Hello folks! This week, we are going to begin a multi-part series on HVAC Control Sequences. We’re going to cover several control sequences and I’ll 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.
Short on Time Listen to the Audio Instead of Reading
Trust me, this will not be nearly as boring as it sounds. If anyone's ever read control sequences before, they're pretty freakin’ dry, but I'm not going to just be covering these word-for-word. We're actually going to be talking about what's going on and the concepts that are going on, along with thoughts and ideas that I've picked up along the way.
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Our course will take you all the way from zone control to complex central utility plants. It will teach you how to control these systems, and how to understand all of the different points and variables needed to interact with as a building automation professional. I encourage you to click the above link and check out the course. It is one of our best-selling courses for good reason; it will teach you exactly what you need to know about HVAC, in the shortest amount of time possible.
Variable Air-Volume (VAV) Boxes
Alright, in Part 1 of this multi-part series, as I mentioned, we're going to 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: 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? One of the questions we ask is, “What is the most important variable in zone control?” A lot of folks will say temperature. I get that I mean, after all, with a VAV box, you're controlling the temperature, but really, you're actually controlling airflow. At the end of the day, nothing's going to happen if you don't have the right air in that box, going out of the box, into the space.
At the end of the day, most of the VAV boxes, with the exception of reheat boxes, really can't do anything to control air temperatures. They could control zone temperature, but they are dependent on the air being conditioned by an air handling unit. hen we're talking about zone control, and we're talking about VAV units, we're really talking about modulating the airflow that is going into the space, so that we can modulate the air changes which are the amount of volume that is changed in a space within an hour.
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, or as an implementer, you are going to be spending most of your time on boxes. That is where most of the issues are going to occur.
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 that oftentimes with boxes, we're looking at a combination of physical install and 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 when there's hundreds of boxes, that is where issues tend to abound. This is when a box, or whomever is installing the controls on the box, or wiring it up, or creating the program, misread the sequence or they don't really think about what they're doing because they're on autopilot. That’s when 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.
Now 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. If you get efficient in this area, you will realize the biggest gains. Getting more efficient on an air handler 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 VAV’s and fan-powered boxes, is typically going to have a much larger effect.
An individual VAV box could typically control to a zone temperature setpoint. When controlling a box you have a process variable, which is going to be the variable you're controlling and a process being a loop of where you have an input and a setpoint. 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. Typically we will have a minimum-cooling CFM setpoint and a maximum-cooling CFM setpoint. Our zone temperature and zone setpoint are going to be compared, and that's going to drive a direct-acting pod loop.
As zone temperature goes above zone setpoint, the output of the loop is going to increase. As zone temperature goes below zone setpoint, the output of the loop is going to decrease. That, naturally, is going to go into a reset block within programming, and that reset block typically will scale between the min and max CFM. Now as that zero-to-100% output of that first PID Loop increases, that is going to change a CFM setpoint.
Okay, so as you change that CFM setpoint then you have another process variable, and that process variable is the Flow Setting. That could either be an external flow sensor or it could be one built into the field controller. Now you've got two PID loops, first of which is driving the zone temp, and the second of which is driving to your CFM setpoint.
Only then can you drive to your damper actuators. So, you would drive a damper actuator, typically a round actuator that's on a little turn shaft. You're going to mount an actuator or a controller actuator to that shaft on the box and open and close that damper to control to setpoint. It's pretty straightforward sequence but you can see where it starts to throw a lot of people for a loop if they're brand new because it's a loop within a loop. That’s something that we really don't see too much outside of the VAV boxes.
Most of the rest of our control sequences are going to be a single dedicated loop per process. 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 setpoint, 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. We are driving to a minimum CFM setpoint, not a zero setpoint. You should typically not have zero CFM as your minimum setpoint on a VAV box, because as you go and turn on your boxes, and if all boxes are satisfied, then 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 are not tripping your high statics, but you're also better able to meet the standards of ASHRAE 62.1, which requires 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.
My recommendation is usually to set a minimum position for all actuators. It’s hit or miss whether you set 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 then I don't have to worry about it. It's one less thing I have to troubleshoot.
Now we move from cooling-only boxes, to boxes with reheat. Cooling-only boxes typically are within the internal parts of a building. We're talking not the external parts of the building, but let's imagine a bull's eye. You have that little circle in the middle and that would be the internal aspect of the building. Inside there, we're going to have cooling-only boxes and these boxes are going to be cooling-only because the theory is that, properly utilized, you're naturally going to heat up inside the building with latent-sensible loads and all of that fun psychrometric stuff. Then the reheat boxes are going to line the perimeter and their sole purpose is to keep the BTU’s, or heat, from escaping the building. Now what you'll do is 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 (SCR reheat), which is basically like hot water reheat except that you’re sequencing electrical stages instead of controlling a hot water valve. But let's talk about VAV boxes with electric reheat.
VAV boxes with electric reheat are still going to have zone temperature and zone setpoint, 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. A lot of folks will initially think discharge air temperatures exist in boxes for controlling purposes, however, 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.
Now 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. So you're going to go from minimum cooling to heating CFM position, and only then, when that is achieved, will you go and start to stage on your electric heat.
How I typically like to do this is, I like to use a single PID loop that is controlling to temperature. Then I will have 0 to 50% of that loop, so this is a reverse-acting loop, and it will drive my minimum to heating CFM. It'll drive a PID loop for min to heating CFM.
Typically 'd have 0-50 coming out of my zone temperature loop, and that would be driving the 0 to 100% output, and I would reset and I would say, that 0 to 50 is going to reset from min to heating CFM setpoint. Then from 50 to 100, I would take that and 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 reset that as a zero to 100% command, or a zero to 10 volt DC command out of the output.
Ok, so that's how we go about controlling electric reheat. Still pretty straightforward, right? We're seeing a couple more things introduced, were de-mystifying the purpose of having a discharge air temperature sensor. We're talking about why our CFM heating setpoint, and our min and max cooling setpoints. We're talking about using resets in order to turn on our airflow for heat first, and then turn on our heating stages or our heating outputs.
Alright, 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 to discuss when and why we use each one.
A parallel fan-power box, with heat, as they typically have a stage of heat in them. It operates pretty identically to our reheat box, by controlling our min and max CFM’s for cooling. Then, as we fall below setpoint, 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 that heat rises, it's going to be warmer in the plenum, and we're going to take our warm plenum air and blend it with this minimum cool air from the primary air damper. Sounds wonderful, right? We're going to get free heat!
After this, going to say, “Hey, you know what, we're still not hitting setpoint.” Then, and only then, will we go and turn on our terminal unit heat, whether that's electric or hydronic. Then we just step up the heat as normal.
Now, here's a couple key things to know. A 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 assumed the fan is assisting in that.
Alright, so then we move up to this concept called series fan-power boxes with heat. Now generally, it is preferred to use parallel fan-power boxes because 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 we are going to control the zone temperature setpoint. We're going to have our primary air damper 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 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. What happens now 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 continues not to reach setpoint. Rather, it is going to get to the point now where the heat is going to start to engage.
I'm just going to talk real quickly about series versus parallel and 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. Additionally, we don't have to run the fan constantly. So that is why you will often see folks wanting to use parallel fans.
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 sound 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.
Alright, folks, so I hope this post 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.
As always, if you have any questions, I encourage you to use the comments section at the bottom of this post. Feel free to ask us whatever questions you have. I also encourage you to check out our Control Sequence Fundamentals course, which will really teach you exactly what you need to know. I mean, this was only 20 minutes worth of material we covered! We spend a fair bit more time on each subject in that course. I really encourage you to check it out!
Alright folks, in our next post we will discuss airside systems, like fan coils and rooftop units. Thanks, a ton! Take care!