The “more is better” mindset does not always apply in MEP engineering projects. Over-engineered components often increase project costs without providing any real benefits, and there are many cases where excessive capacity in fact has negative consequences on performance and service life. Another type of over-engineering occurs when the system used for a specific application is too complex, and a much simpler solution would have been possible without compromising performance.
General Disadvantages of Over-Engineering
Regardless of the specific application, over-engineering drives up project costs without offering significant benefits: oversized components are more expensive, and the associated labor cost also increases because equipment becomes more difficult to handle.
When mechanical, electrical and plumbing systems are too complex for the application at hand, there is also a higher chance of error during construction, due to the introduction of unnecessary components. This increases the chance of MEP engineering professionals and others involved in the project having to deal with change orders during project construction.
Over-engineering also brings performance issues that are specific to each type of building system. HVAC installations tend to suffer the most: an over-engineered system can be just as problematic as an undersized one, if not more.
Oversized Electrical Circuits
The main issue with oversized electrical circuits is their high cost. In fact, performance is improved: oversized conductors reduce both heat dissipation and voltage drops. The problem is that these benefits are not enough to justify the drastic increase in costs:
- Copper is expensive.When you consider that a typical building has thousands of feet of electric circuits, the cost of oversized conductors adds up very quickly
- Conduit diameter is increased.Electric codes establish a maximum fill percentage for conduit, so increased conductor capacity also involves larger conduit and accessories.
- Labor costs are increased.Since they are more difficult to handle, larger conduit and circuits typically require more man-hours of work. In most cases, specialized tools may also be needed.
When the extra costs of oversized conductors are considered, they far outweigh the benefits. Oversized conductors are particularly common with energy-efficient HVAC equipment – they are often specified based on “rules of thumb” that only apply for older and less efficient equipment.
The NEC and other electric codes may establish a maximum allowable voltage drop. It varies depending on the application, but in most cases either 3% or 5% is used. In these cases, conductor diameter should be raised so that voltage drop is brought to acceptable levels, but any further increases are unnecessary.
Using various supply voltages in the same installation is an excellent way to optimize conductor diameters. Keep in mind that power transmitted is proportional to both voltage and current, but only current defines conductor diameter. If a piece of electrical equipment draws too much current at 240 V, it makes sense to increase rated voltage to 480 V – this reduces line current, allowing smaller conductors to be specified. Of course, these are design choices that can only be determined by qualified professionals.
Oversized Electric Motors
In the case of electric motors, over-engineering tends to bring far more issues than with conductors. When subject to part-load conditions, electric motors display two main types of negative behavior:
- They suffer a drastic reduction in efficiency when the mechanical load on their shaft is much lower than their rated load. For example, a motor loaded at 80% does not suffer an efficiency drop, but for values under 50% the effect becomes significant.
- Power factor is also reduced when a motor is loaded lightly. Utility companies normally establish a minimum power factor for their consumers, and there are extra power bill charges for falling below that value.
Of course, another drawback of oversized electric motors is the drastic price increase. Motors can be among the most expensive pieces of electrical equipment, and oversizing them only reduces efficiency and power factor.
When specifying electric motors, special consideration must be given the voltage rating, since it determines the characteristics of all circuits and breakers located upstream. Large motors may justify the use of voltages such as 480V or 600V to prevent excessively high currents.
There are many types of air-conditioning systems, including mini-split units, packaged terminal air conditioners (PTAC), packaged rooftop units (RTU) and heat pumps. However, over-engineering tends to bring a common set of performance issues:
- Oversized compressors run in shorter and more frequent cycles, which is detrimental for their components and results in increased maintenance expenses. Keep in mind that compressor motors draw an inrush current that is several times their rated value each time they start – ideally, they should not cycle more than necessary.
- Air conditioning systems have the goal of controlling both temperature and humidity, but many types are cycled on and off based on temperature alone. Since oversized units reach the temperature set point faster, they are unable to extract enough humidity and the resulting environment is cool but humid. This is uncomfortable for occupants, and may bring health issues as well.
Compressors are not the only AC system components that bring performance issues when oversized. In system configurations that use air ducts, over-engineering also brings several negative consequences. For example, oversized ducts involve displacing a large volume of air, which drives up the CFM and power requirements of blowers.
In chiller plants and other types of AC installations that use hydronic piping, the extra cost associated with over-engineering can be particularly high. Other than being expensive, oversized piping requires more pumping power, increasing the nameplate capacity of both pumps and motors.
For air conditioning installations that will be subject to gradual capacity increases, variable refrigerant flow (VRF) systems can be a great choice – their modular nature offers great flexibility to size their capacity precisely depending on building needs. Chiller plants also offer flexibility, but are better suited for larger capacity increments than those typical of VRF systems.
For heating systems that are based on heat pumps, the same logic of air conditioning installations applies: oversized compressors suffer from frequent cycling and normally experience a diminished service life.
In the case of oil and gas boilers, the main drawback of over-engineering comes from short cycling: a phenomenon that occurs when an oversized boiler meets heating demand too quickly and then shuts down. To better understand the impact of short cycling, consider that boilers operate in a four-step cycle: pre-purge, firing interval, post-purge and idle period. When the firing interval is short, several negative consequences arise:
- The boiler radiates heat from its enclosure through the entire cycle, including the two purge phases and the idle period. Oversized boilers waste more energy in the form of radiated heat.
- During the pre-purge and post-purge steps, fans are used to displace any flammable mixture of gases that may have been left in the boilers. Both purging stages consume energy.
Although gas and oil boilers can cycle depending on the load, doing so is very inefficient. A superior alternative is to use two or more boilers of reduced capacity, which offers the flexibility to meet varying load conditions with energy-efficient operation. If there is a large demand for heating at any given moment, for example on Monday mornings during the winter, all boilers can be used simultaneously. Then, some of the units can be shut down to avoid short cycling losses.
The misconception that a larger boiler is better dates to the time when fireplaces and chimneys were used for indoor heating: a larger chimney offered greater flexibility to accommodate fires of any size. However, modern boilers operate on completely different physical principles, and the assumption no longer holds.
Concluding Remarks from an MEP Engineering Professional
Over-engineering can be favorable in specific applications where a high safety factor is required, but in most cases, it only drives up MEP engineering project costs without a significant return on investment. In fact, oversized systems typically come with a higher cost of operation due to inefficient operation and frequent maintenance expenses. Hiring the services of a qualified design firm is the best way to ensure MEP installations are engineered properly.