Episode Description:
In Episode 474 of the Smart Buildings Academy Podcast, we delve deep into the intricacies of 100% outdoor air units (OAUs). For building automation professionals, mastering these systems is critical, especially in high-stakes environments like operating rooms, clean rooms, and data centers. Whether you’re a seasoned technician or just starting your journey in HVAC controls, this episode unpacks the nuances that can make or break the performance of OAUs.
Join us as we explore how to prevent common pitfalls and enhance system performance. You'll walk away with practical insights and a fresh perspective on state-based control strategies that could revolutionize your approach to HVAC systems.
Key Topics Covered:
- The vital role of state-based control in HVAC troubleshooting and programming.
- Common issues with 100% outdoor air units in extreme weather conditions.
- The interplay between preheat, cooling, and reheat sequences.
- Ensuring safety and efficiency with damper status and static pressure controls.
- Humidity control strategies for maintaining precise environmental conditions.
Dive into this episode to gain actionable knowledge on optimizing OAU performance and to hear insights that could transform how you approach control sequences.
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
- Control Sequence Fundamentals HVAC Course
Podcast Video
Transcript
Phil Zito 0:00
This is the smart buildings Academy podcast with Phil Zito. Episode 474. Hey folks. Phil Zito, here and welcome to episode 474. Of the smart buildings Academy podcast. In this episode, we're going to be continuing our series with Part Four on HVAC control sequences. We're going to be diving into 100% outdoor air units. Now the time I'm recording this, this is December 20, almost, and it's very cold outside, and that is the prime time when people start messing up 100% outdoor air units. You start getting frozen, coils, preheats not working, all sorts of issues. And then you got summertime too, right, where people are struggling to deal with humidity. These 100% outdoor air units are absolutely critical for spaces like clean rooms or operating rooms. So understanding how to control these and how to work with these systems is something that can be challenging for a lot of new technicians and a lot of new mechanics. We're going to dive into a lot of the different aspects of 100% outdoor air units, how they work, how they're controlled, what you need to be careful with, and much more in this episode, as always, everything's available at podcasts, at smart buildings, academy.com forward slash 474, once again, that's podcast smart buildings. Academy.com forward slash, 474, and if you found this episode valuable, please, if it's on Spotify or Apple podcasts, leave us a review. Give us five stars. If you think we've earned it on YouTube, please subscribe, hit the like button and comment that helps the algorithm. And if it's on LinkedIn or Facebook, please repost this with your thoughts and share it with your network. With that being said, let's dive into the episode. In previous episodes, we talked about space control, we talked about air handlers, we talked about fan coils, we talked about rooftop units. We talked about mixed air, single path, variable and constant volume units. And now we're looking at closing out the air side. One thing I want to talk about that we didn't really get to finish up in the previous episode was mixed air control. We talked about it briefly, but not a whole lot. Now we talked about how there are low temp alarms, and that when it's triggered, that will put you in a low temp state, something I really want you to think about as you move through any of these sequences, is a state based mindset. So where I see a lot of folks get in trouble, especially with the air side stuff, not so much with the hydronic but very much with the air side stuff, is not understanding state based thinking. So when you think of an air handler, you can be in a cooling state, you can be in a heating state, you can be in a ventilation state, you can be in a dehumidification state, a re humidification state. There's multiple different states, and what I want you to think about when you think about states, is, what state Am I in? What parameters would trigger me to be in that state, and then what should be happening in that state. This will really help you program. This will also really help you troubleshoot. So for example, if you are in a dehumidification state because your return air humidification or humidity level, or your space humidity level is too high, then you can expect your control system to potentially be doing simultaneous heating and cooling. You can also expect that you're going to be trying to drive the air to saturation. That way, you can then drain out the moisture and then heat the air back up prior to sending it downstream. That's how typical dehumidification sequences work. So when we think about our low temp and our mixed air control. There's several different variables that can take place. We have low temp control, which is where the airstream incoming is too cold and you don't want to freeze coils. So you start to close outside air dampers where most of the cold air is coming from. You start to open up, preheat valves if they exist, and reheat valves if they exist to full position. You close cooling valves typically, and you run the fan, and that's what you'll typically do in order to make sure that you don't freeze coils. And everything's all well and good. But there's also mixed air temp control in which we are either using a preheat or we are using a outdoor air damper. I see a lot of folks mix this up, because what they don't realize is that there is both a heating component, typically, and a cooling component to mixed air control. The cooling component to mixed air control is typically using your economizer to drive your mixed air temp down. So you've got a mixed air temp set point, you've got a mixed air temp and you're opening up your economizer to bring in cold air to drive the temperature down. You also, though, have preheat which typically will come before the averaging mixed air temp sensor, and you will drive a preheat valve open in order to go and. Condition air. Now you will sometimes see this on mixed air single path units, but you will especially see this as we move into our 100% outdoor air unit sequences. So let's take a look. I'm gonna go through our 100% outdoor air sequence that I have here in front of me, and we're gonna start to go through it. And the sequence has the basic safeties you would expect, high, low static, smoke and fire detection. But it also, and it and it also has, you know, free stat, high, low humidity, those kind of things. But it also has an outdoor air damper status. This is really important when we're using 100% outdoor air units, because that fan needs to draw air from somewhere. And if you don't have 100% outdoor air damper open, then the outdoor air damper is not going to be providing air flow, and that air flow then is not going to get inside the unit, and the unit is going to start to suck in the sheet metal. I've seen this happen. This is kind of the inverse of if you had fire isolation dampers downstream from a fan, and those were closed, and that supply fan was blowing air, and that air had nowhere to go, so it would pop the duct work. Well, this would suck in the duct work. So we need to be cognizant of that our outdoor air dampers are open upon our fan signal to start, and if the fan doesn't start, then the dampers are going to close and we're gonna have a fan failure alarm. But if we don't get damper status, then we are going to actually go and turn off the fan, because that is one of our biggest issues. Now, it's all well and good to have a damper status, but sometimes those damper statuses can fail. There's a bunch of issues that could potentially occur that could cause us to have no damper open, even though we have status saying it is open. That's why we always typically want to have a low static pressure safety cut off. And that low static pressure safety cut off is going to make sure that, hey, if our static starts to drop too low, that meaning that we have an issue with our fan and the damper most likely being closed, we then will shut off the fan. So that's kind of our safety there. Then on these outdoor air units, what we will tend to have is both preheat, cooling and reheat. We tend to see these outdoor air units used where we can't recycle air, so something like an operating room where you constantly want fresh air, you don't want recycled air, or maybe a containment room, or maybe an area where you're dealing with electronics and you really want to deal with the quality of air. In this scenario, we will often have cooling valves, we'll have preheat valves, we'll have potentially a humidifier, and we will also have reheat so fan speed control is hit or miss with 100% outdoor air you're either going to have constant volume or you're going to have variable volume. Now we can have variable volume with 100% outdoor air units, especially if we're serving operating rooms that have a
Phil Zito 8:15
have a supply box and an exhaust box in the operating room, and they are trying to manage a positive pressure scenario in the or so that nothing can come in, right we're pushing all the air out. And in that case, we would typically have a vav and the operating rooms, and then we'd have VFDs on the 100% outdoor air unit in order to drive these discharge static pressure, so that these VAV Units can properly supply flow. And what we would typically have interlocked with these units, especially if we have variable return air or return boxes, is we would have a exhaust fan interlocked, not part of the unit, but interlocked with the operation of the unit, and that's how we're going to go and make sure that we are varying our exhaust to either do our volumetric offset or just to maintain positive pressure. Most times, we're going to be looking at space pressure and maintaining that. Sometimes we'll do it through a volumetric offset. Most of the time, like I said, we'll have like, a TSI monitor that will measure pressure of outside the space and inside the space, and will maintain a positive pressure scenario, all right? So that's fan speed control and pressure control cooling valve, pretty straightforward, right? We're going to go and drive that to a discharge air set point. Now, typically, what we will do is we will drive a discharge air set point, and we will drive it to maybe like 5553 52 degrees, depending on our humidity projections, and at which point we're going to reach saturation and have the effect of draining the humidity out of the. Airstream to the point where we're at our relative humidity goal that we want when we reheat the air, because the air is typically going to be reheated by the boxes within the space. Now we also have a preheat control. Now this typically is solely existing to re or preheat the outdoor air Airstream as it enters the space. So you may see steam preheat in scenarios where you have to use 100% outdoor air unit, but the outdoor air is really, really cold, and you want to be able to overcome that Airstream. So you'll see steam which is capable of introducing a massive amount of BTUs into the airstream rather quickly. The Con. The one thing you have to worry about, though, is if the outdoor air is really low relative humidity already, and you reheat that Airstream with your preheat, you're going to have to have a humidifier to add moisture back to the airstream, because remember, with psychrometrics, we are going to already be at a pretty low amount of moisture as the temperature gets lower. So even like a 3050, 60% relative humidity is going to have much less moisture at zero degrees outdoor air than it is at 70 degrees outdoor air, just due to hotter air being able to contain more moisture content. So what's going to happen is we're going to have this preheat and as we especially if we use steam and we really heat it up really fast. One, we're going to have really tight control, one thirds, two thirds valves to make sure that we're controlling our steam and we're not overheating the Airstream. But two, we're going to want to make sure that we are introducing humidity after this, because we're going to bring that temperature up and most likely that relative humidity is going to drop substantially, which brings up humidification control. Typically, we are going to have a humidifier, and that is going to go and either look at the space temp and determine the average humidity there, and we're going to drive humidity based on or not space temp, the space sensors, and drive space humidity, or we are going to drive it off of the actual air stream itself. It's hit or miss. See, the thing is, if you're doing your sequencing of your supply and exhaust boxes, right, and you don't have a whole lot of things introducing moisture into those spaces because you're properly pressurized, then really you can control humidity at the airstream level, at the supply Airstream, and be pretty good. The reason I tend to avoid averaging is that you could have swings if you're not properly pressurized. However, with averaging, you can also use that as well and just say, Hey, we're properly pressurized, our spaces are going to be pretty tight on humidity. And we're going to use an averaging scenario where we look at the average relative humidity and then drive that as our set point for our for our supply, Airstream, and we can do that. Another thing we can do that I see very commonly is resetting based off average. So that's another scenario. So I'm giving you a lot of scenarios. I'm not necessarily telling you which one is my preferred but like I said, there's a static set point for the supplier stream. There's looking at the average value across the spaces and using that for the supply set point. And then there's resetting where maybe you take like a 60% relative humidity average, and that or 60% relative humidity setting, and that is when you're at 45 average. And then you can do like a setting of 85 when you're at 30% relative humidity, space average. This also can be used. You can use this as a kind of linear scaling reset, like we do with hot water supply temp or you can do maybe, like a trim and response, which we do with pressure, where every, you know, 510 minutes, we adjust the set point up. That's really up to whoever writes the sequence. It's also up to you, depending on how tight you want the control, how responsive you want the control. So, you know, I gave you four different scenarios, right? I gave you just a static set point for relative humidity. I gave you matching whatever the average is. I also gave you a reset scenario that's linear, and I gave you a reset scenario that is trim and response, I really hope at this point in the episode. And by the way, folks, if you haven't listened to the past four episodes, I really encourage you to, because these episodes build upon one another, and I'm starting to go into more advanced sequencing and controls theory and things like that. So it's my hope that at this point in the episode, in the series, that this. Technology that I'm utilizing is familiar to you. I know I'm talking really fast because I'm trying to cram a lot into 30 minutes, but I also want you to be comfortable in that you're taking this information in and you're really starting to understand kind of the theory behind all of these controls sequences that were enacting. All right, so that's pretty much it for a 100% outdoor air unit. Sometimes for 100% outdoor air units, you'll have an ERV, which is it's like an energy recovery unit. And what it is, it could either be for humidity or for BTUs, and you have this wheel that will then and for humidity. It's typically a wheel with deco sent on it and and or some other chemical, and it will reclaim moisture and then reintroduce moisture into the airstream, or remove moisture and introduce it to the exhaust airstream, just depending on what you're sequencing. But there's also where you could have heat pipe, where you have a set of coils in the incoming Airstream and on the outgoing Airstream. And basically you'll take some of the heat on the exhaust airstream during a cold scenario, and you will take that heat and transfer it to the incoming outdoor air air stream, thus heating up the air. That outdoor air stream will absorb the heat and it'll transfer now that chilled water back to or chilled glycol back to the exhaust stream, which then will once again absorb heat from the exhaust stream, and that's kind of a energy recovery scenario that you may see in a 100% outdoor air unit. So that's that next up we have mixed air units, okay? Or mixed air units makeup areas. I always want to call them mixed air units Maus, but I know they're not called that. It's just so freaking tempting. But makeup air units are pretty straightforward, at least on the surface level. They can get complicated. You gotta forgive me here, if I'm like, snorting and making noises, I've getting over a head cold here, and it's, it's like, oh my goodness. I went recently to Disney World with my kids for eight days, and I managed to avoid my eight year old who was like, really sick, like, nastily sick, and I managed
Phil Zito 17:35
to avoid it all the way up to the last day. And on the last day, she finally broke my immune system and got me sick, and now I got a head cold. So that's freaking awesome, right, right on Thanksgiving, but anywho, so forgive me if I'm like coughing and making noises. But with that being said, so makeup, air units, you'll see these a lot in high rises. You'll see these a lot in kitchens. You'll see these a lot in buildings where you're exhausting air flow and you need to make up airflow into the space. Now there's kind of two ways of makeup air units. You can have makeup air units that precondition air and pipe it into the building, or you can have makeup air units that just supply air into the building or into the airstream. It's kind of two different ways. In a lot of hotels, in a lot of apartments or high rises, you'll see make up air units used for the corridors, and then you'll see mixed air single path units with VAV Units utilized for the spaces. And the whole point of a makeup air unit is simply as the air is exhausted out of the space to kind of re pressurize the space or building so that you maintain positive pressure. Um, you also see these in kitchens, where you have a dedicated kitchen exhaust and you are using a makeup air unit that is running in parallel makeup air units, they have a lot of different things going for them, right? You can have just pure makeup air units, which is the fan and an outdoor air damper, and it's bringing in air at a static CFM value, and that is balanced so that it pressurizes the building. Then on the complete flip side, you have makeup air units, which are variable, which have mixed air returns, which have face and bypass dampers, which is something we haven't really talked about here yet. So let's start to kind of unpack the most complicated and then we'll go from there. So with face and bypass what that is, is you have a damper that faces the coil and a damper that bypasses the coil. So bypassing would just straight up continue the airstream through this kind of little dip. Typically it'll go either up or down, and it'll be a bank of dampers. Top one will open up and will face the coil and. And allow you to go through the coil. Sometimes it's the bottom one that will face the coil. It just depends how the unit's built. And then the bypass damper, whichever damper that is that opens up to bypass the coil. Typically it'll either be a cooling or a heating coil. And so when you need to control discharge air temp, you will face the coil and pass it through. And when you need to not control discharge air temp, you'll bypass the coil. Pretty straightforward, right? So makeup air units and then the whole 100% outdoor air damper versus a mixed damper. It all depends on what you're using the makeup air unit for. If you're using it for making up pure fresh air, which a lot of scenarios are, then you'll typically have a pure, 100% outdoor air damper, and you will pull through that. If you're using this as a kind of mixed air solution, you don't really care about the 100% outdoor air you don't care about meeting ASHRAE 62 requirements. You're not using this for that purpose. In that case, you can have a mixed air damper, and it'll just open, typically based on controlling a mixed air temp. So there you go. Folks managed to keep this down to 20 minutes, and with that, we're pretty much done with the air side of things. And in next week's episode, we're going to start to look at chilled water systems. We're primarily going to focus in on air cooled. We may get into water cooled, chilled water systems as well. All right, folks, I hope you enjoyed the episode. I hope it was impactful for you. I hope that it educated you 100% outdoor air units. I hope you're coming out of this with a greater idea of how these systems work. This is just a small step into the world of HVAC control sequences. We have our bas 200 course, which if you really want to dive deep into control sequences, controls, point mapping, one line document or one line diagram creation, then definitely you can find out more about that at pockets, at smart buildings, academy.com. Forward slash 474, you can also find out about our free technical skill assessment, which has now been used by over 25,000 individuals, not 24,000 anymore. We've done a lot more this year, so we are creeping towards that 30k number, and we'd love to have you be one of the people who take our free skill assessment and really figure out what gaps you have around building automation, HVAC, it and much more. Thanks so much everyone. As we come towards the end of 2024 when I'm recording this, I am immensely thankful for all of you who have been with us throughout the years, who have listened to these episodes and who have given great feedback that has improved the quality of these episodes. Some really cool stuff coming in 2025 we're going to be having our instructors start recording episodes, so you're going to be getting a lot of different views and perspectives and probably some better quality episodes, because they're way better than I am. So thank you everybody, and I look forward to seeing you in the next episode. Take care. Bye.
