by Greg Longo and Blair Richardson
Developers of commercial properties are always concerned with the first cost associated with development. This often results in the selection of less energy-efficient HVAC equipment,
resulting in higher utility costs and reduced comfort for the future tenants. First costs for HVAC typically include equipment cost, ductwork, and labor required for installation of a complete working system. However, energy costs cost are often considered secondary to the first costs. This can lead to excessive operating costs and reduce profitability for building operators. The best approach is to look at the building from a life cycle perspective.
There are many different options when it comes to HVAC systems. For this comparison we are going to be looking at packaged rooftop units and variable refrigerant flow (VRF) systems and show that VRF can be a cost-effective alternative.
A constant volume packaged rooftop air handling system is one of the most common pieces of HVAC equipment used in commercial construction. This equipment typically consists of a natural gas furnace, condensing unit, cooling coil packaged into one piece of equipment located on a roof, and associated ductwork distribution system. Equipment efficiencies are typically the lowest allowable by the energy code. The cost of a complete rooftop system is approximately $20 psf.
Variable refrigerant flow (VRF) systems typically consist of an outdoor condensing unit, an associated system of refrigerant piping, and multiple indoor fan coil units. The condensing unit acts as a heat pump to provide heating and cooling. The indoor units come in a variety of different styles to fit the aesthetic of any space and have many flexible mounting options. The system capacity modulates based on the heating or cooling demand on the building. Because this system modulates between zero and 100%, efficiency is much higher than code minimum. Additional efficiency can be gained though the heat recovery function of some systems. The cost of a system is approximately $30 psf.
In accordance with the energy code, a load calculation is created for every project. This takes building location, construction, internal loads, occupancy schedules, etc. into consideration. The peak load is the highest expected heating and cooling requirements for the year. This typically indicates the minimum heating and cooling capacity required for the system to meet the owner’s needs. From the load calculation we can then create an energy model to simulate how each type of system will operate under these established conditions. The results from this energy model show the yearly energy use and help determine the best value with respect to energy cost, first cost, and maintenance.
The packaged rooftop system typically serves a single zone in either heating or cooling mode. This can cause thermal comfort issues as the seasons change. Because there is only one ductwork distribution system, there is no ability to modulate the amount of heating or cooling in a space. Spaces that need cooling year-round require supplemental systems to provide air conditioning during the heating season, and conversely for spaces that require heating year-round.
The VRF system breaks the building down into multiple zones to provide effective heating and cooling to spaces based on the real-time demand. This allows better much better control, significantly improved efficiency, and the ability to transfer heat between spaces without needing a large ductwork system. With the systems improved efficiency, we can also eliminate the code requirement for 100% outdoor air economizers. This drastically reduces the size of the ductwork distribution and reduces the overall footprint of the system.
Every owner needs quality data to make an informed decision. It’s not just about the cost of the equipment anymore. A well-designed VRF system may be marginally higher in first cost, but will have significantly lower annual energy cost and provide greater comfort when compared to traditional packaged rooftop units. By modeling the buildings and equipment we can show that VRF can save up to 20% on yearly energy cost, resulting in a simple payback of approximately five years.
Greg Longo, PE, is senior project manager and principal, and Blair Richardson, PE, CEM, CGD, LEED, is senior mechanical engineer at BL Companies headquartered in Meriden, Conn.