Summer, I’m both sad and glad to say, is starting to wind down. The first day of fall is just a week away. Cooling season is not completely over and heating season has not begun. This in between seasons , in the commercial market place, is where an economizer really can come into play.
A system with an economizer can make use of the cooler temperatures outside to cool the space without having the need for mechanical cooling or compressors to run. If properly set up, the economizer will cool the space with just the outside air. Since the compressor is not running, and the fact that the unit is cooling the space with out that compressor, hence the name — ECONOMIZER — cooling without the cost of an operating compressor. GREAT IDEA!
Economizer are usually put on systems (1) to provide FREE cooling, (2) to meet code required outdoor air to the space (minimum position), and (3) in recent years to help maintain Indoor Air Quality (IAQ). But, the initial idea was item 1 — free cooling.
So depending on the manufacturer, there are various ways the economizer operates. The most common is on a call for cooling, the Y1 call goes back through the economizer logic module. At this point, the logic module looks at 2 things, outdoor “enthalpy” (total heat) and mixed air or discharge air temperature and decides if the economizer dampers should open or not. If the outdoor air is above enthalpy set point, mixed air or discharge air do not matter since the Y1 call to terminal 1 with now just be sent back out of terminal 2 and the compressor is turned on.
If the outdoor air is below the enthalpy set point, and the mixed air or discharge air is above its set point (usually fixed at 55 degrees F), the economizer opens and brings in fresh air to provide free cooling to the space. the Y1 signal from the thermostat stops here and mechanical cooling is not running. The mixed air or discharge air sensor now will “modulate” the economizer damper to maintain the 55 degree F temperature to the space. As long as the temperature is above 55 degrees F, the economizer dampers are wide open. If the mixed air or discharge air drops below 55 degrees F, the dampers will start to open and close to maintain the 55 degree F temperature by blending the return air with the outside air. If the mixed air or discharge air continues to drop below 55 degrees F, the outdoor dampers may close all the way even though there is still a call for cooling and it is still in an economizer mode.
Now, if the economizer cannot keep up with the load in the space, the thermostat will send the Y2 signal to the unit and that then allows a compressor to be turned on to assist the economizer with the cooling.
Here is where a lot of contractors make a BIG mistake. On a unit with a single compressor and an economizer, they install a single stage cooling thermostat. BAD IDEA! With just the Y signal coming to the unit and that going to the economizer, if the economizer cannot keep up with the load in the space — the user is “out of luck” because that is all the unit is going to do — provide economizer cooling since there is no way to turn on the mechanical cooling. So, a lot of contractors still use the single stage thermostat and jumper Y1 & Y2 together — problem solved right? NO!!! Above enthalpy set point just the compressor is going to run — that’s normal operation. However, below enthalpy set point, a Y call will open the economizer AND turn on mechanical cooling whether it needs it or not. This can lead to low pressure trips, freeze stat trips, and other lock outs. The contractor needs to consider the economizer as a stage of cooling whether the unit has 1 compressor or 2 compressors. In other words, any unit with an economizer needs, at minimum, a 2-stage thermostat to function properly.
In my next post, I am going to get into troubleshooting the economizer. Reason being — Economizers are the most mis-understood, mis-diagnosed, and most often gutted control system on commercial packaged equipment. Because of that — I’ll try to make diagnosing and fixing as simple as possible, and most importantly — how to by-pass it when it doesn’t work without gutting and cutting everything. Hope you keep reading.