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4 min read

Learn Series: How Sequenced Control Works

By Phil Zito on Aug 31, 2016 6:00:01 AM

Welcome to the third Learn Series article! Man I did not expect the response I got from this series.

Thank you so much for everyone who has been e-mailing me telling me that this series is helping them unpack concepts they've been struggling with! SBA Nation you all are friggin awesome!

Well enough of the emotional stuff, let's get down to business!

In today's article I will be discussing

  • How Sequenced Control Works

Sequenced control is the next step on our control mode journey. As you may recall I kicked this series off with a bang covering PID loops.

In the first several articles of this series I am going to take us through a journey discussing the control modes. Control modes are the different processes that a BAS can use to control systems. So far we've covered PID loops. The control modes are:

In this episode we will discuss sequencers. You are going to learn:

  1. What sequencers are
  2. How they work
  3. When they should be used

You ready, let's roll!

What are Sequencers

Sequencers, what are they?

Sequencers exist to sequence devices on and off. Have you ever looked at your home thermostat and it said something like "Cooling Stage 2 On". Well my friends you are looking at a sequencer in action.

Sequencers take a control signal and "stage" on and off devices. There are two types of sequencers, logical and physical.

Logical Sequencers exist in software. They look at a software point and based on that point they turn a BAS controllers physical inputs and outputs on and off, they "sequence" them.

Physical Sequencers are a physical control board that takes a voltage signal and commands a series of outputs on and off. Typically this board will have a set of dip switches or potentiometers that allow the technician to adjust the on and off timers for each stage.

Speaking of stages let's talk through how a sequencer works..

How Sequenced Control Works

When a sequencer controls something it is often said that the device is being "sequenced". Sequenced control is much more common than you would think. I find that folks often times associate sequencers to electrical heating or direct-expansion (DX) cooling stages.

But if you've worked in the BAS world for any amount of time you will know that sequenced control is used for other control scenarios like central plant staging.

So how are devices sequenced? A sequencer, whether it is logical or physical, will turn on and off devices. The sequencer looks at an input signal, usually 0-10VDC (Physical) or 0-100% (Logical) and then based on the signal it turns on and off stages.

Now you couldn't just have the sequencer turning on and off stages willy-nilly, that would be very bad for devices. Because of this the sequencer has a concept called minimum on and minimum off timers.

The minimum timers will state that an output must stay on for a certain period of time and then must stay off for a certain period of time. Imagine the chaos if a chiller was repeatedly turned on and off because an output kept drifting between 49% and 51%...

Now you are going to see why I showed you PID loops first. So, let's say your PID loop for discharge air control is putting out a 50% signal. This signal could then feed into a sequencer block (logical) that then commands your 3-stage (3 separate outputs) electrical heat on and off.

When Should Sequencers be Used?

I got asked recently on LinkedIn under what scenarios should PID loops be used. That question made me realize that I didn't address when each individual control loop should be used. In order to avoid that mistake I wanted to clarify when sequencers are a good fit.

Here is my tried and true rule for when to use a sequencer and it has served me well throughout the years.

Sequencers should be used, when you have multiple, binary devices to control that are all part of the same system.

Let's unpack that.

Multiple is quite simple it means more than one.

Binary means the devices have two modes, and only two modes, on or off. Electric heat, binary, heating valve... not quite.

All part of the same system, you don't want to sequence two different heating systems. You do want to sequence 3 chillers that are part of the same central plant.

Got it?

Make sense?


So, now you know how sequencers work. Next time you're standing on a rooftop looking at a storm wall coming at you while the owner of the building tells you "Your not getting off this roof until you get my cooling to work" you'll understand how staged cooling works...

I hope you are enjoying this series. My mission at BAM is to help 20,000 new BAS professionals enter our field by 2025 and this series is one the ways I am addressing that goal. In next weeks learn series I will discuss how to set up and program logical sequencers.



Please me know your thoughts on this article, and any questions you may have, in the comment section below.

Phil Zito

Written by Phil Zito

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