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Microelectronic controls for HVAC.

The future of heating, ventilating, and air conditioning (HVAC) controls lies in microelectronics. Microprocessors--in which logic patterns are electronically etched onto computer chips--have increasingly become the brains of building comfort systems, replacing old-fashioned mechanical and pneumatic controls.

Essentially, a microelectronic revolution is sweeping the HVAC industry. Today, a well-designed microelectronic control system can frequently be installed at the same or lower cost than a conventional building automation system, besides adding substantially greater capability and reliability.

Microelectronic controls, which initially came onto the HVAC scene about eight years ago, are now capturing the large new buildings market and are making major inroads into existing buildings as HVAC systems are modernized or replaced.


Microcomputers are also making a number of changes in HVAC systems. These are their three basic functions:

* As a standalone device, the microprocessor controls the desired performance capacity of a unit. In the case of a chiller, for example, it can adjust the level of cooling it produces--its percentage of capability--based on loads that the building or outside temperatures are applying.

* Microprocessors can monitor an HVAC unit's safety and operating parameters to ensure that they are not exceeded. Sensors can report measurements of temperature, pressure, voltage, amperage, and air velocity. If these measurements approach or extend beyond established upper and lower limits or drift from stipulated setpoints, the microprocessors are programmed to analyze and diagnose the problem, stop the unit, or report to a central computer so that it can take further action.

* Finally, each microprocessor can communicate the operating status of the unit it controls and all the information it is evaluating. A microprocessor is factory installed into each HVAC unit and communicates over a twisted wire pair (simple telephone wire). With each control having its own serial communications "address," the entire HVAC system can be linked to a central computer for reporting and overall control purposes.

Certainly the key piece of equipment for air conditioning a building is the unit that produces the chilled water on which the building temperature depends. A microprocessor on a building chiller can provide extremely close chilled water temperature control while providing over 50 monitoring points and more than 50 diagnostic messages . . . all actions not practical with old-style controls.

In addition to basically controlling the chiller operation, the microprocessor constantly monitors more than 50 points of information. To list but a few, they include chiller system diagnostic status, daily peak load, chilled water system setpoint, system run hours, present system load, condenser control sensor, evaporator or condenser flow, compressor status, condenser pressure, and so forth.

Today, every HVAC unit can cost-effectively have its dedicated microprocessor control. Microprocessors can direct air handlers to deliver the right amount of air at the right temperature to cool a space. In turn, a microprocessor-based control on each variable air volume (VAV) terminal maintains precise space temperature control.

In a different system, a microprocessor controls commercial self-contained units--used often in high-rise office buildings--to manufacture their own cooling for an entire floor or several floors. The same type of controls can also apply to fan coils, heat pumps, and so forth.

Actually, microprocessor controls can extend beyond typical HVAC units to other equipment that may be integrated with a building's comfort system. A modern system will monitor the status of the boiler and cooling tower, even lighting and, at times, be able to control their performance.

Not only is performance enhanced, but maintenance can be substantially reduced. In fact, controls can provide optimization of the entire HVAC system, not just each unit individually. For example, in a system, the microprocessor can determine which box is widest open. It can be used to reset the air handler's static pressure and leaving air temperature. In turn, the system can see which air handler's cooling valve is open and can be used to reset the cooling tower's setpoint.

This concept, called "system optimization," is not cost effective with older control schemes, but offers a major opportunity for energy saving and improved comfort.

With microprocessors, there are no springs, bellows, gear systems, or lever arms whose performance will degrade as the system ages. Compared to mechanical and pneumatic air-driven devices, microprocessors are smaller, substantially more accurate, and need no periodic maintenance.

Factory mounting

In addition to the microelectronic revolution, there is a second major move afoot in the industry--factory mounting. When controls are built into units in the factory, reliability is heightened, quality control is inherently superior, and total installed costs are reduced compared to field-mounted controls.

When factory productively and labor rates compete against field productivity and labor rates, the former will win. Factory installation also provides single-source responsibility, with one provider of control, HVAC unit, and building automation system.

Perhaps the key benefit of microprocessor controls in HVAC systems is their ability to spot current or even upcoming problems, report them via automatic alarms, and diagnose their nature and the measures necessary to eliminate them.

Again using the chiller as an example, one microprocessor task is to evaluate pressure and temperature inside the condenser. If the control sees condenser pressure and/or temperature increase at a rate that is programmed to find unacceptable, it immediately commences to limit the chiller's capacity, decreasing the amount of cooling. At the same time, it flashes a warning to the central computer, which itself is programmed to check the entire system and locate the cause(s) or probable cause(s), advise its operator of the problem, and know what procedures to follow to resolve it.

Preventive maintenance is another important capability. Microprocessors can keep track of a unit's starts and run times and advise when maintenance or new components are required.

Along with these diagnostics is a powerful service tool called the saved status report. This not only tells the operator what the problem is, but if a unit happens to fail, it will "freeze" the status report so that a serviceman knows exactly how the machine was operating up to one minute prior to the failure. This is a diagnostic tool that can frequently be a source of a quick resolution to the problem, saving hours on "stakeout" in an equipment room waiting for a failure to repeat itself.

Microprocessor-based systems can permit automatic restart as soon as a diagnosed condition clears or, if preferred, require a manual reset before a unit's operation can resume.

Although each microprocessor is standalone in its control of an HVAC unit, it is normally tied into an overall building automation system controlled by a central computer. Night setback is one example where, during winter, the building is microprocessor-controlled at an energy-saving, reduced temperature when unoccupied. At the termination of night setback, the heating system is optimally started, achieving proper temperature only at the time of occupancy.

On evenings and weekends, an override mode can restore occupied temperature control to a specific zone with a tenant billed for that HVAC usage, a feature which many owners have found produces paybacks on an entire building automation system in less than nine months.

In electrical demand limiting, a major cost saver in many areas of the nation, the automating computer is programmed to take appropriate action, turning off selected loads when the building electric demand and cost approaches a programmed limit.

Whether microelectronics are used to ascertain exactly how HVAC equipment and all components are performing, or participating actively in that performance, microprocessor control technology has become the essential future of HVAC systems.

Gene Smithart, director of marketing of heavy machinery for the Trane Company, has been with Trane for 18 years. He has managed a number of Trane's key departments, including customer direct service software, building automation, and systems marketing.
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Title Annotation:heating, ventilation, and air conditioning
Author:Smithart, Gene
Publication:Journal of Property Management
Date:Jan 1, 1991
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