Building Design and Construction

Control of Computerized HVAC Systems

Control of HVAC systems ranges from simple thermostat ‘‘on-off’’ control to control by sophisticated electronic and computerized systems. Many variations of electronic and computerized systems are available for system control and energy management.
Programmable Thermostats. These are available for control of the total environment.
They provide energy savings and improved comfort levels and are widely  utilized to control many types of commercial unitary products. Different programmable types are available for heat pumps and for single- and two-stage heating and cooling equipment.
Programmable thermostats conserve energy by automatically raising or lowering the space temperature to preprogrammed settings several times each day. Building temperature adjustment is performed during occupied or unoccupied hours, again conserving energy.
Other types of thermostats are available that produce energy savings:
• Reduction of overshoot on recovery from the setback temperature, limiting the rate of temperature rise
• Selective recovery from the setback temperature by utilizing lower stages (singlestage and multistage only)
• Selective recovery from the setback temperature using a heat pump in lieu of an auxiliary heat source (heat-pump thermostat)

• Prevention of room temperature from deviating from a setpoint under varying load conditions
Microprocessor-Based Systems. Computerized energy-management systems are available to control complex HVAC systems. These microprocessor-based systems are capable of saving enough energy that they can pay for themselves in 2 years or less. Energy management systems are available in many variations and with many features. The most common include the following capabilities: equipment scheduling, duty cycling of equipment, temperature compensated control, demand limiting, optimum start / stop, and night setback.
Equipment Scheduling. Running HVAC equipment on a continuous basis, including periods when operation is not required, results in excessive and nonproductive energy consumption. These periods include lunch hours, evenings, weekends, and holidays. By using a scheduling program, the control panel promptly shuts down HVAC units and other units at preset times. A typical scheduling program may provide four start / stop times for each load on an 8-day cycle. It will also allow automatically for holidays, leap year, and daylight savings time.
Duty Cycling. Duty cycling applied to HVAC equipment utilizes an on-off control to reduce operating time and increase energy savings. The duty-cycling program will permit up to eight cycle patterns for each load. On-off protection timers are provided for each piece of equipment to assure equipment safety.
Temperature-Compensated Control. Maximum occupant comfort is ensured by use of a computerized control that provides equipment cycling with automatic temperature override. Cycling will be reduced or stopped, depending on whether room temperatures fall outside present comfort levels. Hence, the guesswork of selecting proper cycling strategies is eliminated.
Demand Limiting. As much as 50% of bills for use of electrical energy may be attributed to kW demand charges. A demand-limiting program may save a substantial amount of utility energy costs over a short period of time. A computerized control, in particular, can limit costly demand charges by use of a predictive demand program that anticipates electrical demand peaks. When the control panel senses that a peak is about to occur, it simply turns off selected loads on a priority basis until the peak subsides, thereby reducing the energy bill.
Optimum Start /Stop. Energy can be wasted by HVAC equipment that is scheduled to start and stop on worst-case conditions. This scheduling results in operations that begin too soon and run too long. Computerized control monitors indoor and outside temperatures and determines the most efficient start and stop times without sacrificing individual comfort. Optimum start / stop also allows independent control of individual comfort zones.
Night Setback. Utilization of night setback temperatures saves energy by automatically converting the specific comfort zone to a predetermined nighttimetemperature setpoint. The setpoint overrides the daytime setting for every load or zone. HVAC equipment will operate to maintain the nighttime setting until morning warm-up or cool-down is required. Night setback also allows independent control of each zone.

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