If you have been following my last two posts, by now you should understand what an economizer is; how it works; and how to properly set it up. Now, as promised, we’ll get into troubleshooting the economizer and its various components. And, if I am successful, after learning all of this, economizers will cease being the most “gutted” control system on an HVAC unit.
Let’s start with something simple and work our way up to the logic module itself. Let’s look at the mixed air/discharge air sensor. Depending on the manufacturer, the economizer will have a sensor attached to T & T1 on the logic module. the purpose of the sensor is to maintain a 55 degree discharge or mixed air into the space. I prefer systems with discharge air sensor because they also take into consideration DX cooling. With a discharge air sensor, the outside air + DX cooling is maintained at 55 degrees. With a mixed air sensor, 55 degrees is maintained onto the coil, but if mechanical cooling is on, the air into the space can be well below 55 degrees creating uncomfortable areas in the conditioned space.
Regardless of where the sensor is located, this is probably the simplest component to check out because you can do it with a simple ohm meter.
Attach your ohm meter to the 2 wires on the sensor and, using the chart, you can see if the sensor is within calibration and reading correctly. If it is “off the chart” or way out of calibration compared to the actual temperature, then you need to change the sensor. This is one component that rarely fails.
Next, we’ll look at the enthalpy sensor. This is attached to So and S+ on the logic module. This sensor IS POLARITY CONSCIOUS … + on the sensor must go to + on the module. To properly check out the sensor, you need to have a meter that reads milliamps (mA). To read MILLIAMPS on a meter, the meter is placed in series on one of the connections.
- Jumper R to G on the unit control board. This will provide 24 volts to TR & TR1 on the logic module.
- Measure the temperature and humidity where the enthalpy sensor is located with a psychrometer to give you a base line to compare the reading to the chart.
- Insert a DC ammeter in series with either SO or SR, depending on which sensor is being checked.
- Make sure to connect the positive lead from the meter to S on the sensor and the negative lead to either the wire or S on the control
- Meter should indicate a reading between 4 to 20 mA if the sensor is operating properly
- If the meter indicates “0”, the sensor may be wired backwards.
- If the reading is out of the 4 to 20 mA range — replace the sensor.
These are the two basic components attached to an economizer module. These control the opening and closing of the dampers based on discharge temperature and outdoor air being suitable for providing “free cooling”.
The next step in economizer trouble shooting is making sure all the proper voltages are present where they are suppose to be on the module.
- power at TR & TR1. This should have 24 VAC any time the blower is running. No sense powering an economizer if it can’t deliver air to the space so the blower (G) circuit is what is used to power the economizer.
- TR is the 24 VAC “hot” and TR1 is the 24 VAC common. The module is POLARITY CONSCIOUS.
- Next we need power at TR1 (C) and “N”. This is the “occupied/unoccupied” input to the module. In the unoccupied mode (no power to N) the minimum position pot is disabled. The economizer will still work on a call for cooling but you cannot set the minimum position without power to “N”. This usually comes from an “occupied” command from a thermostat or time clock.
- Next. depending on the manufacturer, we look at terminals 1 & 2 and then possibly, 3&4. Some manufacturers only route Y1 through the module. Others route both Y1 & Y2 through the module.
- Terminal 1 is nothing more than Y1 into the module. This tells the module to look to see if the outside air is suitable for free cooling. If it is — the “free cooling” LED will be on and the unit should be in an economizer mode. If it is above enthalpy set point, the free cooling LED will be off and 24 VAC will be present at TR1 and terminal 2 sending the Y1 call back to the main control to turn on mechanical cooling. (It really is that simple — TERMINAL 1 = Y1 IN AND TERMINAL 2 = Y1 OUT.)
- If your manufacturer uses this, Terminal 3 is Y2 IN. Here is where the “logic module” earns it name. If Y1 is in an economizer mode and the Y2 call comes in on terminal 3, the module switches the Y2 input to terminal 2 (output) to energize FIRST STAGE of cooling. If the economizer is not in a free cooling mode, then the cooling calls are Terminal 1 = Y1 in, Terminal 2 = Y1 out. Terminal 3 = Y2 in and Terminal 4 = Y2 out. It is that simple. Only in an economizer mode does Terminal 3 — Y2 in leave the module as Y1 OUT.
In my next post, I will attempt to show how to diagnose the module itself and the actuators. I figure, for now, this is probably a lot to absorb so I’ll leave it here and, hopefully, finish this subject with my next post — STAY TUNED!