Economizers — Revisited Part 2

In my last post, I explained how economizers should work and the importance of installing a 2-stage thermostat on ALL economizer applications.  I also stated  that — Economizers are the most mis-understood, mis-diagnosed, and most often gutted control system on commercial packaged equipment.  Part of that comes from the economizer not being properly set up.  On the logic module there are a number of pots (potentiometers) that need to set in order for the economizer to function properly.  A lot of issues can be resolved if a little time is taken and a proper start-up / set-up is performed just as on any piece of equipment.  It also helps to understand how these pots work so you understand why you need to set them up in the first place.

Proper Set up of the Economizer Control Module

ExH- This setting is adjustable from 2 to 10 volts DC. It controls the normally open set of contacts between EF & EF1.  This setting controls at what point the power exhaust is energized.

Min Pos-  This setting is adjustable from 2 to 10 volts DC.  When terminals Tr1, Tr and N receive 24 volts AC the control will operate the dampers to a minimum position to bring in outside air.

DCV Max & DCV Set– These two potentiometers are used to position the dampers in response to an input from the CO2 sensor placed across terminals AQ1 & AQ.

ABCD Enthalpy Setpoint–  This setting is used by the module to determine under what conditions the module will allow free economizer cooling.  By region and preference is where the setpoint should be.

Lets Start at the top and work our way down to make sure we set it up correctly.

logic module

Exhaust Mode

The mode of this operation is used when the economizer is used with a power exhaust fan. If the voltage output to the actuator exceeds the adjustable setpoint, the ExH LED will energize and the normally open set of contacts between terminals EF and EF1 will close. At this should happen — The coil for the ER relay (Most units uses a separate relay to switch the line voltage to the exhaust fan motor)  should energize.  The ER relay will close it’s set of contacts and energize the power exhaust fan motor. Note: there is a 45 second delay in closing the EF and EF1 contacts. The reason for the delay is so the exhaust fan does not put the building into a negative pressure since the damper blades / motor move very slowly and may not be open yet.

Adjusting Minimum Position

  • Ensure that you have 24 volts located on terminals TR & TR1 and on terminals N & N1.
  • *Ensure that the factory jumper is in place between terminals P and P1 on the W7217 control.
  • *Make sure that the thermostat system setting is in the off position.  Turn the thermostat fan switch to “ON”.
  • *Adjust the minimum position potentiometer on the W7212 control to achieve the desired percentage of outside air.

The formula to determine how much outside air is being supplied is as follows:

(OAT X OA%) + (RAT X RA%)= MAT

OAT= Outdoor Air Temperature  OA%= Percentage of required outdoor air      RAT= Return Air Temperature    RA%= Percentage of required return air        MAT= Mixed Air Temperature


Our building requires 15% of outdoor minimum air, our outdoor air is 50 degrees and our return air in the building is 78 degrees:

50 degrees X 15% = 7.5 degrees    +    78 degrees X 85% = 66 degrees          7.5 degrees +  66 degrees = 74 degrees.

If the mixed air temperature is higher than 74 degrees you do not have enough outdoor air, if the mixed air temperature is lower than 74 degrees you have too much outdoor air.

Demand Ventilation (IAQ  Indoor Air Quality)

This mode of operation allows the W7212 control to respond to a 2 to 10 volt DC signal to terminals AQ1 & AQ.

The most frequent used control used is an external CO2 detector located in the building space.If the CO2 (Carbon dioxide) level becomes too high in the space the CO2 detector will begin to increase it’s DC signal to the W7212 control.

If the CO2 signal becomes higher than the DCV setpoint the demand ventilation will then begin to drive the actuator open.  The voltage output by the W7212 control to the actuator is limited by the DCV Max potentiometer setpoint. If the outdoor dampers open enough the power exhaust will energize to relieve the building internal pressure.

Freeze ProtectionDuring demand ventilation, if the discharge air temperature drops below 45 degrees, the voltage output the actuator will be reduced to the minimum potentiometer  (min pot) setpoint.

Logic Module 2IMPORTANT — If you are not using an IAQ sensor, both the DCV SET and DCV MAX pots need to be turned to the left (Counterclockwise — 2 VDC) on the Gen 2 W7212’s  if there is no sensor connected.  Otherwise if it sees less than 1 VDC it will assume the sensor has failed and will force the economizer to the DCV max position. Make sure this is done if an IAQ sensor of any type is not installed

Enthalpy Set Point

The A, B, C, D setting is based on the outside air temperature versus the  relative humidity.  In the midwest most settings are set to D (and I always remember it as “D” for dummies.  It works for me and I never forget).”D” will only let the economizer start to open when it is 63 degrees with 50% humidity outside.  Up here in Chicago, this is a GREAT place to start economizing. Here is the chart showing the relation of A, B, C, D to temperature and humidity and for use in setting up your economizer’s set point.

Enthalpy chart

As you can see on the chart — as HUMIDITY GOES UP — TEMPERATURE  GOES DOWN (and visa-versa).  This means that the economizer will not open until the temperature is much lower when the humidity is high and the other way around when the humidity is very low. Example: As I said above  — “D” will allow the econ to open at 63 degrees 50 % humidity but when the humidity is at 90%, now the temperature has to be down to 53 degrees for the econ to open (a lot of “total heat” in the air so it opens at a lower temperature).  Conversely, if the humidity is only 10%, now the economizer will start to open at 73 degrees (less total heat in the air so it opens at a higher (temperature).   Remember, enthalpy is TOTAL HEAT and is a combination of temperature AND humidity so based on the “total heat” we make maximum use of the outside air for cooling based on these set points.

Hopefully all of this makes sense and hopefully it shows how important a proper set up is even for something  as “simple” as an economizer.

Starting in the next post, I will start to address each component and how to troubleshoot it. But before that, I felt it was important to understand how an economizer works and how to properly set it up and that is why I did these first 2 posts. The more you understand — the less likely you’ll be to “GUT” the economizer out of the system  [:>).   Stay tuned for part 3 coming soon!

About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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