Episode Description:
Are HVAC control sequences leaving you puzzled? Whether you’re a technician, programmer, or designer in the building automation industry, this episode is your starting point for mastering the critical systems that shape modern smart buildings. Join us for Part 1 of our eight-part deep dive into HVAC control sequences, focusing on one of the most common systems you'll encounter: VAV boxes. This is the knowledge you need to solve issues faster, design smarter, and build your confidence with one of the most misunderstood aspects of BAS.
Key Topics Covered:
- VAV Boxes Demystified: Discover the core principles behind variable air volume (VAV) boxes and their essential role in zone temperature control.
- Troubleshooting Made Clear: Learn why most BAS issues occur at the box level and how mastering this can maximize efficiency across systems.
- Control Loops Simplified: Unpack the unique “loop within a loop” concept for VAV box control, essential for effective programming and operation.
- Transition to Reheat: Explore the differences between cooling-only boxes and those with reheat, including key considerations for setpoints and sensor placement.
- Fan-Powered Box Insights: Understand the contrasting roles of series and parallel fan-powered boxes and when to use each for optimal performance.
This episode is packed with practical insights and concepts that will change the way you approach HVAC systems. If you're looking to deepen your expertise and stay ahead in the field, this series is for you.
Click here to download or listen to this episode now.
Resources mentioned in this episode
- Free Technical Skill Assessments
- Smart Buildings Academy Workforce Development Program
- Smart Buildings Academy Courses
- Upcoming November Facilities Webinar!
Podcast Video
Transcript
Phil Zito 0:00
This is the smart buildings Academy podcast with Phil Zito episode 471 Hey folks, Phil Zito here and welcome to episode 471 of the smart buildings Academy podcast. In this episode, we are going to be talking about something that I found to be particularly interesting, which is HVAC control sequences. So you all are in for a treat. We have a eight part series that's going to be released over the next several weeks. And this eight part series is on HVAC control sequences. So if you do not know HVAC, if you do not know control sequences. If you do not know H fact control sequences, this is going to be the series for you. Everything will be available at podcast smart buildings academy.com forward slash 471 once again, this podcast dot smart buildings academy.com forward slash 471 all right now, before we start the episode, depending on when you're listening to this, it's that time of the year. It is the one time of the year when we have the smart buildings, Academy, Black Friday extravaganza. Things are going to be a little different this year. First off, we're going to have three discount codes. We're gonna have discount code one, which is Black Friday 40, all caps Black Friday 40, 40% off our self paced courses. We're gonna have the code Niagara 25 we've partnered with think tech to bring the self paced TCP certification and bundle it with our training courses. So 25% off those courses using coupon code Niagara 25 and then we have our workforce development program, 30% off this with coupon code workforce 30 you can only get these discounts through our E commerce, which is available at courses that smart buildings, academy.com Once again, that's courses dot smart buildings, academy.com now here is the catch. These coupon codes are only good Friday through Monday. And there are limits. There are only 200 of the Black Friday, 40 coupon codes, 100 of the Niagara 25 codes, and 50 of the workforce, 30 codes. These are the best discounts you'll ever get on our training programs. They're available for only these four days, first come, first serve, once these discount codes are used up, or once Cyber Monday is over, this sale is over. So do not miss out on this opportunity. Go to courses, dot smart buildings, academy.com, this Friday and use coupon code Black Friday. 40 for 40% off our self paced courses Niagara. 25 for 25% off our Niagara certification bundles, or workforce 30 for 30% off our workforce development program. All right, folks, with that being said, let's get into the episode.
Phil Zito 3:22
So what we're going to do is I'm going to read through some control sequences, and I'm going to provide you my thoughts as we go through these sequences, so that you can better understand what exactly we should be doing when we have control sequences as building automation professionals. Now trust me, this will not be near as boring as it sounds, because when I thought to myself, I'm going to be reading control sequences. How in the world is that going to be at all entertaining? Because if anyone's ever read control sequences before, they're pretty freaking dry. But what we're going to be doing is, I'm not gonna just be reading these word for word. We're actually gonna be talking about what's going on and the concepts that are going on, and just thoughts and things that I've picked up along the way. All right, so in this episode of this multi part series, as I mentioned, we're gonna start going through sequences, and what sequences Should we start with? Well, why don't we start with the most common system we are going to interact with as a BAS professional, and that is vav boxes, also known as variable air volume boxes, because guess what? They vary the air volume going into the space. If we think about, how do we control a space, right? One of the questions we ask, and I'm giving you all a freebie here, on our skill assessment, as well as on our tests, that several people get wrong, surprisingly, is what is the most important. Variable in zone control, and we say, is it temperature, is it humidity? Is it CO two, or is it air flow? And a lot of folks will ask temperature, or they'll select temperature, and I get that. I mean, after all, with a VAV box, you're controlling the temperature, but really, what are you controlling to you're actually controlling air flow, because at the end of the day, nothing's going to happen if you don't have the right air in that box, going out the box into the space at the end of the day. Most of the VAV boxes, with the exception of reheat boxes, they really can't do anything to control zone temperature, or rather, air temperature, is what I mean to say. They could control zone temperature, but they are dependent on the air being conditioned by an air handling unit. So when we are talking about zone control, and we're talking about VAV Units, we're really talking about modulating the air flow that is going into the space so that we can modulate the air changes. And air changes are the amount of volume that is changed in a space, typically in an hour. So we'll say air changes per hour. And usually that's like four to six for average spaces. And when you move into the healthcare environment, you can go up to 10 to 20 air changes an hour. All right, so what does all of this have to do with boxes? And what do these things matter to us? Well, see, here's the deal, as a technician, as a programmer, as a designer, as a implementer, you are going to be spending most of your time on boxes. That is where most of the issues are going to occur, because here's the deal, if an issue occurs with an air handler or an issue occurs with a central utility plant, it's pretty easy to isolate. It's pretty easy to figure it out and fix it. If you have a halfway decent HVAC knowledge and you understand how to program the issue is, is that oftentimes with boxes, we're looking at a combination of physical install. We're looking at a combination of control points and programming, all of which in themselves, are not very difficult. I'm not at all making it sound like programming a box or installing a box or commissioning a box is hard. It should not be hard. It should be the easiest thing you do. The problem is, is when there's hundreds of boxes, and that is where issues tend to abound, is when a box, or whoever's installing the controls on the box or wiring it up, or creating the program, they misread the sequence, or they don't really think about what they're doing, because they're on autopilot, and you run into mass scale issues. Now, granted, these can be fixed, but when you have to go to each box one on one, that can be a problem. So we're going to focus in on boxes, and we're going to focus in on a lot of concepts that come from boxes, purely because this is where you're going to spend most of your time, and if you get efficient in this area, you will realize the biggest gains. Getting more efficient on an air handler or central utility plant is usually only going to benefit you on a handful of systems per site, whereas getting more efficient on troubleshooting, understanding and installing VAVS and fan powered boxes is typically going to have a much larger effect. So vav boxes, a individual VAV box could typically controls to a zone, temperature set point. So you have this thing called a process variable, which is going to be the variable you're controlling in a process. A process being a loop of where you have a input and you have a set point, and those are compared utilizing typically a PID loop, which then drives some sort of output. Now, what's unique about vav boxes, unlike fan control and cooling valve and reheat valve control, is that with vav boxes, we are often utilizing a loop within a loop. So typically what will happen is we will have a minimum cooling CFM set point and a maximum cooling CFM set point. And so our zone temperature and zone set point are going to be compared, and that's going to drive a direct acting PID loop. So as zone temperature goes above zone set point, the output of the loop is going to increase as zone temperature goes below zone set point, the output of the loop is going to decrease. That naturally is going to go into a reset block within the programming and that reset block typically will scale between the min and max CFM. So as that zero to 100% output of that first PID Loop increases, that is going to guess what? Change a CFM set point. Okay, so as you change that CFM set point, then you have another process variable, and that process variable is the flow setting, and that could either be an external flow sensor or could be one built into the field controller. So now you've got two PID loops. First. Which is driving to zone temp, and the second of which is driving to your CFM set point. And only then can you drive to your damper actuator. So you will then drive a damper actuator, typically a round actuator that's on a little turn wheel, or rather a little turn shaft, and you're going to mount a actuator, or a controller actuator, to that shaft on the box, and open and close that damper to control to set point. So it's pretty straightforward sequence, but you can see where it starts to flow. Throw a lot of people for a loop, if they're brand new, because it's a loop within a loop, and that's something that we really don't see too much outside of vav boxes. Most of the rest of our control sequences are going to be a single, dedicated loop per process. So when we are controlling a cooling only box, all we can do is open to minimum flow or maximum flow, and in between to control the zone temperature. As zone temperature reaches set point, the damper will go to its minimum position. I want you to understand that wording, because that's really critical, and it causes a lot of issues when people don't properly understand it minimum position, meaning not zero, but minimum position. Okay, here's the deal we are driving to a minimum CFM set point, not a zero set point. You should typically not have zero CFM as your minimum set point on a VAV box. The reason of which is that as you go and turn on your boxes, and let's say all the boxes are satisfied, and you turn on the fan. You don't want a ton of air slamming at dampers that are all at 0% and then you're tripping your high static safety on your air handler before the boxes can even respond. By having a minimum flow position you are then allowing one airflow to get into the space, so that you're not tripping your high statics. But you're also better able to meet the standards of ASHRAE 62.1 which require you to have a certain percentage of fresh air within spaces in order for those spaces to be occupied. Now, how do you do that? You do that by having a minimum CFM flow. That's why you will often see areas that are not consistently occupied being set to zero CFM and areas that are consistently occupied being set to a minimum CFM. However, my recommendation is usually to set a minimum position for all actuators. You know, it's it's hit or miss, whether you do setting unoccupied spaces to zero and then occupied spaces to a minimum, or if you set everything to a minimum, I prefer setting everything to a minimum, because that way I don't have to worry about it. It's one less thing I have to troubleshoot. So from there, from cooling only boxes, we make the leap to boxes with reheat. So cooling only boxes typically are within the internal parts of a building. So we're talking not the external parts of the building. But actually, I should be more clear, because external and internal parts, you may be like, what Phil you're saying a box is outside the building? No, no, no. What I'm saying is inside the core of the building. You have, it's kind of hard to describe on a podcast, but let's imagine a bullseye, right? A big bullseye. We've all seen bullseyes, right? We know what a bullseye looks like. So you have that little circle in the middle that would be the internal aspect of the building right. And inside there we're gonna have cooling only boxes. And these boxes are going to be cooling only because the theory is that properly utilized, you are naturally going to heat up inside the building with latent, sensible loads and all of that fun psychrometric stuff. And then the reheat boxes are going to line the perimeter, and their sole purpose is to keep, you know, the the BTUs, keep the heat from escaping the building. And so what you'll do is you'll warm the exterior zones often, because there's thermal losses on the exterior zones. So now, as we move to a VAV box with electric reheat, or with hot water reheat, or with a silicone rectifier reheat, which is also known as SCR reheat, which is basically like hot water reheat in how it's controlled, except for your sequencing electrical stages instead of going and controlling a hot water valve. But let's talk about vav boxes with electric reheat. So vav boxes with electric reheat are still going to have zone temperature. You're still going to have zone set point, but now what you're introducing, typically, is discharge. Air, especially if you are wanting to make sure that your heating is working, that's the primary purpose of discharge air. By the way, a lot of folks will initially think discharge air temperatures exist in boxes for controlling purposes. Typically, they exist in boxes for high limit safeties as well as to detect that the heating is working, you will still want to drive your heating loop based off of your zone temperature. So what you'll have, right? You'll have your zone temperature, and it will drive between min and max cooling CFM set points when it's In cooling mode, and when it drops below a certain threshold, it will start to go from its minimum position to its heating position. Okay, so you're gonna go from minimum cooling to heating position, heating CFM position, and then, and only then, when that is achieved, will you go and start to stage on your electric heat. So how I typically like to do this is I like to use a single PID loop that is controlling to temperature, and then I will have zero to 50% of that loop, right? So this is a reverse acting loop. Zero to 50% of that loop will go and it will drive my minimum to Mac to heating CFM, and it'll drive that loop. It'll drive a PID loop for min to heating CFM. So I'd have zero to 50 coming out of my zone temperature loop, and that would be driving a zero to 50, or that would be driving a zero to 100% output, and I would reset, and I would say, okay, that zero to 50 is going to reset from min to heating CFM set point. Then from 50 to 100 I would take that and I would put that into a sequencer block, and I would stage my electric heat. Now, if I was using hot water reheat or SCR reheat, then I would just simply take that 50 to 100% and I would stage that, or I would reset that as a zero to 100%
Phil Zito 17:14
command, or zero to 10 volt DC, typically command out of the output. So that's how we go about controlling electric reheat, still pretty straightforward, right? We're seeing a couple more things introduced. We're de mystifying the purpose of having a discharge air temperature sensor. We're talking about why maximum CF or our CFM heating set point and our min and max cooling set points, and we're talking about using resets in order to turn on our airflow for our heat first, and then turn on our heating stages, or our heating outputs. All right. From here, we move on to what are called Fan powered boxes. There's really two types of fan power boxes. We have a series fan power box and we have a parallel fan power box. Let's go through the parallel fan power box first, and then we'll talk through the series fan power box, and we'll talk about when we use each one and why we use each one. Okay, so a series or, sorry, a parallel fan power box, right with heat. Typically, these parallel fan power boxes will have a stage of heat in them. It operates pretty identically to our reheat box, right? We're going to control our min and max CFMs for cooling, and then, as we fall below set point, we are going to go and drive ourselves into a new state. However, this new state is going to be that first we turn on this fan that is parallel to the damper. Okay, so it's parallel to the damper, and what it's doing is it's drawing warm air from the plenum. The theory is, is that heat rises. It's going to be warmer in the plenum, and we are going to take our warm plenum air and blend it with this minimum cool air from the primary air damper. And that's going to be wonderful, right? We're going to get free heat. That's all good. And then what we're going to do after that, right? Is we are going to say, hey, you know what? We're still not hitting set point then. And only then will we go and turn on our terminal unit heat, whether that's electric or hydronic, okay? And then we just step up the heat as normal. Now here's a couple key things to know if you're thinking about this. The first thing you need to know is we no longer have a heating CFM. The theory now is that with the assistance of the fan and the minimum. Cool air from the primary air damper. We are going to satisfy the CFM requirements to get a proper heat transfer off of the coil, so we're not going to fluctuate between minimum and heating CFM in this sequence. It's going to be assumed that the fan is assisting in that. All right, so then we move up to this concept called series fan power boxes with heat now, generally it is preferred to use para parallel, if I could speak right, fan power boxes. And the reason why is that, as we just saw, the fan is going to be turned off when we're not in heating mode. Now, with series fan power boxes, we're going to be running the fan constantly, because here's how it's going to work, right? We are going to go and control the zone temperature set point. We are going to have our primary air damper, and it's going to modulate to allow the cool air to come in as needed. It's going to be blended with warm air circulated from the plenum by the fan, which is going to always be running. So essentially, what happens is that the air that's delivered into the zone is at a constant volume and variable temperature. So this becomes a little weirder of a sequence in that we are varying temperature by controlling our damper while at the same time providing this constant volume of air. It's a little difficult to follow verbally, but if you go and draw a line, you'll start to see that, oh, this fans in line. It's in series. It's going to draw air, and that's why it's creating this constant volume air flow scenario. And what happens now, right is that when it starts to get cold within the space, the damper, once again, modulates to the minimum position, allowing less cool air to be blended with this plenum, air that is still constantly going and then the zone is going to reach set point, or the zone continues not to reach set point. Rather, it is going to get to the point now where the heat is going to start to engage. Here's the thing. I'm just going to talk real quickly about series versus parallel. Why we even use fan power boxes in the first place? The first reason we use fan power boxes, by the way, is it reduces the fan having to work at the VAV unit, especially when we're using these fan power boxes on really long runs. So rather than having to have the fan work quite as hard, we offset that with running a smaller, little fan and pulling from the plenum. Now the nice thing about parallel fan power boxes over series fan power boxes is obviously we are not blending hot air with cold air that we don't need to do. We can actually just close the damper down if we are satisfied. And additionally, we don't have to run the fan constantly. So that is why you will often see folks want me to use parallel fans, another addition or another reason why that whole fan decision happens and actually in favor of the series fan power boxes, in lieu of the parallel fan is noise. So oftentimes, folks will choose series fan power boxes because they're always running, and it almost simulates a white noise, kind of noise, rather than every time the box goes into heating mode, it slams a fan on, and that noise can be quite jarring. So that is another factor that folks consider when they're looking at utilizing series fans versus parallel fans, all right, folks. So I hope this podcast helped you better understand when you would use certain forms of terminal units, why we utilize terminal units, the thought process behind different types of terminal units and some of the concepts around controlling them, both in programming as well as just in sequencing. All right, everybody. I hope you enjoyed that. This is the first of eight episodes that we're going to be dropping that are going to be going in depth about HVAC sequences. So if you are struggling to figure out HVAC sequences, you are going to want to go and listen to these episodes. Once again, everything will be available at podcast dot smart builders academy.com forward slash 471 once again, this podcast at Smart builders academy.com forward slash 471 and if you're listening to this and you want to save money. Me on courses, because I realize there's a lot of you who want to pay out of your pocket. This is the only time you can afford our courses, because we do charge a premium for our courses. It's just how it is. They're really good quality. I'm not going to charge them for cheap. So once again, go to Courses. Dot smart buildings, academy.com, starting the Friday after Thanksgiving until midnight on Cyber Monday. And use coupon codes Black Friday 40 to save 40% off of our self paced courses, Niagara 25 to save 25% off of our Niagara training bundles, or workforce 30 to save 30% off of our workforce development seats. These coupon codes will not last. There's only couple 100 of them. They will go quick. Make sure you take advantage of them. Pick out your courses now so that you can be ready tomorrow when this sale goes live. Thanks a ton, everybody, and I'll see you again in the next episode. Take care.
