A project was undertaken to evaluate the effectiveness of the HVAC systems at a major office complex in the Northeast. Performance data on the major mechanical components was gathered from the building energy management system on an hourly basis. This data was collected over an entire operating year and used to build a benchmark of operation for the HVAC system. During that time, two workshops were held with the building management and operating personnel to review the data collected, and to identify specific opportunities to improve the operation of the HVAC systems. This report is a summary of the information presented and recommendations identified.

Mechanical equipment in the building consists of 300 water-source heat pumps, and a central plant including two boilers, a chiller, two cooling towers, and two make-up air handling units equipped to condition the outside air prior to introduction into the space. Operating data from this equipment has been assembled and used to create a benchmark of operation for each component. There are 22 constant volume boxes that are supplied from the make-up air units to provide fresh air to zones within the building. Data collected from these boxes includes space humidity and CO2 levels, and will be compared with standards previously defined to determine the overall effectiveness of the HVAC systems. Operating data from 40 heat pump units has been collected and used to model the operation of all heat pump units.

Return air temperature data from the heat pump units was collected and compared to a standard to maintain 68 deg F to 76 deg F during the occupied periods. Although some units are showing low levels of acceptable performance, the average space temperatures recorded over the year appear to be controlling within acceptable standards.

The standard for acceptable humidity levels is 30% to 60%. Humidity data was collected on all of the constant volume boxes. That data indicates that the humidity levels were marginally in the acceptable range during the cooling season, but fell out of range completely as the ambient temperatures and humidity levels dropped. A review of the chart showing average humidity vs. outdoor temperature indicates that the average humidity in the building is consistently lower than desirable, creating uncomfortable conditions in the colder months and using excess energy for dehumidification in the warmer summer months. Control of humidity can account for up to 60% of peak cooling load in the building. Once again, the problem appears to stem from heat punp units that operate in the cooling mode for most of the year. These units can discharge air in the high 40 deg F range, providing excessive dehumidification throughout the year. This excess dehumidification can be very costly, but unfortunately this problem cannot be resolved due to the limitations of the existing system design.

The amount of fresh air being introduced can be indirectly measured through monitoring of CO2 levels. The levels of CO2 being measured at each box are well within acceptable standards (<1000 ppm). At our six month review, the CO2 data showed that the average levels did not vary, indicating that there was probably an excess of fresh air being introduced into many parts of the building. At that time, a strategy was implemented to more aggressively control the fresh air being introduced into the building. The results of that effort can be seen in the graph.

The following is a list of recommendations that were developed based on a review of the data collected and discussions held in the workshops. Some of the recommendations are proposed to improve the quality of the indoor environment. Others are identified to reduce operating costs. An estimate of potential savings is included where applicable, along with a description of each project.

Reset Heat Pump Loop Temps - By monitoring heat pump performance, an optimization strategy can be developed for the condenser loop setpoint. Condenser water temperature may also have an indirect impact on average space humidity levels.

Reduce Fresh Air Minimums – The amount of fresh air that was previously being introduced into the building was excessive for the building occupancy. We recommend that new minimums be established based on actual building occupancy.

Cycle Heat Pump Fans in Winter – The heat pump fans can be programmed to cycle the supply fan on a call for heat. Heat pump fans should run continuously in the cooling mode.

Chiller Plant Optimization – >A chiller plant optimization program can be implemented including the installation of VFD’s on the condenser water pumps and cooling tower fans. Condenser water temperatures will be optimized to improve chiller performance, reduce condenser water pumping horsepower while minimizing tower fan horsepower.

 Install Heat Pump Shutoffs – The primary heat pump condenser water loop includes a 250 hp pump that runs continuously. Installing shutoff valves on the individual heat pumps will allow the pump to be modulated based on building load.

Install Heat Pump Cooling Tower VFD’s – Fan energy savings can be realized by installing VFD’s on the heat pump loop cooling tower fans. The fans can be modulated to match building load.

Modify Make-up Air Unit Sequencing – The make-up air units have sequencing that includes humidification, dehumidification and reheating for humidity control, often when the building humidity levels are already within the acceptable ranges. Modify the control sequencing to directly control building humidity at optimal levels. Also, increase the fresh air flow in the shoulder months in an economizer cycle mode.