ASHRAE Journal - August 2011 - 32

Multizone systems can be retrofitted by removing the heating coil and installing a larger lower pressure drop cooling coil. The heating/cooling zone dampers are removed to eliminate damper restriction. VAV boxes with reheat coils can be installed in zone ducts. The savings for this conversion are similar to the other constant volume reheat systems with reduced airflow, elimination of damper leakage and decreased static pressures through the removal of damper and coil restrictions. Reheat is provided at the VAV box.

Early VAV Systems
The first VAV systems were designed before the advent of VFDs. Several methods were used to vary the airflow to the space. Some of the first VAV systems used pinch-off VAV boxes but then allowed the fan pressure to build up as the boxes closed. The fan speed remained constant and the airflow was reduced as the static pressure increased and reduced the airflow. This technique was called riding the fan curve. Sometimes discharge dampers were installed at the discharge of a unit and produced the same flow reduction. While the reheat airflow was reduced, the fan horsepower did not decrease as much as current systems that slow the fans down. Inlet vanes were the next improvement in VAV systems. Adjustable vanes mounted on the inlet of the fan wheel would

open and close to control the duct static pressure. This improvement reduced both the airflow and the horsepower requirements. As inlet vanes age, the inlet dampers have a tendency to stick and become locked in place. Both of these types of early VAV systems are good candidates for VFD installations to reduce airflows. Savings can be calculated when the specific systems are converted to VFD control. The control systems have the following factors: • Constant volume system: 1.00; • Discharge dampers or the “riding” the fan curve: 0.88 factor; • Inlet vanes: 0.62 factor; and • VAV systems with a 70% average fan speed or (0.70)3: 0.34. Removing the inlet vanes and installing a VFD will save the following in fan energy. 60 hp × (0.62 – 0.34) × 746 kW/hp × 4,368 hours per year × $0.08/kWh = $4,379/year Inlet vanes should be removed. An inlet vane that is locked open will still impose an unneeded restriction and reduce the efficiency of the fan.

Inadequate Fin Tube Control
Fin tube radiation is a common method of heating building exterior zone spaces in ASHRAE climate Zones 5 through 8. Fin tube radiation is typically sized to provide heat to offset the envelope losses from the building walls, windows, roof, slab and in some cases, infiltration. Because these losses are dependent on outdoor temperatures, the control strategies used to regulate the fin tube heat typically reset the hot water temperature based on the outdoor air temperature. Under this method of control, the heat output of the fin tube varies as the water temperature is changed. While there was usually some form of control for these systems, it was rudimentary. Entire fin-tube hot water temperatures were reset based on outside air temperatures. In more advanced systems, individual exposures were reset based on a single room temperature. Sunlight adding heat through windows, equipment and light generated heat was usually not factored into the heat requirements of spaces. Because of inaccurate control, these systems typically overheated the spaces and then allowed the overhead air system to provide space cooling. This simultaneous heating and cooling of the space is an energy wasting method of operation. Comfort and energy savings can be obtained by combining the space fin-tube and VAV box control. On a call for heat, the first part of the control sequence reduces the airflow from the VAV box, the second part opens the reheat valve to provide space heat from the VAV box and the third part opens the fin tube heat when the reheat valve is at the 60% to 70% open range. This sequence eliminates the simultaneous heating and cooling seen with limited fin tube control.
August 2011

The Standard for Green Buildings has a New Companion—

189.1 User’s Manual

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Standard 189.1-2009, Standard for the Design of High-Performance Green Buildings, a jurisdictional compliance option of the International Green Construction Code™ (IgCC), covers the key topic areas of site sustainability, water-use efficiency, energy efficiency, indoor environmental quality, and the building’s effect on the atmosphere, materials and resources. It provides the basis for creating greener buildings, and everyone benefits: • Architects and engineers get clear guidance on designing high-performance green buildings. • Governments demonstrate leadership and accountability • Developers and contractors get greater construction efficiencies and can offer higher value
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To order, visit: www.ashrae.org/standard1891andUM
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ASHRAE Journal - August 2011

Table of Contents for the Digital Edition of ASHRAE Journal - August 2011