We Don’t Need No Stinkin’ Sequences of Operation

Sequences of Operation papersSome people would argue that Sequences of Operation are the most important thing to know when programming a controls system. They would say knowing what the engineer’s intent for the use of the system or piece of equipment and the capabilities of the unit are key to providing an efficient and capable controls system. They even think you need some sort of manufacturer’s documentation available for the equipment being controlled.

Those people have no spirit of adventure!

The truth is… THEY are absolutely correct and most of the time I happen to be one of THEM. It makes all too much sense that you have to have an idea of what the equipment was designed and made to do. Then it is up to you as the controls guy to figure out any better ways to accomplish the same effect and add in atleast one of these things:

  • Added comfort
  • Efficiency and Energy Savings
  • Lower Maintenance Costs
  • Cool Factor (yeah really)
This is a continuation of my Liebert Mini Mate Retrofit. If you haven’t watched it I would check it out first.

We Got Comfortable With The COOL Factor


In our little Liebert Mini Mate retrofit we figured we hit on two of the above bullet points; Added Comfort and Cool Factor.

Comfort was accomplished to the delight of our 2 servers and phone system tower because they had been enduring 90°F room temps in the closet since the Liebert system had been off due to mechanical issues of a refrigerant leak.

Needless to say having the A/C running and doing what we wanted it to by programming it ourselves added some cooling comfort.

I mean c’mon… we’re a controls integrator and were running our unit off a stand alone thermostat. What!

The Cool Factor wasn’t about temperature though, it was that we could now play with the unit and get email alarms if we wanted to and not have to hear the stupid Liebert Thermostat blaring beeps every time the compressor short cycles because it’s getting too cold, too fast.

So now that we added a Delta Controls DAC-606, we now have it on our Building Automation System and can take a look at it from any computer… <insert movie trailer guy’s voice> … a n y w h e r e  in  the  W O R L D!

It’s supposed to make cold air, DUH.

So I knew very little about any sequences to this unit and most of the time neither will you if you are walking up on an existing unit of a small project where no engineers are involved. This is why i say most of the time I want sequences, but… I’ve done this long enough to get the gist of mechanical systems and units like this one and what *I* want it to do. But what should you do if you don’t?

Liebert-Mini-Mate2-Installation-Operation-and-Maintenance-Manual-1-1.5-Tons-50-60HzGOOGLE IT! So for this article, I googled ‘liebert mini mate 2 operation manual‘ and as you can see there are plenty of options right on the first page. You can do this with any kind of HVAC unit out there, though sometimes it helps to have the first few characters of the model of unit and not just a broad manufacturer and model name.

When looking for manuals always go for the PDF results as those are usually the official manuals offered up on websites and such.

Here is the Manual I chose which was 2nd on the list: Liebert Mini-Mate2™ Installation, Operation and Maintenance Manual – 1-1.5 Tons, 50 & 60Hz

If you look in Section 4 on page 37, you can read what the manufacturer has decided the various Sequence of Ops should be.

Ol’ Frank Sinatra would be proud. “…and I did it MY way.”

So being that we weren’t too concerned with energy savings for a server closet that housed our company’s data and the humidity levels weren’t much of a concern since the cold air pumping in to such a small area kept it within tolerance… I did it MY way.

Delta Controls DAC-606 Application Controller

It was getting late and we just wanted to get the sucker to run. In the last retrofit video you heard me mention I was only interested in the fan and compressor. With those two outputs we could get the unit cooling and running.

My little sequence was nothing more than utilizing the following inputs and outputs:

  • Space Temperature (Input)
  • Supply Air Temperature (Input)
  • Supply Fan Status (Input)
  • Supply Fan (Output)
  • Compressor or Cooling Stage 1 (Output)

To accomplish this I/O, I used the following products:

This is how I Dooze it…

With those three parts we can control the simplified, let’s-hurry-up-and-finish-so-I-can-go-home-and-eat-something version. I utilized a database (Delta’s controller database file contains all the I/O, variables and programs to make a unit work) that I had already written for use in any kind of package unit, heat pump or fan coil that had a set number of digital or analog cooling/heating stages/valves.

I promise to show you in detail with a look over my shoulder of how that looks like in a future video.

PID Loop Control - Essentially, the sequence of operation is to utilize a proportional only PID Loop looking at space temp and space temp set point to change the Unit Mode to Heating, Cooling or Deadband. Deadband, meaning it is satisfied and the unit is waiting in the OFF position.

Once the PID Loop hits 100% I change the Unit Mode to Cooling. The Proportional Band is set really wide so that the unit will have longer cycle times, but the temperature will have a wide swing as well. looking back on the trend logs we saw the space temp would go from 68° to 78° which is exactly the 10° ΔT (differential temperature) I programmed.

Supply Fan Control – When the Unit Mode is in any mode other than Deadband the Fan is started.

Cooling Stage 1 Control – First off I always build in a safety to not turn on a compressor unless fan status is proven and that is where the current switch comes in. Once the fan status has been proven I allow the compressor to be turned on based on the PID Loop.

Since the programming is setup for the possibility that the unit could have heat I split the percentage numbers of the PID Loop so that at 100% turn on cooling and at 50% turn it off. If the unit had a heat stage then it would come on when the PID Loop hit 0% and back off at 50%. These percentages only work if they are in the proper Unit Mode (cooling/heating).


When controlling DX compressors on an A/C unit, you MUST make sure your sequence/programming does not allow the compressor to start more then 6 times per hour.

This is a standard practice, just ask any mechanical or HVAC/R equipment guy and he will tell ya how frustrating it is that greenhorn Controls Guys don’t know how to control a compressor properly.
BAM! Now go show em what ya know.

If you don’t have any questions, you’re not thinkin’ hard enough…

I have just barely scratched the tip of the iceberg and have tried to keep everything very general and easy to understand. If you have questions or are confused about what I have laid out. Let me know.

Take a second to write your comments below and myself or any one of the experienced engineers and controls technicians lurking about will be sure to feed you with food for thought.

…you won’t do it…

11 Responses to We Don’t Need No Stinkin’ Sequences of Operation

  1. tim February 7, 2012 at 15:58


    I just found your blog recently and I think it’s great. I’m a red seal HVAC/R tech with about 5 years working with delta controls. It’s nice to see some others who are excited about this kind of work. A while ago, we tore the guts out of a large carrier rooftop and used a 1212 (there are a lot of possible points in those things) to control it and tie into the automation. It seems to be working a lot better since it can be run and staged to work with the building.


  2. BACnet February 8, 2012 at 16:44

    I have seen companies take one of two approaches to creating sequence of operation documents for a system they are installing/updating.

    The first type of company is the lazy-approach company. They build the system, get it working and debugged and then they write up documents that explain the sequence of operation. In theory this isn’t the worst approach in the world, but the impetus to complete the drawings and to make them accurate is lacking. It’s not uncommon for the person doing the documentation to get called onto another task before it’s done and we all know the likelihood that they will ever get completed drops off very quickly as the weeks and months pass.

    The second type of company is the professional company. They create the sequence of operation documents *at the start* of the project. Then these documents are what are given to the programmers to code into the controllers and the front end. Along the way if a change needs to be made, it is made first to the SOO documents, then put into the controllers.

    Using the second approach you are always guaranteed to have good documentation and it actually makes the job of implementing the system easier.


    • The Controls Freak February 8, 2012 at 19:15

      Awesome comment BACnet… I need to convince you to Guest Post something now and again. You have some great information to share.

      This little retrofit definitely falls into the first example. All on the fly and used as a training tool for Cody who hasn’t seen small equipment like the Liebert Mini Mate.

      As for projects I have done for clients we either utilize the engineer’s sequence of operations they provide us or we are the one’s writing the sequence of operation for the client in cases where a customer selected engineer is not a part of the project.

  3. Scott February 8, 2012 at 19:58

    I like your blog/site. Very cool and informative stuff.
    I agree that a decent Seq. of Ops. is important up-front so the tech/programmer knows what the systems should do and how to create the local code and global database.
    In the end, the most accurate and professional Seq. of Ops. are the result of “marked-up” submittals that are turned into as-builts by an engineer (or a capable tech). A marked-up set of submittals with tatered and worn pages from a thorough tech are GOLD!
    I’ve been looking for as blog like this. Good to see others passionate about “well constructed” HVAC control systems. I’ve seen the good, the bad and the ugly when it comes to control systems. Take care.

    • The Controls Freak February 11, 2012 at 15:09

      I know what ya mean by the whole marked up and tattered submittals. Then again, in a perfect world those drawings actually make their way back to the office and are transposed on the digital drawings by the controls’ engineering department.

      Then there is an actual record that can persist and not be susceptible to being lost or having coffee spilled on it. ;)

  4. Ryan February 11, 2012 at 05:03

    Did your new sequence have any alarms in it? What are you using the supply air sensor for? Will the compressor still lockout in case of high head pressure?

    • The Controls Freak February 11, 2012 at 15:53

      As for alarms, we did make sure we had some basic alarms such as the fan status and space temperature. But being this little retrofit was for our own use and on-the-fly we didn’t get too detailed, but here are some typical things you would want alarms on:

      • Supply Air Temperature out of range
      • Space Temperature out of range
      • Supply Fan Status
      • Compressor Status

      A supply air sensor isn’t necessary, but we try to include that on almost all units and equipment. It can be used as a sort of status for sub-systems such as a chilled water coil that has a valve actuator with feedback. The feedback proves the valve is truly in the open position, but it doesn’t mean there is cold air being supplied. The cutoff valve could have been left closed and the air the unit is supplying is at 75°F.

      High head pressure… This is a good one. With our HVAC Controls, we don’t try to detect issues such as that. That would fall under the scope of the unit manufacturer or maybe the mechanical contractor who supplied the unit. In this case, that would be something Liebert would have provided, built-in safeties or cutoff switches to prevent further damage to the equipment.

  5. Courtney February 14, 2012 at 05:47

    Good article. In situations where we are controlling compressors, we use the minimum on/off time setting to prevent cycling. Then, even if we have a mistake in the programming, we are protected.

    If you have extra money, you can purchase the Liebert Modbus/BACnet card and bring in all control points and alarms on a Delta system or any other Modbus/Bacnet System.

  6. A.F. February 15, 2012 at 15:53

    Is the retrofit completed now? Is there a follow up video to the first one?

    • The Controls Freak February 18, 2012 at 20:10

      Yes I still have one, maybe two more videos discussing the retrofit. The next one will show a little about how we wired up the new DDC controller to the existing relays and contactor.

  7. Paul Warner March 12, 2012 at 13:49

    I think there is one factor you left out…making the system fool (read operator) proof. You think of every possible way the operators of the equipment can screw something up and they always manage to find a new way.


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