Working With the Honeywell W7212 Economizer Controller; How I Came to Write This String of Posts

What you are looking at is a “breadboarded” up economizer control system on the drafting table in my office, built around a Honeywell W7212 economizer controller, which, I am happy to report, works.   I put it together because Gary, Ron and I are currently working on a project with 14  Variable Flow Refrigeration Systems (VRF), all of which have an economizer package, but none of which have a working economizer package.

When I say none of which have a working economizer package what I mean is that we deployed several data loggers on several units to assess performance over the course of  two different weeks, including a sensor that could measure actual damper movement.  And, after a week of weather that should have triggered all the basic operating modes, demonstrated that nothing moved.   Granted we did not log all of them, but since they are all supposed to be wired the same and the two we logged were selected at random, we are pretty confident that we have a problem.

More specifically, the VRF manufacturer does not directly make and market an economizer for their equipment.  But, for applications where the designer, or code requirements, or both (in our case, it’s the latter) dictate the need for an economizer, a 2nd party vendor has developed a package.  consisting of a mixing box assembly, which bolts to the back of the VRF unit along with a box of controls, by a third party (Honeywell;  this is where the W7112 comes into the picture), which need to be mounted and wired by a 4th party, and interfaced to a building wide DDC system, by a 5th party.   Then, the whole “shootin match” gets tested to make sure it works by a 6th party (that’s where Gary, Ron and I come in).

The control wiring and set up information for the assembly is communicated by a combination of a generic wiring diagram by the 2nd party, item specific product sheets by the third party, and a control shop drawing by the 5th party.  It all seems like enough information, but if you try to put together a working system from it, you run into some challenges.  For instance in this wiring diagram from the economizer package manufacturer …

… it shows a  “Clicks On” type change over sensor wired to the SO and SO+ terminals.  But our project is supposed to use an enthalpy based change-over strategy.  The diagram deals with this contingency via a note with instructs the user to Remove 620 ohm resistor when using adjustable dry bulb or enthalpy strategy.  That seems clear enough until you consider that there are two 620 ohm resistors shown;  do you remove one, or both, and if one, which one.

(I realize that the image is to small to read, but if you click on it, it will open up in a separate window in a more legible form.  Incidentally, that is generally true for any graphics in any of the posts.)

When confronted with a question like this in the field, one often turns to the manufacturers literature for the specific piece of equipment that is in question.  In this case, that would be the W7212, and you would find a 24 page document with a lot of detail and 9 wiring diagrams similar to this one …

… none of which apply to a VRF system let alone this particular make and model.  And while you might assume that the information shown for wiring a single enthalpy sensor probably applies to the VRF system, you might also notice that while all of the Honeywell diagram show wires from the W7212 back to the mechanical refrigeration system in addition to the wires from the system, these wires do not show up on the packaged equipment manufacturer’s diagram.

As a result, you may turn to the project control submittal.

Here you would discover that two pairs of wires, one from a thermostat and one from the building control system, are supposed to land on undesignated terminals in two different locations on the VRF unit.

Bottom line is that  you can probably see why the 4th party (the installer) might be a  bit confused and why nothing works.  The fact is that all of the individual elements work, they just don’t work together.  And, there is some question regarding exactly what “working together” means. 

The project documents specified a packaged solution and to a large extent, rely on the integrity of the package and code requirements to provide the desired functionality and the commissioning process to verify that.  In fact the designer was a bit skeptical of whether or not the package would work and asked that we focus some attention on it during the commissioning process. 

Complicating all of this is the reality of the field and the related “existing conditions”.  Here is a picture of one of the VRF units earlier in the construction process.

The VRF unit is the sheet metal box in the middle of the picture.  The mixing box is the insulated section of duct in the foreground.  The economizer controls have not been installed as of this point in time but you can see the factory wiring harness hanging as a little bundle of colored wires on the back of the VRF unit next to the mixing box.

In terms of height, this is one of the more accessible units on the project.  Many of them are 15-20 feet up in the air over hard ceilings.  So at this point in time (post construction and immediatly prior to occupancy) you end up working on them through an access hatch on a 12 foot ladder.  The unit in the picture above now has a very nice looking but not particularly accessible ceiling under it.  And there is a very nice 20 foot or so long conference table under it that probably weighs more than my house and will not fit through the door with out disassembly, even if you  could lift it.   Bottom line is that troubleshooting and rewiring the installed controllers is not such an easy thing.

Technically, the new construction commissioning process is supposed to be about verifying the integrated performance of the systems and equipment after they are installed and started up.  In theory the start up and verification process should take care of problems like missing sensors, improperly installed sensors, wiring errors, etc.  And while a commissioning provider may review and comment on the contractor’s proposed start-up and verification process and may even witness some of the start-ups (that’s what’s going on in the picture up above), they generally are not involved in every single one with the depth of involvement generally set by budget constraints and system complexity. 

For most providers, satisfactory completion of the start-up and verification checks and other pre-functional testing requirements will be a pre-requisite for functional testing.  That means if you are cleared by the contractor to functionally test a system and then discover that the pre-functional requirements have not been completed satisfactorily, then you check the “test failed” box and move on.   

In the case of our economizer equipped VRF units, given the lack of clear instruction regarding how the various components from the various parties were to be integrated into a working system, we were failing functional tests before we even started and were a long way from being able to test the integrated system.  

Technically, it’s usually not in the commissioning providers scope of work to troubleshoot and resolve wiring problems and the like.  And its certainly not in the commissioning provider’s scope to develop the job specific wiring diagram for a package like the economizer package I am discussing.  But most of us have done that, either in a past life with a different hat on or as a part of a retrocommissioning (existing building commissioning) process, or, because somebody needed to do something to move things along.

Getting things to move forward is the current need on this particular project.  So far everyone was wasting a lot of time discussing failed tests and whose problem it all was.  As a result, Gary and I concluded that it might actually be cost effective to buy a set of parts so I could figure out exactly how they worked, how to wire them, and then verify everything, all from the comfort of my office.  The icing on the cake was that I was about to teach a string of classes where the parts could be used as a demonstrator and hands-on lab experiment.  Thus, I was able to leverage a number of budgets to fund my little research project.

The class experiences have been particularly enlightening;  I would guess that at this point, I have probably been in front of 120-130 people with the demonstrator doing some version of a hands-on interaction  with it.  When I went into it, I thought that some of the difficulty I was having finding documentation and figuring things out was simply because I don’t work with these every day and just didn’t know where to look or who to ask. 

But the evidence suggests that I am not the only one with such difficulties.  For one thing, there are the 14 non-working systems on my project.  And, in the course of my class, I have run into several mechanics who are around this type of equipment a lot, all of whom expressed some level of the same experience I was having, and one of whom was really excited that I planed to do a string of blog posts on the topic sharing what I learned.

One way to look at that is to say its a bit alarming considering that as I have typed this, a number of major equipment manufacturer’s have cranked tens if not hundreds of packaged units off their assembly lines with this controller in it.  And, there are many states where just such a device is required by the energy codes. 

And that’s all well and good because economizers can save energy, but only if they are implemented properly and understood by the field.  And my circumstantial evidence (along with a lot of research) says neither is true.  For more examples, take a look at some of the references in the FEMP Fact Sheet titled Actions You Can Take to Reduce Cooling Costs,  particularly some of the work Pete Jacobs did.

But alarming is only one way to look at it.  As several of my students pointed out, it also represents an opportunity.  After all, the concept is valid and the individual elements appear to work.  The need is in understanding how to integrate them into a working assembly and tell others about that so they can do the same.

So there is the background behind the effort and the reason for this string of posts.  What I hope to do is share with you what I am learning as I experiment with the W7212 and understand its nuances and how it can be made to work with all of the other elements that are part of an economizer process.  So far, I have demonstrated that the equipment can be made to work and developed a wiring diagram.

Note that you can click on this image to see a larger version of it, just like the others.  But, I have also posted a .pdf of it on my Google Documents page if you want to down load it from there.

To be clear,  this represents a generic application using a two stage commercial grade cooling thermostat along with pilot lights to represent the loads/contactors that would normally control compressors, heating elements, and fans.  The notes, and to some extent, the way I drew the symbols, hopefully convey what I have learned so far, which is based on experiments, and reading through various documents and “connecting the dots”. 

The bottom line at this point is that I have been able to make the system work on my drafting table and also have found that there are a lot of subtle but significant nuances to it, which could be the keys to success or failure in terms of achieving the intent of the economizer.  One thing that seems to be abundantly clear is that implementing the simple concept of “cooling with outdoor air instead of mechanical refrigeration” can be quite complicated, especially when one of the parts in the process is a “black box” and multiple parties are involved.

My next step, after experimenting with this a bit more, will be to figure out the VRF specific wiring diagram.  Meanwhile, I will start to look at the various components and share what I have learned so far in a string of blog posts, the first of which will focus on the heart of the system, the W7212.

All of this is based on my experiments and what I have learned from them.  Please chime in if you know something that I fail to cover of feel I have missed something or misunderstood something.  But bear in mind the goal here is to share knowledge and  communicate with each other about how we can successfully use this equipment to deliverer energy savings and efficiency.  Its not to point fingers of blame, complain about products or bash designers or contractors.  There is already plenty of negative stuff floating around out there.  We’re looking to do something positive.

David Sellers

Senior Engineer – Facility Dynamics Engineering

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