HVAC refers to the integration three components of ‘Heating’, ‘Ventilation’, and ‘Air-Conditioning’. There’s a fourth component, ‘Controls’ that pervades the entire HVAC system. Controls determine how all HVAC system components operate in tandem to attain energy efficiency, comfort, safety, and cost-effectiveness goals.
Heating is accomplished by heating the air within occupant’s work or living space. Ventilating maintains an adequate mixture of gases in the air we breathe by controlling CO2 levels, ambient odors, and removing contaminants from occupied spaces. “Clean” air helps keep occupants healthy and productive. Air-conditioning refers to the sensible and latent cooling of air. Sensible cooling involves the control of air temperature while latent cooling involves the control of air humidity. Controls, the fourth component, maximizes HVAC performance and energy efficiency by coordinating all components and equipment while providing occupant comfort and safety.
High-performance HVAC equipment will more significantly impact energy consumption reductions when implemented in conjunction with a complete system design that includes control elements for heating, ventilation and air conditioning. If the comfort zone is extended through natural ventilation and air movement in summer, and through lower air temperatures in winter (made possible by highly-insulated and, therefore, warmer wall and window surfaces), even higher energy efficiencies and cost savings can be achieved.
Heating Control Elements
There are four types of heating controls that increase energy efficiency:
A Modulating Flame control continually adjusts (modulates) the heat input to the boiler up or down to match the heating load required.
A Step-Fired control enables the heat input to the boiler to change in steps, typically high/low/off. the capacity of the boiler can come closer to the required heating load.
A Modular Boiler control system is an energy-efficient measure that assembles groups of smaller boilers into modular plants. As the heating load increases, a new boiler enters on-line, augmenting the capacity of the heating system in a gradual manner. As the heating load decreases, the boilers are taken off-line one by one.
An Oxygen Trim control system continuously adjust the amount of combustion air to achieve high combustion efficiency.
Ventilation System Controls
In recent years, ventilation control systems have become more advanced and complex, but certainly more dependable. Among the advancements are:
Direct digital control (DDC) systems using digital-logic controllers and electrically operated actuators are replacing traditional pneumatic analog controls. DDC systems are repeatable and reliable, provide accurate system responses, and can be monitored from a central computer station.
Constant Air Volume (CAV) systems automatically reset the supply air temperature at the cooling coil to provide the warmest air possible to the space with the highest cooling load, reducing reheat throughout the system.
Variable Air Volume (VAV) control systems serve areas with lower cooling loads. Inlet variable speed fans are used to control air volume.
CO2–based control systems regulate the amount of outside air required for ventilation. These systems monitor the CO2 in the return air and modulate the outside air damper to provide only the amount of outside air required to maintain desired levels.
Air-Conditioning Equipment Controls
Chiller controls that significantly affect the energy efficiency include:
- Variable speed drives achieve good part-load performance by matching the motor output to the chiller load, and by cycling off at a lower fraction of capacity than constant-speed chillers.
- Multiple compressors achieve a closer match of the load than single-compressor chillers by sequencing the compressors as needed.
- Water temperature reset controls raise the water temperature as the demand decreases, allowing for more efficient chiller operation.
Cooling Tower Controls that significantly affect the energy efficiency:
- Variable-speed or multiple-speed fans.
- Wet-Bulb reset strategies, where the temperature of the cooling water is adjusted according to the temperature and humidity of outside air.
- Fans and pumps that use variable frequency drive (VFD) controls to reduce energy use at part-load.
Integrated Chiller Plant controls use monitoring and computational strategies to yield the minimum combined energy consumption for the chillers, cooling towers, fans, and pumps.