ASHRAE Journal - August 2011 - 27

Testing, adjusting and balancing was an inexact science relying on Ak (area) factors and the precise placement of a velometer probe to obtain the velocity. This velocity multiplied by the area factor determined the airflow rate. Since the placement of the probe allowed most any reading desired, it was relatively easy to produce balancing reports where all numbers were perfect. In 1979, I clearly remember being given a meter with five small round holes for air inlets and a dial that read 0% to 100%. I was told that balancing meant having all the readings at 50% and to adjust dampers until the readings were consistent. When excessive flow rates are coupled with inefficient designs, inaccurate controls, simplistic sequences, leaking ducts and improperly balanced systems, energy use is almost always higher than it should be. The opportunity for energy savings in older buildings is significant. Each of the HVAC system types that have been mentioned have specific issues that contribute to inefficiencies. The process of technical retrocommissioning (RCx) can locate these areas of inefficiency.

Constant Volume Reheat
Before Retrofit
Outdoor Air Damper Outdoor Air Intake Optional Pre-Heat Coil Filters Supply Air Fan Supply Air Duct Return Air Damper Cooling Coil Optional Return Air Fan T T Reheat Coils

Exhaust Air Louver

Exhaust Air Damper Return Air System

New Low Leak Outdoor Air Damper Outdoor Air Intake Return Air Damper

After Retrofit
Optional Pre-Heat Coil New Low Pressure Drop Filters Supply Air Fan With New VFD Supply Air Duct Cooling Coil Optional Return Air Fan With New VFD
Enlarge Duct To Lower Pressure Drop

Exhaust Air Louver

Constant Volume Reheat

New Low Leak Exhaust Air Damper

T T Constant volume reheat systems deliver cool air at 55°F (13°C) in sufficient amounts to keep the space at setpoint on a Figure 1: Constant volume reheat system. Reheat boxes were in vogue in the 1970s. summer design cooling day. The system’s reheat coils heat air to keep the space from overcooling. During These systems are perfect candidates for a retrofit where the the month of January in North Dakota, the outdoor temperature constant volume box is removed and replaced with a variable can be –30°F (–34°C). air volume (VAV) damper. When I tell a building owner that the building’s heating The following calculations assume a 25,000 cfm (11 800 system thinks the outdoor temperature is 92°F (33°C) and L/s) constant volume system of 55°F (13°C) air at 8 in. (1992 very humid, and that the system is trying to air condition the Pa) of external static duct pressure and 3 in. (747 Pa) of interspace, but a reheat coil is overriding the cold air and adding nal pressure with a 60 hp motor (45 kW), running 12 hours per heat, I get a stare that says “you’re kidding me.” While it day and seven days per week. Utility costs are $0.08 per kWh may sound idiotic, this is the premise of a constant volume and $1/therm ($0.01 MJ) of gas with efficiencies for boiler at reheat system. 80% and fans at 70%. Constant volume reheat boxes were in vogue in the 1970s. Savings are seen in the following areas: These boxes provided constant volume airflow and included a A typical VAV conversion will reduce the flow of a fan by reheat coil and a damper controlled by a spring. As the pres- 70%. The horsepower reduction is calculated by the following sure in a duct changed, the spring would, in theory, adjust the equations: damper to maintain constant flow. These dampers worked when 25,000 cfm × 70% = 17,500 cfm the system was clean and the springs were still operational. The spring and damper imposed a 1 in. (249 Pa) pressure drop. As 60 hp × (17,500/25,000)3 = 20.58 hp the springs wore out or dampers became dirty, the blades would (60 hp – 20.58 hp) × 0.746 kW per hp × require more than the original 1 in. (249 Pa) pressure to operate. Sometimes, they simply stayed stuck in one position. 4,368 hours per year × $0.08/kWh = $10,276/year

Add New Building Pressurization Sequences

Return Air System

Retrofit VAV Boxes With Reheat Coils

August 2011

ASHRAE Journal

ASHRAE Journal - August 2011

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