Packaged Rooftop HVAC Systems

Packaged rooftop HVAC systems typically consist of pre-assembled, off-the-shelf equipment for space heating, cooling, and ventilation. A number of sizes and designs are available that are found on the rooftops of office buildings, retail stores, malls, schools, manufacturing plants, and a host of other commercial and industrial facilities. Single-package rooftop units can cover all the heating and cooling needs of a facility, or they can be

Packaged Rooftop Source: www.pnl.gov

A packaged rooftop unit contains all of the components found in a typical HVAC system. This includes the compressor, evaporator coil, condenser, heating unit, supply fan, exhaust fan, and filter. All of the components are contained in a single self-contained packaged unit.

Heating

Natural gas and electric heating options, and the choice between the two, is based primarily on the cost of the unit, as well as operating and installation costs. Natural gas heating units typically have higher overall installation costs when gas piping is included, but are generally less expensive to operate. The added cost of piping natural gas to these units may be quickly recovered, depending on the fuel costs, and the efficiency of the equipment. The chart below compares the cost of an equivalent amount of energy (million Btu) for electricity and natural gas.

*For estimating purposes only. Note that these costs can vary significantly.

Any calculations regarding annual operating costs will need to consider the efficiencies of the heating systems, operating hours, and comparative fuel costs.

Cooling

Commercial packaged rooftop cooling systems in the 5 to 30-ton range are often distributed in several locations along a rooftop, depending on the size of the building and the overall required cooling load. The American Society of Heating, Refrigeration, and Air Conditioning (ASHRAE) published the following voluntary standards (ASHRAE Standard 90.1) for packaged rooftop cooling systems.

The incremental cost of a high-efficiency unit is in the range of $100 to $200/ton, indicating that the premium for a 5-ton high-efficiency unit might be $750. The savings in the energy costs over a 2 to 3 year period often justify the expense of the higher-efficiency unit and can yield a significant overall operating cost savings for the HVAC unit's useful life.

In addition, the energy efficiency of the HVAC unit can decrease over time due to insufficient coil and fan cleaning, degraded insulation, compressor wear, and overall system deterioration. The rooftop unit is constantly exposed to weather elements, and may also be exposed to higher temperatures due to the solar absorption characteristics of darker colored roofing materials in proximity to the HVAC unit. These conditions make for an extremely difficult operating environment, particularly in the more humid regions of the United States. For this reason, the construction of the higher-efficiency packaged rooftop HVAC systems is generally more substantial than the standard systems meeting minimal code requirements, indicating that they might hold up better under severe conditions.

Options

The following are optional HVAC features:

• Temperature and time controls that incorporate standard programmable thermostats or digital controls, are common features of packaged rooftop systems. A seven-day time clock function can provide flexibility to operate the HVAC according to occupancy schedules and nighttime temperature setbacks.
• Desiccant dehumidification systems can be added to lower the humidity of the incoming outside air. Energy savings can be achieved in many applications due to the lower demand on the direct expansion cooling component of the system (reduced compressor load).
• Economizer units can be added to take advantage of the free cooling. A series of dampers and controls allows the outside air to be used for cooling when the outdoor temperature is moderate, such as during summer evening hours or during cooler days in the Spring and Fall.
• Fan controls can include variable air volume options to achieve greater energy efficiency. Comfort zoning and increased flexibility can also be achieved in facilities that have several packaged rooftop units. These can be tailored to meet specific building area needs.

Right-Sizing Your HVAC System

Equipment oversizing can also result in shortened equipment life, poor humidity control, and higher operating costs. All of these concerns have a direct bearing on total ownership cost, occupant comfort, employee productivity, and profitability of any business or organization. There are several reasons that HVAC systems are over-sized:

• Reliance on old nameplate data.
• Reliance on rule of thumb formulas, which should only be used for general estimating purposes.
• Lighting efficiency upgrades that have reduced internal heat generation within the building.
• Insulation upgrades that have reduced building envelope energy losses.
• Use of full load nameplate data of heating load generators (such as computers and office equipment), even though the equipment rarely, if ever, operates at full load.

Over-sized equipment can not only result in higher operating costs, but also increase the initial capital cost. An over-sized unit can require more frequent cycling of the compressor, which can lead to premature equipment failure and increased electricity use. It may also require larger fans and ductwork, which can cause variation in occupant comfort because of the larger surface area and the greater probability of system air leakage. Furthermore, the coordination between the economizer and the compressor may be exacerbated.

Usually, maintenance costs and comfort problems increase each year until the unit is about 20 years old. Then, the owner has to decide between a major overhaul of the existing unit or a complete replacement. The decision to overhaul a system is dependent on the quality and efficiency of the original unit, compared to new designs. If the decision is made to replace the HVAC rooftop unit, care should be taken to properly size the unit to the current cooling load requirement of the building, rather than matching the old design. This analysis should be left to a qualified HVAC consulting engineer, experienced contractor, or trusted HVAC equipment supplier. Always involve two or three equipment suppliers to compare design strategies as well as pricing.