High Performance HVAC - Two Steps to Energy Savings

The design of an HVAC system for a sustainable building is a two-step process. First, your team must determine how to reduce or eliminate as many loads as possible. As a result, the second step of specifying HVAC equipment means a smaller and therefore less costly system.

Step #1: Low-Cost Strategies for Reducing Energy Loads

In terms of reducing or eliminating loads, start by creating a tighter envelope with adequate insulation and the right glazing to optimize performance. Use daylighting to reduce heat from well-controlled, high efficiency electric lighting. Finally, select high efficiency plug loads and control them wherever possible.

Climate responsive design also helps reduce loads. Good mechanical design takes advantage of whatever climate/site resources are available to offset building energy needs.

A simple low-cost solution is often overlooked. Use fresh outside air for cooling whenever possible. The operation of the air system on “fan only” without additional cooling is called an “economizer” for the obvious reason that it saves on energy costs, which can be up to 50% in Western states. The use of make-up air units with energy exchangers can greatly extend the value of the economizer cycle and deal with the high humidity typical to the Southeast. The results of a survey which examined rooftop systems less than five years old showed that 68% either did not have an economizer or the economizer was not operating properly. (Source: NBI/PIER)

Night venting also takes advantage of local climate. By running the air system at night when temperatures are lower, you can pre-cool the building against the next day’s heat load. In some cases, natural ventilation can eliminate the need for a cooling system during periods of the year. The same system can be used in winter to transfer heat from the core to the perimeter to reduce startup heating demand. Oregon Health Sciences University (OHSU) utilized such a strategy as discussed earlier in this document.

Natural ventilation such as operable windows can be effective in buildings with lighter loads. When used in conjunction with radiant heating and cooling (“cool beams”), natural ventilation and hybrid ventilation systems can offer superb economics. For larger built-up systems, evaporative cooling is available in drier climates, using about half the energy of compressor cooling. For smaller systems, a hybrid rooftop unit combining evaporative and compressor cooling is currently in beta testing by NBI.

Building Orientation Impacts Energy Use and Loads

n northern climes, orienting the building to take advantage of solar heating during the winter and daylighting year round makes perfect sense. Of course, adequate mass must be built into the structure to effectively use the heat gained. In southern zones, orienting the building to reduce solar heat gain, especially from the western afternoon sun, and utilizing an effective daylighting strategy that avoids heat gain, can save substantial energy (see Rinker Hall above).

With the appropriate site, a ground or water source heat pump system maximizes efficiency by taking advantage of the heat capacity of the earth/water on site. In some cases, local geothermal sources may be available.

Step #2: How To Select The Optimal HVAC System

Once loads have been reduced or eliminated, you will be selecting an HVAC system that is smaller in capacity and therefore less costly than a conventional system. Make sure your mechanical designer allows for the reduced loads.

System Component Selection: Once your design team selects a system, the HVAC components specified must be the most cost-effective and efficient available. Using low first cost but inefficient equipment results in excessive operating costs and lower net operating income. Criteria for choosing your equipment and recommended efficiency levels for various kinds of equipment can be found in the NBI’s Core Performance manual, Section 2.9 Mechanical Equipment Efficiency Requirements

High Performance HVAC System & Lessons Learned

Here are some thoughts to consider when designing an HVAC system for your
sustainable building:
•    Start by reducing loads to envelope, lighting and plugs via
•    efficient components
•    Use site resources such as sun, wind and geothermal to offset or eliminate HVAC loads
•    Use high efficiency HVAC equipment with Variable Frequency Drives (VFD)
•    as needed
•    Provide separate HVAC units for process loads (e.g., data centers)
•    Use adequate lighting controls
•    Make sure the building gets commissioned

Key Questions to Ask Your HVAC Team

HVAC typically represents a third or more of the building’s energy use and directly affects the comfort and productivity of the building’s occupants, so it is key to both energy efficiency and sustainable design. Important questions you and your design team need to consider:
•    How do you get the engineers involved early in the design process where they can make a difference?
•    How do you structure engineering fees to incent the engineer to deliver smaller, more efficient solutions?
•    Who is responsible for the performance goals being met and how does that affect the team’s willingness to push the envelope?
•    How can the building be zoned to both aggregate similar loads and separate dissimilar loads?
•    Where can you get high efficiency equipment?