WNA, Inc.

PROJECT:
Gas Fired Boiler Efficiency Study
Navy Federal Credit Union
Vienna, Virginia

Facts

Size: 1,000,000 square feet

Project Type: Feasibility Evaluation

Services Provided: Mechanical and Plumbing System Analysis and Recommendations to Improve the Boiler System Efficiency

Features

Two gas fired Cleaver-Brooks boilers each with 10 million BTUH output

Building heat loads estimated at 6.5 million BTUH

WNA was retained to examine the existing gas-fired heating water boiler system operation and to identify potential modifications to the system to increase the overall energy efficiency of the heating system.

Installed in 1995, the two gas-fired water boilers in the mechanical room generate hot water to meet the space heating requirements for the entire NFCU Headquarters Facility. Each of the boilers is rated at 12.5 million BTUH input and 10 million BTUH output. Both boilers are equipped with a combination gas and fuel oil burner.

The Headquarters facility was thermally modeled using a computer simulation and the results of the peak heat load calculations were compared with the existing boiler plant capacity. The calculations indicated that the existing boiler plant has significantly more heating capacity than required, and the building only needs a fraction of the installed capacity to maintain comfortable indoor conditions during the winter season.

Because the existing boilers appear to be oversized, the hot water temperature set point is reached quickly after the burner fires. As a result, the burner short-cycles to maintain the desired hot water temperature. This short cycling results in reduced operational efficiency and increased energy consumption. Based on the results of this evaluation, several recommendations were identified to reduce energy consumption and costs:

Modify one boiler to burn both gas and fuel oil so the gas service can be changed from "uninterruptible" to "interruptible."

Reduce outside air ventilation rates to match actual facility occupancy to save energy in both the winter and the summer seasons.

Increase the loads on the existing boilers to improve operating efficiency by shifting the existing four domestic water heater loads to the main boiler system through a heat exchanger during the winter season. This would make the boiler burner operate above its lowest turndown range and minimize the short cycling.

Replace one of the existing boilers with a smaller boiler having a capacity equivalent to 60% of the peak heating load and allow the facility to operate the smaller boiler at a higher firing rate for most of the heating season.

Add a high efficiency boiler to provide high efficiency heating during most of the heating system. Condensing high-efficiency boilers can achieve a high seasonal efficiency and result in a significant reduction in annual gas consumption.