figure 1, this standard operating working pressure may not be enough. In our case, the building is 140-foot tall. Assuming that the system is being filled with water at the bottom of the building, the pressure of the make-up water system, aka the cold fill pressure, will need to be at least 65 psig (60 psig to overcome the height of the building plus 5 psig at the top of the system). If the chilled water pumps have a relatively high design head, i.e., 75 psig, then the system’s designed operating pressure just downstream of the pump will be 140 psig. Although 140 psig is below 150 psig, some engineers or owners may not feel comfortable with a 10-psig safety at design conditions. It will then be up to the engineer to decide if all chillers (i.e., permanent and temporary) will need to be rated for the next higher up standard ASME operating pressure.

Another scenario that could be considered when designing for operation under temporary conditions is the loss of normal power while the central plants are being connected to a temporary boiler or chiller. Under this more extreme scenario and unless the entire emergency power system is sized to support all building HVAC systems at full capacity, load-shedding strategies will most likely need to be implemented. Figure 6 shows a possible HVAC system load-shedding strategy; floors 3-6 will be designated as noncritical floors, while floors 1, 2, and 7 are designated as critical.

A temporary rollup generator has been one of the most common types of portable equipment that is widely familiar and utilized across market sectors. This has been changing in recent years with improved attention toward preventive maintenance, protecting an initial investment as well as current environmental and utility uncertainties and considerations. All these factors have helped lead to increased interest in resilient design that both helps to prevent an owner’s assets from damage and to recover them from damage. In terms of architecture and design, this could take on many different forms and strategies, one of which is designing buildings for the use of temporary rollup HVAC equipment as described above. 



Temporary HVAC equipment connections, such as a chiller or boiler supplied on a portable trailer bed, can be fed from normal or emergency power or both. This decision should be made early in the design process, as it not only affects electrical connection and feeder sizing but could also require close coordination with an emergency switchboard or emergency paralleling switchboard, normal switchboard, and a possible transfer switch. Depending on the building’s construction and configuration, future retrofitting for additional overcurrent protection and feeders could be both difficult and costly. 

The decision between which power sources are best suited for a temporary HVAC supply is often a factor of various components, including client input, the level of critical functionality of the building, and available power source options and capacities. The first critical factor is determining if the emergency system has the capacity to support the temporary HVAC equipment or if it can be designed to include these loads. If this can be done, it’s recommended that that design team create a full inventory of the necessary components of providing a normal and emergency solution in order to get an estimated cost analysis to evaluate and discuss in detail with the client. Providing a dual-feed system would require additional conductors/conduit, an additional breaker, and a transfer switch. Depending on sizes and locations, these could be very costly additions. The transfer switch can also be provided as automatic or manual. As all rollup temporary equipment will require planning and procurement, a manual transfer switch is appropriate for a normal and emergency feed scenario.

The connection point between the building systems and the rollup equipment is best done with a Cam-Lok/Posi-Lok receptacle-enabled safety switch. These switches are often referred to as quick-connect safety switches, company switches, or cam lock connection switches. The switches are offered with many different features, such as single or double throw, various voltage/pole, wire/neutral configurations, and fusible or non-fusible. They have the load-side terminals factory-wired to the receptacles. When specifying not only the switches but all components, it’s important that all equipment is listed and labeled per National Electrical Code (NEC) 110.3. 



Other elements besides where a trailer is able to park during operation include height obstructions that could affect equipment performance, protection of the switches from both external elements and weather (it’s recommended to use National Electrical Manufacturers Association [NEMA] 4X where possible, but NEMA 3R would be acceptable as well), and lengths of cables to ensure distance from trailer to hookup is reasonable and that connection is within sight of the disconnecting means. There are many different wiring packages available from manufacturers, which are commonly provided in a cable box that contains various male and female pigtails and cam-type connectors. This cable should be UL 1581 and ideally would surpass NEC 400 requirements to maximize allowable applications. Ambient temperature correction factors should also be considered when determining portable cabling. It’s important to work with a reputable manufacturer, as there are many code requirements related to portable power. Temporary wiring methods alone require compliance with various NEC articles including but not limited to 525.20 Portable Cords, 525.31 Equipment Grounding, 250.114 Equipment Connected by Cord and Plug, and 590.2 Approval. Working with a professional vendor will help to ensure all applicable codes are met on the supply side of the equipment. 

The electrical sizing is one of the most important aspects of temporary HVAC because without the proper power supply the temporary equipment will not be able to properly function. This typically only requires close coordination with the mechanical design. Once final load values are determined, the equipment supplier usually provides specific minimum circuit ampacity (MCA) and maximum overcurrent protection (MOP), the maximum wire size lug(s) that are acceptable, and even a specific circuit breaker trip rating. 

During this coordination period, it’s important for the electrical engineers to work closely with the mechanical engineers and to understand what equipment could realistically and commonly be provided to the site in order to confirm possible electrical requirements between various vendors. Factors such as the local market, existing service contracts, or even client preferred manufacturers could affect which brands of equipment should be looked into. 

Due to the variable nature of temporary equipment (what is available and when), look closely at the manufacturer-provided information and choose wire/conduit and breaker sizes to maximize that potential. For instance, it might make sense to simply oversize wire/conduit for a worst-case condition because it would not require any additional work. Design an adjustable electronic-type circuit breaker so trip ratings can be switched as needed. The breaker could also be adjusted as needed for short circuit coordination. It’s critical to run short circuit coordination studies with the breakers related to the temporary system to ensure compatibility and avoid any false tripping once the HVAC systems are connected as well as to ensure safe and coordinated connections between equipment. 

With temporary HVAC equipment, it’s important to recognize the new and additional loads being added to the existing power systems. Ideally, these portable rollup loads would be, at worst case, a one-to-one replacement of existing HVAC loads for equipment that is being temporarily supplemented. However, due to the possible complexities and interconnections of the existing building systems, this may not be the case, thus it’s important to consider load-shedding strategies. 

To simplify control strategies, any load-shedding strategies to existing building equipment should be provided to ensure  conductors and overcurrent protection for the temporary HVAC equipment have minimal risk for overloading and tripping. This also helps to streamline the electrical design portion of temporary HVAC by focusing on providing the required power for equipment loads as recommended by manufacturers and leaving the control strategies to reside within the HVAC design. 



With all of the varying elements in temporary HVAC equipment, it’s critical to work closely with the mechanical engineers, architect, and owner. This will help to determine the power strategies that maximize operation efficiencies, owners’ project and budget goals and maintain the integrity of the overall project design. While coordination is ongoing, the electrical engineer must also ensure the associated electrical systems being designed are compliant with all applicable NEC articles and UL listings to provide a code-compliant and safe solution for the customer. As temporary power is often provided in unfortunate situations, maintaining a simple and safe solution is as important as supplying the temporary HVAC.