The HVAC Factor: Reducing Water Consumption

By Neil Maldeis
Originally published in the April 2012 issue of Today’s Facility Manager

Conserving natural resources is a fundamental part of the high performance building approach that is helping boost operations in new and existing buildings throughout the United States and worldwide. While energy conservation gets most of the attention, facility managers (fms) recognize there are also opportunities to make buildings more efficient and environmentally responsible by reducing water consumption. Comprehensive building audits should include assessments of current water use and a process to identify conservation opportunities.

Water conservation is a component of the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) certification process, with organizations needing to reduce consumption by 20% to 30% to earn points in the Water Use Reduction category. Meanwhile, the Federal High Performance and Sustainable Buildings guidelines for existing federal buildings mandate a 16% reduction in the indoor use of potable water from a 2007 baseline by the end of fiscal year 2015, and a 50% reduction in landscape water use.

Fms aiming to reduce water consumption rightly look first at the biggest water consuming areas. These can include industrial processes, laundries, kitchens, restrooms, and landscaping. Many fms are also taking steps to reduce water use in heating, ventilating, and air conditioning (HVAC) systems as part of an integrated strategy. Figures vary by building type, size, and geography, but air conditioning and heating can account for 10% to 25% of a building’s water consumption, according to a series of studies by the San Jose Environmental Services Department.

Heating and air conditioning systems are traditionally designed as “closed” systems that consume little or no water in normal operations. In large commercial HVAC installations that are properly operated and maintained, water for cooling towers accounts for almost all of the HVAC system’s water consumption.

Cooling towers need to make up water that is lost in three ways: by evaporation; from intentional bleed off (required to remove suspended and dissolved solids left behind when pure water evaporates); and from drift, water that escapes the system in the form of mist or droplets carried by the airflow.

As part of building wide improvements or as a discrete project, fms can reduce water consumption and improve the performance of cooling towers with a relatively small capital outlay that pays for itself over time. For example, according to a 2011 life cycle cost study commissioned by the California Statewide Utilities and Standards Program, it would cost about $3,500 to install prescribed cooling tower improvements at a typical office building and operate them for 15 years. The measures would deliver more than $11,000 in water and chemical cost savings during that time period. The study projected a much greater return on investment in a more water intensive building type such as a school, hospital, or factory.

Strategies for improving cooling tower performance and reducing water consumption include:

  • Use existing building automation system technology to control tower operations, including automatically shutting the tower down when possible without impacting occupant comfort.
  • Monitor water levels in the cooling tower reservoir to prevent overflow. Check fill valve settings and operation, and replace old float style fill valves with more efficient parts. Install an overflow alarm to alert the operator in case of sump overflow.
  • Install drift eliminators to redirect airflow and reduce water loss.
  • Recognize that evaporation rate is directly tied to cooling load, with evaporation loss of about three gallons per minute per 100 tons of cooling. Finding ways to reduce cooling load will reduce both energy and water consumption.
  • Use turbidity sensors, chemical monitors, automatic bleed systems, side stream filtration, and other methods to maintain ideal water quality levels in towers. Consider alternative make-up water sources such as wastewater from industrial processes.
  • Evaluate potential of hybrid cooling towers, dry coolers, heat sinks, and other less water intensive methods to replace or supplement current cooling towers as part of a broader upgrade.
  • Engage an energy engineer, energy services company, or other third-party advisor to conduct a critical system audit that includes the cooling tower and other HVAC system elements.

With the broader adoption of high performance practices, fms have an increasing number of tools and processes at their disposal to reduce water consumption in their facilities.

Maldeis is energy engineering manager for Trane, a global provider of indoor comfort systems and services and a brand of Ingersoll Rand. He is responsible for the technical development, support, and review of performance based contracting solutions and activities on a national basis. He has nearly 30 years of experience as a mechanical/project engineer in the building construction and energy conservation fields.