HPAC Magazine
Feature Article

Digital controls optimize performance

The airside economizers of today bear little resemblance to their predecessors.

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May 1, 2014 by ADRIENNE THOMLE

In the 1970s, the concept of airside economizers was introduced to the HVAC industry. The first economizers included two remote bulb temperature changeover controllers with a minimum position potentiometer for adjustment of the outdoor air (OA) dampers. One temperature changeover controller was placed in the outdoor air and the other in the mixed air. They were usually set for 70F and 55F, respectively. As the outdoor air temperature rose above 70F, the OA damper would close to a minimum position that provided ventilation for the occupants. The controller would modulate the dampers to maintain 55F in the mixed air (MA) section of a rooftop unit.

Why the specific temperature, you might ask? Fifty-five degrees Fahrenheit was chosen as the best air temperature to deliver to the occupied space. Though the concept of using cooler outside air for “free cooling” started decades ago, the idea did not take off until the oil crisis of the mid-seventies. Due to the oil embargo, building owners, like everyone else at that time, needed to cut down on their energy use. Using cool outdoor air for cooling buildings achieved that.

The accuracy of the humidity sensors used with the old systems would not even be considered in today’s world of digital technology. But in their time, they worked well to cool buildings without using mechanical cooling when the outdoor conditions were right. They were not controlled with a space thermostat and were not considered “integrated economizers,” meaning they were not integrated with the space thermostats’ demands for cooling.

A reliable and popular stand-alone non-demand control ventilation economizer in the HVAC industry was introduced in 1985 and is still being used in many applications where DCV is not used. This analog logic module was the base for all current economizer logic in the HVAC industry. It is used with a two-stage thermostat and has output for two-stage mechanical cooling.

There were a few basic requirements for an airside analog economizer controller in a commercial building:

• 24 Vac power.

• Input for an OA sensor.

• Input for a return air (RA) sensor (optional).

• Selection of the changeover control point for when to economize or not.

• Selection of the minimum position for the damper for ventilation when the space requiring cooling is occupied.

• Input from Y1 call for cooling from the thermostat.

• Input from Y2 call for cooling from the thermostat.

• Output to drive the damper actuators.

• Output to stages of mechanical cooling.

Analog economizers were designed to be integrated economizers with a two-stage cooling thermostat required in the space being cooled. When the OA is good for free cooling the LED on the logic module will light. On a call for Y1 from the stat, the economizer will open the OA damper and close the RA damper to maintain a MA temperature between 50F and 55F. On a second call for cooling (Y2) the logic will turn on one stage of the mechanical cooling. The economizing continues to boost the mechanical cooling.

When the OA air is not good for free cooling, two stages of mechanical cooling will be used.

A common misapplication for the economizer is not using a two-stage thermostat. A one-stage cooling thermostat will not allow the mechanical cooling to supplement the economizing and the space may not be satisfied causing occupant complaints and the economizer being disconnected.

All analog controllers used the A, B, C, D and E curves for single enthalpy changeover. Due to the analog design of the controllers and sensors, the “curves” were fairly approximate and are based on a simple psychrometric chart. The curves are only used for referential (single) enthalpy changeover where the reported conditions from the OA sensor are compared to the position relative to the selected curve.

Outdoor air conditions that are warmer and higher in humidity than the selected “curve” are not good for economizing and the economizer will not allow for “free cooling.”

For differential enthalpy, the economizer controller determines the “best” air to use based on the OA and RA conditions reported by the sensors and the changeover curve is not used.

In the late 1990s, the first analog economizers using demand control ventilation were introduced. These economizers were more complex and required longer setup time. The limits of an analog device created complexity for the technician.

 As more features and control functions are added, the analog controllers are becoming more difficult to use and do not provide fault detection or simple diagnostics for the HVAC service technicians. The challenge is to keep the economizers operating to satisfy the ever rising demand for more energy savings in buildings and to achieve the goal of net zero buildings.

The need for product innovation and sensor accuracy never ends. There will always be a desire for reduced energy costs and improved control with low payback period. Using technology developed in the 1980s is like using a block cell phone versus your smartphone today. Technology has advanced and so have economizer controls.

The first step in the continued innovation of economizer controls is achieving the energy goals at an affordable cost to the small building owner.

Reports suggest that 40 to 80 per cent of the existing economizers in the field do not work. There are a number of reasons for this: technicians do not understand the economizer operation and disconnect it, economizer controls are misapplied, many applications are using a one-stage thermostat, and/or the sensors are wrong for the application or out of calibration. We have all seen a block of wood in damper or a disconnected actuator so the dampers cannot open or close. But the reality is it takes more time than the technician has to properly diagnose a non-working analog control system. The industry has changed from the analog controls of the 20th century to digital controls that will allow technicians to save time in set up and troubleshooting.

These new economizer controllers use state of the art LCD displays to tell the operator the status of the device. Are the conditions good for economizing? Is the system in the economizing mode? What are the conditions being reported to the controller by the sensors and actuators?

The systems can be easily set for the proper amount of ventilation based on the cfm required by the occupants, meaning there is no more guessing for minimum damper position with one finger or three fingers to set the dampers.

Menus on the displays save time for the technician by walking them step-by-step through the set up and confirming their changes have been made in the controller.

Troubleshooting is now easier. The controllers will tell the operator that there is an alarm or the information can be displayed using a thermostat in the space to alert the occupants to call for service. The fault detection and diagnostics of the controls make it easy for the operators to tell the system status and correct any issues that may be increasing
energy use by the building.

Incorporation of demand control ventilation in the control reduces the cost of extra modules and wiring in an attempt to make the contr
ols simple.

The digital economizers of today are easy to install, use, maintain and troubleshoot. Soon the issues raised regarding airside economizers will no longer exist. Sensor accuracy has vastly improved so more control features are available to save the most energy possible in every climate zone.   <> 

Adrienne Thomle is global product manager with Honeywell. A recognized expert in airside economizer controls, Thomle holds six patents and has several pending patents related to economizer control in commercial buildings.

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