Working with the Honeywell W7212 Economizer Controller; Nuances of the Economizer Control Process, Part 1 –Background

 

This post is the second in a series I started a while back  on the Honeywell W7212 controller.  I recently did a lab session that was built around the controller and having had questions from several of the attendees who have been thinking through the nuances of the controller and its application, I thought it might be helpful if I answered the questions via a string of blog posts. 

The Scenario

To set this up, I am going to describe the lab that triggered all of the questions because it paints the background for them and will  place them in context. 

The scenario behind the lab is one that we are running into out in the field right now, especially in California where Title 24 requires economizers on equipment that might otherwise not be provided with one in other states.   Specifically, the  lab revolves around a system with a VRF evaporator fan coil coil that is large enough to require an economizer. 

As a result, the designer specified that the VRF units be provided with an economizer. But, since the VRF vendor does not make an economizer, the contractor had to turn to a third party to provide the economizer package.

There are a number of vendors that can do just that.  But  their package includes not just a mixing plenum and dampers.   It includes the parts for a control system, in this case the Honeywell W7212 system, most of which is shipped loose.

So in theory, everything you need to make a working economizer is there (and in the case of my first exposure to this problem, a few things you didn’t need were also there).  The problem is that there is no one document provided that shows specifically which parts to use, where the sensors should go, and how everything should be interconnected to make a working system.

An Opinion

In my opinion, the designer should have  provided the document that showed all of the parts interconnected into a working system.  After all, it is there seal on the drawing and they specified the various parts.  Plus, if they don’t do that,  they are missing out on one of the most important (and also fun) parts of the design process, that being the development of the control system design.

Here is the diagram I created to wire up the system where I ran into this issue, which triggered the first blog in this series.

There is a .pdf of the diagram on my Google Drive if you want a closer look (you can zoom in on the small font for instance). 

Plan B if You Beg to Differ

I guess at its core, my opinion is that as the engineer of record, you need to own the control system design.  I personally take great pleasure in doing that by developing the necessary wiring diagrams, piping diagrams, logic diagrams, point list, system diagrams and other documentation necessary to deliver a working system to my clients.  

But some may not feel as  inclined or even comfortable with those details, and I get that.   But, I think that means you still need to be sure the base is covered by:

• Clearly stating how the integrated system is to work, and
• Providing the contracting language necessary to ensure that some entity involved with the project is responsible for preparing the documents I mentioned above in a project specific format, exactly matching the equipment supplied for the project, and
• Enforcing your requirements.

The Lab Exercise

Generally speaking, air side economizers are one of those things that are simple in concept by fairly complex when it comes to implementation, which is why I think there are so many questions that come up once you start to think about it.  I try to set that up in the lab by giving the students a pile of parts and asking them to imagine that they are Fred and Peggy Sue, an imaginary team of service electricians charged with wiring it all up.

To some extent, I am just trying to let the lab attendees “walk a mile” in Fred and Peggy Sue’s shoes, which I think can be an important perspective to have if you really want to solve a problem.  What Fred, Peggy Sue, and the lab attendees quickly discover is that:

• In the documentation provided, there is no one piece of paper that shows how to wire it all up.
• In the documentation that is provided, there is actually conflicting documentation regarding how to wire things up.
• There are more parts than you need but no guidance on how to pick the right one.
• There is limited information on where various sensors should be installed.
• There is nothing that explains what an economizer is, let alone an integrated or non-integrated economizer.
• Time constraints (for Fred and Peggy Sue, the amount of time their boss gave them to do the work and for the lab  attendees, the time we have for the lab session) will likely force decisions that may or may not be the right ones, all in the interest of getting something installed and wired up and moving on.

So that is the background and foundation for the discussion that occurred in the lab, which triggered the questions and discussion that will follow in the blog posts that are next in this series.

 

David Sellers
Senior Engineer – Facility Dynamics Engineering


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