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Economizer Systems and DDC HVAC Control

Direct Digital Controls Systems in HVAC control has taken leaps and bounds in the last few years in being user friendliness, ease of designing systems, troubleshooting system problems, and increasing efficiency over the older energy managment systems. When discussing increasing efficiency of systems the idea of economizing always comes up. Economizers can be found in many different forms like using outside air when conditions are right to cool inside spaces, recovering heat loss from the relief air leaving the building, or making full use of the heat a boiler produces by recovering heat from the flue gases. There other ways to capture and conserve energy and for the sake of brevity we'll cover the basic way many buildings make use of economizers. That is using the outside air to cool the inside spaces instead of using mechanical cooling for the same thing.

Outside Air Intake

First of all we have to understand that the mechanical code book requires so many changes of air per hour in commercial buildings. Many building owners rely on a manual fixed damper system to meet these requirements. Other systems have stand-alone dampers that will actuate to the open position based on certain rudimentary input devices. The accuracy of these devices and the function they serve are rarely set up properly and hardly ever calibrated to ensure they function as designed. Typically, within a year of installation they are disabled and the damper is closed off permanently in some way.

The second realization we must look at is the fact that, depending on geographical location, a building owner can save lots of money with a properly designed and calibrated energy management and control system that will allow the economizer to work through the HVAC systems direct digital control system. That a good energy management DDC system has the ability to be monitored from a remote location over the internet. That it collects trend data and proves energy savings using charts and graphs not mentioning the fact that the final proof will be the lower utility bill. That devices that are out of calibration or broken can be observed from the remote location and action taken to repair such devices. A good DDC energy managment system will do all this and more.

Outside Air Temperature and Humidty Sensor

To properly set up an economizer so that it works as efficiently as possible we must have at least three input variables to control it. CO2 monitoring inside the space, an outisde air temperature reading, and an outside relative humidity reading. The CO2 will override everything alse if it reaches a specified setpoint in the DDC program. By monitoring CO2 inside the spaces we eliminate the need to have a minimum outside air damper position to meet the code requirements. That means that as long as the CO2 inside the spaces is below a specified amount the unit can operate and condition the inside air without any influence from the outside air because of minimum positions on the outside air damper. That saves energy cost and safely monitors the building spaces for CO2 levels. The next thing we need to make the economizer effective is an enthalpy calculation on the outside air. To get an enthalpy reading we must know what the temperature and relative humidity are outside. The DDC program will crunch the numbers and determine the enthalpy value of the outside air. Based on a predetermined setpoint the DDC program will lockout mechanical cooling and open the outside air damper to bring in cool air for the purpose of cooling the space. This feature alone has the ability to save lots of money on energy costs.

*Air Quality sensors which measure VOC's can also be used instead of CO2 sensors for this process.

One other feature which we can monitor and control with a good energy management DDC system and the economizer is building pressure. Using pressure transducers the building pressure can be controlled through DDC and the outside air damper and the relief air damper. This can be beneficial to buildings that typically have problems with dampness and humidity levels inside the building from infiltration air. The system can be designed to maintain a slight positive pressure in the building so that damp infiltration air is eliminated. A good energy management DDC system can save lots of energy dollars and pay for itself very quickly if it is set up properly.

Additionally, it doesn't hurt to set up pressure transducers on each floor of the building to monitor building pressure. The building pressure can maintained at a slight positive pressure using the economizer and/or a make-up air unit to control the building pressure.

DDC Related Links

DDC Controls Page 1 | DDC Controls Page 2: The Technical Aspects of DDC Controls | DDC Controls Page 3: DDC Controls Variable Air Volume System VAVs | DDC Controls Page 5: Programming and DDC Logic | DDC Controls Programming Example | DDC Controls Technician Descriptions

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