When specifying a building's mechanical and electrical equipment, it is easy to assume that extra capacity is a good thing, but it can be as counterproductive as having undersized facilities. Firstly, extra capacity has a higher initial cost, which leaves the building owner incurring unnecessary expenses. There are also many types of equipment that face performance issues when over-designed, which can range from low energy efficiency to a shortened lifespan. In a nutshell, oversized systems have a higher price and a higher operational cost.
This article will provide an overview of the building systems that are most commonly overdesigned, pointing out the pitfalls that should be avoided and the negative consequences that can arise from excess capacity.
1) Air ducts
Air ducts play a very important role in HVAC systems that use rooftop units or chillers with air handling units, and designing duct systems properly is critical to ensuring high performance. For New York City buildings, duct systems must meet the requirements set forth in:
- The New York Building Code
- The New York Mechanical Code, which devotes its entire Chapter 6 to duct systems
- ASHRAE Fundamentals Manual
- SMACNA (National Association of Sheet Metal and Air Conditioning Contractors) HVAC Duct Construction Standards
Proper duct design relies on controlling two key variables: air velocity and pressure drop. When these variables exceed the ideal design range, the duct system becomes noisy and inefficient. Ideally, a duct system should be designed for a pressure drop of 0.08 inches of water for every 100 feet of length, and the air velocity should be kept below 1,200 feet per minute.
Both pressure drop and velocity decrease as the cross-sectional area of a duct increases, but there is an upper limit to the size of the duct that can be increased.
- The system becomes more expensive, for the simple reason that material and labor costs are higher.
- The design process for other construction systems can be complicated by the fact that ducts take up a lot of space. It may even be necessary to reduce the ceiling height, disrupting the architectural design.
- Oversized ducts increase fan power because a greater volume of air needs to be moved through the system.
An ideal duct design keeps pressure drop and air velocity below design values while optimizing the cross-sectional area of the ducts.
2) Boilers
Even in buildings where high-efficiency boilers are used, heating is typically one of the highest energy expenses – this is especially true for New York and other northeastern cities, which have cold winters. There are boiler systems for virtually any type of energy input, including fossil fuels like oil or natural gas, electricity or alternative sources like sunlight and biodiesel.
Given the critical role they play in buildings, as well as the potential negative consequences that can arise from system malfunction, the New York City Department of Buildings has a dedicated Boiler Unit. This unit oversees the installation and operation of boilers and performs an annual inspection cycle for all systems currently in operation. Boilers are covered in the New York Building Code, Mechanical Code, and Plumbing Code, and the Fuel Gas Code also applies if the boiler is combustion-based.
Redundancy is desirable in boiler systems, but installing two boilers each sized for the total building load can result in a lot of unused capacity and a high initial cost. The best design recommendation is to size each boiler for 60% of the total building load, which provides a favorable degree of redundancy as well as an extra 20% capacity that can be used during the initial heating of the building. It is highly recommended to avoid “rules of thumb” as they can result in boiler systems two to three times larger than necessary.
Boilers could eventually be replaced with variable refrigerant flow (VRF) systems, which offer much greater energy efficiency as well as greater flexibility to meet diverse heating loads. As the name implies, VRF systems use refrigerant to supply or remove heat, and the speed is controlled according to the building's current load. The advantages of VRF systems are significant:
- They can use heat pumps, which operate in both heating and cooling modes, replacing traditional AC boilers and condensers. The latest heat pump models offer efficiency comparable to a chiller in cooling mode and in heating mode they can provide savings of over 70% compared to a resistance heater.
- Heat is transported in refrigerant lines, which are much more compact than water pipes and air ducts.
- VRF systems are modular, which allows them to be expanded incrementally according to the needs of the building – packaged boilers, chillers and rooftop units are limited in this regard by their fixed capacities.
VRF systems are very popular in Japan, where they were developed, as well as in Europe. They are relatively new to the US market, but their installation cost is comparable to that of a traditional chiller-based system.
3) Air conditioning
The performance of air conditioning equipment improves in direct proportion to how the unit adapts to the application. There is a common but erroneous belief that it is preferable to oversize the unit so that it can cool indoor spaces more quickly while also reducing compressor operating time. However, this brings a lot of performance issues. Furthermore, although the compressor runs less time, it also consumes more energy than a properly sized unit, so energy savings are minimal to no.
Air conditioning systems are covered in the New York Building Code and the New York Mechanical Code. The ASHRAE Fundamentals Manual is also a solid reference when designing any HVAC system.
Poor humidity control
An effective air conditioning system doesn't just remove indoor heat; It also keeps humidity within a range comfortable for humans. When an AC system is oversized, it can reach the required internal temperature in a shorter period of time and then the compressor shuts down – or slows down in modern units with variable speed capability. The problem when AC units operate this way is that there is not enough time for indoor humidity to reach acceptable levels, which results in a cold but damp indoor environment, similar to a refrigerator. In addition to causing discomfort, this is a situation that can result in respiratory health problems for the occupants.
Excessive compressor cycling
An oversized AC unit has spare cooling power, so it can bring the indoor temperature up in less time than a properly sized system. This means the unit will tend to operate in a series of short bursts, which can wear out electrical and mechanical components, reducing their lifespan. On the other hand, a properly sized compressor operates for longer periods but without frequent starts and stops, which is the intended mode of operation.
When AC installations are compared based on their total cost of ownership, considering both energy and maintenance expenses and their useful life, an optimal capacity system is superior to both undersized and oversized systems.
Discomfort for occupants
Because an oversized AC unit has excessive cooling power for the application, sitting just below an air duct opening or in front of an evaporator can be very uncomfortable. Evaporator fans are typically sized proportionally to the AC unit, so the air is cold and moves quickly, causing a cooling effect.
4) Electrical Energy
Many devices and systems used in buildings run on electrical power, and this includes many types of mechanical equipment. When specifying electrical installations, the same principle applies as in mechanical systems: components must have the right capacity for the application, no less and no more. Electrical installations in New York City are required to comply with the following codes:
- New York Building Code, in particular Chapter 27
- New York Electrical Code
- NFPA 70: National Electrical Code
- For emergency and backup power systems, compliance with NFPA 110, NFPA 111, and the New York Fuel Gas Code is also required.
Oversized feeders and branch circuits
Undersized electrical circuits tend to fail quickly, sometimes in a matter of hours, but oversized circuits have no negative consequences in terms of operation. In fact, an oversized electrical circuit has less voltage drop and energy loss than a properly sized circuit. However, this benefit pales in comparison to the increased project costs that accompany oversized electrical installations.
Electric motors
The case of motors is different from that of conductors, and there are several negative consequences when they are oversized:
- Engine efficiency is reduced under partial load conditions. For a given mechanical load, a properly specified motor operating at close to full load will be more efficient than an oversized motor.
- The power factor is also reduced. The oversized motor consumes a high reactive current, which can contribute to power factor charges on the energy bill. The extra reactive current also consumes capacity in transformers, distribution boards and circuits, without contributing to the transmission of useful energy.
When motors will be frequently subjected to part-load conditions, the best option is to use technologies that allow speed control: Electronically commutated motors (ECMs) can be used for fractional power applications and variable frequency drives (VFDs) ) with three-phase motors can be deployed for larger loads.
If a building's engines are modernized, three aspects must be considered to achieve the best possible results:
- Adequate power.
- Upgrading to a higher efficiency level, for example NEMA Premium.
- Implementation of automation and speed control measures.
In chiller-based air conditioning systems, exceptional synergy can be achieved if the chiller plant has variable-speed multistage compressors, while the water pumps and associated AHUs use speed-controlled motors.
5) Sprinkler Systems
A complex building layout typically means that the fire sprinkler system will have a piping layout of similar complexity as well as a high number of sprinklers. When teams responsible for architectural design and fire protection design work in isolation, complex sprinkler systems are a common consequence. These systems have many disadvantages:
- The initial cost increases, both in terms of materials and labor.
- Pumps must be sized larger to provide adequate water pressure and flow for a system with greater piping length and sprinkler count.
The following are some design recommendations for optimizing sprinkler layouts, reducing their complexity and initial cost:
- Coordinate fire sprinkler design and architectural design so that interference from ceiling features such as offsets and soffits is minimized.
- Merge smaller rooms into single areas whenever possible, because each time a small room is added, the sprinkler count increases by one.
When designing sprinkler systems in New York, it is important to observe the requirements set forth in the following standards and codes:
- New York Building Code, in particular Chapter 7 (Fire and Smoke Protection Features), Chapter 9 (Fire Protection Systems), and Appendix Q (Modifications to National Standards).
- New York Fire Code
- NFPA 13 – Standard for Installation of Sprinkler Systems
- NFPA 13D – Standard for Installation of Sprinkler Systems in One-Family and Two-Family Homes and Prefabricated Dwellings
- NFPA 13R – Standard for Installation of Sprinkler Systems in Low-rise Residential Occupancies
It is important to note that Appendix Q of the Building Code takes precedence over national standards, introducing modifications specific to New York.
Final Observations
There are many reasons not to oversize buildings' mechanical and electrical systems. In most cases, oversized systems are as problematic as undersized systems, or even more so, causing performance issues and extra maintenance expenses. Even when there are no performance issues associated with extra capacity, it represents a higher initial cost that must be borne by the building owner.
Hiring professional MEP designers is the best way to ensure that all construction systems are optimally specified for the application in question. Compliance with applicable codes and standards is mandatory for a building to be approved, but these documents often specify only the minimum requirements – with professional design, a project can meet code requirements while maximizing the performance.
