Economizers are one of the most misunderstood, misdiagnosed, and most often “gutted” control systems on a HVAC unit. Yet, when you understand them, they really are a simple control system.
There are 3 basic reasons for using economizers.
- They provide free cooling when the outdoor temperature is capable of cooling a space. It costs less to bring in fresh air than to run a compressor — hence the name economizer
- They provide fresh air to the space to meet code requirements. You can set the minimum position on the economizer to provide the code required fresh air.
- They now have capabilities to utilize IAQ (indoor air quality) sensors to control contaminants in the air in the space.
Basically, an economizer has 2 sensors, an outdoor enthalpy (temperature and humidity) or dry bulb (outdoor temperature only) and a discharge or mixed air sensor. Economizers only function when the indoor blower is energized (no sense trying to bring in fresh air if the blower isn’t running) and the “G” from the thermostat is what “powers” the logic module.
When a call for cooling comes in, the “Y1” is directed to the economizer logic module where it looks at the enthalpy sensor and decides if the outdoor air is suitable for cooling. If it is, the economizer opens, bringing in the cool outside air and locking out the compressor. Mechanical cooling can be brought on with a Y2 call from the thermostat if the economizer cannot keep the space cool enough. If the outdoor air is not suitable for cooling, the logic module sends the Y1 call back to the main control and energizes the compressor.
A limiting sensor is provided to keep the air from being too cold going into the space. That sensor is the discharge or mixed air sensor. It’s function is, whenever the unit is in “free cooling” with the economizer, the sensor maintains a discharge or mixed air temperature of 55 degrees. It does this by modulating the return air and outdoor air dampers to blend the air to maintain the 55 degrees.
This is a very simple explanation of how economizers function. Trouble shooting, is really just as easy. You need to check the enthalpy sensor and the discharge air sensor to make sure they are reading correctly. You need to check for power at TR on the logic module (G from the thermostat come in here). You need to check to see if there is a call for cooling at the module terminal 1. If you have a VOM meter, some jumpers, and a little patience, you can diagnose and repair any economizer out there.
I know this seem pretty simplistic, but, if you take your time; use your meters; you will find out how simple they really are to diagnose and repair.
Attached is a quick reference diagnostic for troubleshooting economizers. Hopefully, this will help you fix that control system instead of “gutting it”.