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HVAC Energy-Saving Measures Compatible With IAQ

Many HVAC energy conservation measures are compatible with IAQ or can be made compatible. This article describes measures that are compatible or incompatible, as well as adverse effects to avoid.

 

Preventive Maintenance (PM) of HVAC

Preventive maintenance will improve IAQ and reduce energy use by removing contaminant sources (e.g., by cleaning coils/drain pans), and ensuring proper calibration and efficient operation of mechanical components (e.g., fans, motors, thermostats, and controls). Data from many buildings throughout the United States show that a properly commissioned building with controls and equipment functioning properly can save 5%-15% in total building energy cost.

 

Air-Side Economizer

Economizers use outdoor air to provide "free" cooling. Economizers potentially improve IAQ by helping ensure that the outdoor air ventilation rate meets IAQ requirements.

 

Economizers are not practical or advisable in hot-humid climates. Except in dry climates, moisture control must be incorporated. For example:

  • On/off set points could be calibrated to both the temperature and moisture conditions of outdoor air to avoid indoor humidity and other problems.
  • Outdoor air may be dried using desiccants prior to entering the indoor space.
  • Economizers may shut down when a preset outdoor air temperature is exceeded.
  • Economizers may need to be disengaged during significant outdoor air pollution episodes if that is a problem, or an air cleaning capability could be applied to the outdoor air prior to it entering the occupied space. Economizers can reduce annual HVAC energy costs in cold or temperate climates.

Night Pre-Cooling

Cool outdoor air at night may be used to pre-cool the building while simultaneously exhausting accumulated pollutants. This is called building flush. However, the cool outdoor air may also have a high moisture content and could humidify the building at night, so caution is needed. In addition to preventing microbiological growth, controls should stop pre-cooling operations if the dew point of the outdoor air will cause condensation on equipment.

 

Reducing Demand Charges

Night pre-cooling and sequential startup of equipment to eliminate electrical and other demand spikes are strategies compatible with IAQ. Energy load-shedding strategies, which are potentially incompatible with IAQ, may involve changing the space temperature set points, or reducing outdoor air ventilation during occupancy. These can create IAQ problems and complaints.

 

Supply Air Temperature Reset

Supply air temperature may sometimes be increased to reduce chiller energy use in VAV systems. However, fan energy will increase because more air is required to provide the same cooling. Higher (lower) supply air temperatures in a VAV system will increase (decrease) supply airflow - this may increase (decrease) outdoor airflow in systems that provide a constant percentage of outdoor air. A higher supply air temperature also reduces dehumidification potential and could create excess indoor humidity.


CO2 Controlled Ventilation

CO2 controlled ventilation varies the outdoor air supply in response to CO2 which is used as an indicator of occupancy. CO2 controls may be useful for reducing energy use for general meeting rooms, studios, theaters, educational facilities, etc., where occupancy is highly variable, and irregular.

A typical system will increase outdoor air when CO2 levels rise to 600-800 PPM to insure that maximum levels do not exceed 1,000 PPM. The system should incorporate a minimum outside air setting to dilute building-related contaminants during low occupancy periods. CO2 sensors must be calibrated periodically and set points may need to be adjusted based on outdoor CO2 levels around the building.

 

IAQ Incompatible Operational Measures

Attempts in the past to save energy have needlessly compromised IAQ and resulted in occupant complaints and/or serious illness. This section highlights measures to avoid, and demonstrates that the potential energy loss from avoiding these measures is not significant.

 

Reducing Outdoor Air Ventilation Below Standards

Applicable ventilation standards usually specify a minimum continuous outdoor airflow rate per occupant, and/or per square foot, during occupied hours. Standards are designed to insure that pollutants in the occupied space are sufficiently diluted with outdoor air. Reducing outdoor airflow below applicable standards can degrade IAQ and has low energy saving potential relative to other energy saving options.

 

Reducing HVAC Operating Hours

Delayed start-up or premature shutdown of the HVAC can evoke IAQ problems and occupant complaints. The loss in energy savings from avoiding this strategy is not significant in a well-run building.

  • An insufficient lead-time prior to occupancy can result in thermal discomfort and pollutant-related health problems. IAQ problems can last for several hours, as the HVAC system must overcome the loads from both the nightly setbacks and from current occupancy. This is a particular problem when equipment is downsized.
  • Shutting equipment down prior to occupants leaving may sometimes be acceptable provided that fans are kept operating to insure adequate ventilation.

Relaxing Temperature/Humidity Set Points Below Standards

Occupant satisfaction and productivity is highly sensitive to temperature and humidity conditions. In addition, thermal discomfort can exacerbate pollution-related problems compounding the complexity and cost of diagnosis and remediation. The loss in energy savings from avoiding this strategy is not significant in a well-run building.

HVAC Energy-Saving Measures Compatible With IAQ

Created on February 18th, 2011.  Last Modified on January 15th, 2015

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