How does Tracker control the economizer in HVAC units?

Issue:

How does Tracker control the economizer in HVAC units?

Resolution:

Trackers Application for HVAC Economizer Control Decision

Application

This application is designed to coordinate economizer activity for HVAC devices serving a common building. The problem this application corrects is that of several HVAC units controlling their economizers differently. This application does so by taking responsibility for the HVAC units OK to Economize decision.

A HVAC unit bases its OK to economize decision based on several factors. The primary factor is outdoor air temperature; however, there are secondary factors such as outdoor air humidity and return air conditions. The HVAC unit is programmed with a threshold for when it is OK to economize or not. The HVAC unit then compares the temperature and humidity input factors against the programmed setpoint and a decision is made to economize or not. When the HVAC unit is economizing, the temperature of the supply air is maintained by using the cold outside air as opposed to the use of mechanical refrigeration. This is a method to save energy when the outdoor air conditions are favorable.

It is usually the customers desire to have all the HVAC machines either economizing or not simultaneously. It is usually not acceptable to have one machine economizing and bringing in 100% outside air while an adjoining machine is using mechanical refrigeration. Unfortunately, even if the machines are programmed with the same OK to Economize setpoint, there is error amongst all the different sensors used by all the machines. Consequently, even though all the HVAC machines are exposed to the exact same outdoor air conditions, there is no unity in their OK to economize decision.

This application solves this problem by forcing Tracker to make the OK to Economize decision for all participating HVAC units. To do so, Tracker must have a valid outdoor air temperature sensor input.

Limitations of this application

This application is limited in several ways.

Tracker must have an outdoor air temperature sensor wired to its input. Tracker cannot use a communicated outdoor air temperature from a different source.

Tracker has only one setpoint. From that setpoint all participating HVAC units will be controlled. If a building has different internal loads between spaces, the operator will be forced to choose which outdoor setpoint satisfies all spaces.

Tracker uses only the outdoor air temperature to determine the OK to economize decision. Since Tracker does not have an input for humidity, Tracker cannot calculate the enthalpy of the outdoor air.

PC software setup fields (Note:this application is not available from the operator display)

SCC setup: Economizer Control Decision[Local, Tracker]

Default = Local

Note: This field is only available if Economizer / Ventilation Type = 2 (Modulation Economizer), otherwise this field is grayed out

Tracker setup: Economizer Enable Temperature[45F  65F]

Default = 55F

Sequence of Operation

If Economizer Control Decision at the SCC setup editor is edited to Local, or the Economizer Control Decision is set to Tracker and Tracker does not have a valid outdoor air temperature input, the SCC will be in control of its own economizer decision; however, Tracker still sends a minimum position setpoint.

If the Economizer Control Decision is edited to Tracker, and Tracker has a valid outdoor air temperature sensor input, Tracker will control the economizer as follows:

Routine runs every 1 minute or whenever Economizer Control Decision is changed.

If the outdoor air temperature is below the Economizer Enable Temperature  1C, Tracker will enable the SCCs economizer.

If the outdoor air temperature is above the Economizer Enable Temperature + 1C, Tracker will disable the SCCs economizer.

If the outdoor air temperature is within 1C (the deadband) of the Economizer Enable Temperature, Tracker will leave the SCCs economizer in whatever state it was in previously.(Either Enabled or Disabled)

Added Notes:

When a ZN517 is set to use Tracker Economizer control, it will ignore its own outside Air Temperature Sensor--it will function even without an OA Temp sensor.

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Notes:

Changes:

Other Question/Problem/Cause Statements:

• How does Tracker control the economizer in HVAC units?
• Does Tracker need it's own outdoor air temperature sensor for economizing?
• Can you control Reliatel Economizer from Tracker 12?

Facts:

• TRACKER BMTK
• Tracker BMTK

Original wave: wave13701

Document Identifier: DOC-86560

Document Identifier: 86560