Hey folks, Phil Zito here and welcome back. In this post, we are going to be talking about demand control ventilation. So, we're going to cover a lot of different aspects of demand control ventilation. When I actually went and first started researching this for the post, yes, I actually did research before this post, I started to find out that there is a lot more to demand control ventilation than what I had connected with it in the past. That's the thing like, I actually have seen these technologies, and I have implemented many of these technologies, but I never really connected them as part of a holistic demand control ventilation strategy.
My hopes are, by the end of this post, you'll have a different appreciation for demand control ventilation that is more than just monitoring CO2, and then controlling a PID Loop based on CO2 driving to a setpoint. That it can be much more than that, and it can be much more integrated than that.
So, what is this thing called demand control ventilation? Well, essentially, you vary the ventilation based on the demand. Pretty straightforward. But what ventilation, what demand? Well, it all comes back to two pretty well-known standards in our industry, which are ASHRAE, 61, and ASHRAE 90.1.
So ASHRAE 62, what it defines is the minimum ventilation rate, and it goes pretty deep. You can actually see these on the read-only versions of the standards, you can find them on their website on the read-only section. If you dig into them, you'll start to realize that depending on the space type, the occupancy load, and the occupant’s density, you can have varying requirements for outside air. So, for example, a classroom is going to be different than an auditorium, is going to be different than a cafeteria, etc. And you know, a cafeteria, it's very transitory occupancy. So, you'll have occupancy in big periods, and then you'll have no occupancy, which makes it a prime opportunity for some demand control ventilation strategies.
What a lot of people also don't realize is that areas that have permanent occupancy density can also be great opportunities for demand control ventilation. But once again, what is the ventilation, what is the demand? The ventilation is outside air, so as humans breathe, they take in air, and they breathe out carbon dioxide. Carbon dioxide is also known as CO2, and as CO2 builds up within a space, it replaces the oxygen percentage of the air. People start to breathe that in, and if there's not enough fresh air, either in the form of exhaust or in the form of supplying the actual fresh air, you have to get more fresh air in otherwise these people start to get really tired. It can be very noticeable when you go above 1200 to 1500 parts per million, you start seeing people yawn. Anything above 2000 parts per million, people can actually start to get very sleepy.
So, what is the demand part of things? Now we know the ventilation is outside air. What is the demand? It is the load; it is the occupancy load. So, demand control ventilation is a strategy that seeks to vary the ventilation based on the demand.
So, how do we do that? One thing I want to point out here is that this strategy is typically done to save energy. For the longest time, ever since COVID hit, we've been focusing on improved ventilation. So, you've seen increased ventilation rates. Now that those things are kind of settling down, depending on what municipality you're in, we are starting to see a focus again on energy efficiency. So, people are having to retool their increased ventilation sequences, and one of the best ways you could do that is by implementing a demand control ventilation sequence.
Now, as I mentioned, the standards 62.1 and 90.1 require some form of ventilation control. So, 62.1, Section 6.2, is ventilation procedure rates, and that's ASHRAE 62.1. It talks about things like the dynamic reset, talks about DCV. We'll see that DCV, demand control ventilation, is not necessarily the same thing as dynamic reset, but it also can be the same thing at the same time.
Then 90.1, 2016 is the one I remember, is where they introduced ventilation controls for high occupancy areas. If I remember correctly, it was like anything larger than 400, maybe 500 square feet, and I think like 25 people per every 1000 square feet, was kind of one of the requirements. They would say like, if you're doing that, and you have an air economizer with control and outside air flow greater than I think it was, like 2500 cfm, then you had to go and like implement some ventilation strategies, and they talk through them. Don't quote me on these things. They're in the standard and I'm going based off memory here.
So, you're starting to see this anywhere that is being held to 90.1 and 62.1, so, basically all new builds. Now what are some areas that would have potentially that greater than 25 people per 1000 square feet? Common areas would be like anywhere where you're going to have a high density, so classrooms, computer labs, auditoriums, lobbies, cafeterias, etc. These would all be areas, and then when you break those areas out, you have some of them that are permanent occupancy, and some of them that are like transitory occupancy. That’ll become important when we talk about some of the DCV control triggers. The sequencing is not terribly hard to do, but the triggers that bring you to that point, those can require some interesting integrations, as I've experienced in my past life.
So, if we talk about ways to do this, there's things like occupancy schedules. Those are like kind of the least reliable because you're assuming asset demand. So, we're going to go from ease of implementation to accuracy of demand. As we get closer to accuracy of demand, we are going to get more difficult except for like CO2 sensors.
So, occupancy schedules, you can look at when the system is occupied and implement a DCV strategy when the system is only occupied. There's no reason to open up the economizer for fresh air when it's not occupied. No one's in the space.
Another technology is CO2 sensors. So, CO2 sensors, or carbon dioxide sensors. They usually have like a five-year life, three to five years on the sensor element.They do provide a greater accuracy around demand. And in my opinion, they're kind of the sweet spot as far as accuracy of demand with ease of implementation.
You also have other things like occupancy sensors and people counters. People counters being the most difficult because you usually have to use some form of API. They've come a long way and there's a lot of, I believe, Israeli technologies around people counting that were pretty advanced. They're costly and they can also be difficult to integrate. You can also do BAC access control. The problem is, people piggybacking and not swiping their card, so unless you have like an RFID, you're going to be kind of out of luck as for density.
So, the golden standard that I want to kind of focus on here is CO2. So, we'll talk about CO2, we'll talk about how to implement it, we'll talk about placement strategies, we'll talk about single zone, and we'll talk about multi-zone. Those are kind of the two things that tend to challenge people.
Single-zone DCV with CO2 is actually pretty straightforward and pretty easy. You either have a CO2 sensor in the space or in the return duct. I personally prefer the space, it's closer to the breathing of the people, and it's not conditional on enough flow, to draw through the return to give you an appropriate sample. So, like if you are full blown economizer, granted, if you're a full blown economizer, you shouldn't have an outside air issue. But you know, if you had increased economizer, you may not be drawing enough return flow to indicate that the CO2 is built up.
By having it in the return, it is nice if you have multi-zones, which I’ll cover in just a bit. As far as single zones go, the big thing with that is, you put the sensor up, you get it mounted, then you have to figure out at what threshold of CO2 am I going to trigger a CFM response. There's two ways you can do this. I've seen linear resets and I've seen PID loop control.
So, linear reset would be like, at 500 parts per million, we deem that based on occupancy density, based on equations that I won’t get into here, we've determined at 500 parts per million, we should have this amount of outside air. Then at, let’s say, 1300 parts per million, we should have this amount of outside air. Now, this requires you to have a flow sensor. This requires you to then have a PID loop with a linear reset and your linear reset is going to say, you know, 500 parts per million is 1000 CFM, 1200 parts per million is 2000 CFM outside air. You have an airflow sensor, and that sits across the economizer, and as the economizer opens up, the air goes across that, you get the sample of your outside air, and you can control it that way.
Now the benefit of that, you get very precise control. You're controlling exactly to a specific outside air CFM. The downsides of that, airflow sensors are notoriously difficult for some technicians to set up and depending on placement and amount of probes they can be inaccurate based on air turbulence, especially as dampers start to partially open. There can also be an increased cost. However, if the air handler is large enough, that cost could potentially pay for itself, because the difference between conditioning 1000 CFM of outside air versus 5000 CFM of outside air, over the period of a year, may be miniscule, but conditioning 10,000 versus 30,000 CFM of outside air on a massive air handler that is a significant cost reduction.
Remember, DCV is primarily about energy. If you want to achieve indoor air quality, then just go to the enhanced ventilation standards that have been placed out, open your damper open wide, and you'll get a ton of fresh air, and everyone will be happy. As long as your outside air damper isn't like next to a loading dock, or next to a source of you know, PM2.5, so that's particular matter 2.5, then you should be solid.
Now the other way I mentioned for a single zone control is to do a PID Loop reset. So, PID Loop is where we have our demand control ventilation static setpoint. The setpoint doesn't scale. So, we may say 800 parts per million, that's a common setpoint for DCV, 800 or 1200 parts per million are common setpoints. So, we would say 800 parts per million, we would measure the CO2 as our process variable. 800 parts per million would be our setpoint, it would go into a PID loop, and as we went above setpoint, this would be a direct acting loop, we would have an increase in PID loop output. That PID loop output would go to a high select that would be coming on the outside of our economizer PID loop. It would select either the economizer output, or it would select our DCV output if our DCV output was higher than economizer.
So, that's kind of how we would program that. We would have two separate PID loops, and that's a very common way of doing it. That's less complex, there's less sensors involved, but you get less granularity of control, and you're kind of doing a set and forget, which if you have variable occupancy, may not be required. So, just things to be aware of, things to be cognizant of.
Now, when you come to multi-zone, this is where things can get like a little hairy. Let's say that you have an air handler, and this air handler is feeding 10 VAV boxes, and these VAV boxes are feeding 10 different office spaces. Now, how do you implement DCV? Well, there's two ways you can implement DCV. You can implement it with a common return, which is the lowest priced solution, but you're also going to get variable results. Or, you can implement a CO2 sensor in each space, with CO2 sensors, averaging $200 to $400 cost, and that's before markup. So, with those costing that for 10 spaces, you're looking at $2,000 to $4,000 of cost.
Are you going to save enough energy? And this is where you just run a basic energy calc, right? What would it look like if we reduced 20% outdoor air use? What would that look like? You have to do that math, but I'm just spit balling numbers here. You can then do an ROI calc and figure out if it makes sense and what your simple payback is, for putting one in each space. That will obviously give you much more accuracy as far as control because then what you do is you totalize up your delta from CO2, and you can actually write a sequence where you can start to vary the airflow, either on the VAV, or vary the damper. There's different ways you can approach it.
Obviously, varying the airflow on the VAV may cause temperature imbalances and temperature misses and discomfort, but you will have proper ventilation. Another option is to add up the overall CO2 demand from these different spaces, totalize that up, and use that to drive a setpoint. You'll still be a little bit off because of you know, each little VAV is going to change.
So, there's some calculations you have to do where you look at CO2 and then you do calculated CFM to figure out basically what percent you need based on the CO2 density for the cubic foot of the space, and then based on the volume of air you’re providing, how much do I need to go and offset that. So, that's an equation you can look that up. I think that's in 62.1, if I remember correctly. So, that's one way of doing it.
The other way, as I mentioned is just doing a common return CO2. Assuming these 10 spaces that I just mentioned, have a common return, then you could put a CO2 sensor in the return and you should get a blended average. Now the problem is, you're going to under ventilate some spaces, you're going to over ventilate other spaces, because you're not going to be getting specific values. It's going to be a blended rate of CO2. That's assuming that you don't take air from other spaces as part of the return. So, just be aware of that. But you know, those are kind of the approaches you can take to actually deal with demand control ventilation.
So to summarize, demand control ventilation is all about controlling the outside air ventilation in response to demand. Now, that being said, we have multiple ways of doing that, as I mentioned, the preferred way is CO2 sensing, although you can do occupancy counting, you can do occupancy sensing, or you can do scheduled control of demand control ventilation. Demand control ventilation can be really implemented in one of two ways.
- You can do a static CO2 setpoint and then reset the damper based on that CO2 setpoint. So, you do a high select.
- Then you also have where you can vary your outside air flow setpoint based on your CO2 density, and then you would do that with a linear reset, which then you would have that as your setpoint and your process variable would be your airflow monitoring station.
So, those are kind of your approaches there. With a single supply, single return, single zone, it's fairly easy, you just put a CO2 sensor in the space or in the return, I prefer space mounted. If it's a multi zone, you have a little more difficulty in that you have to either have a CO2 sensor in each zone or in a common return. If you do have it in a common return, you're going to under and over ventilate, just be cognizant of that. If you have it in the space, you're going to have to do some equations to basically calculate CO2 density based on the cubic feet of the space to give you a good idea of what you need to totalize up as far as your outside airflow setpoint.
Alright, folks, I hope this helps you better understand demand control ventilation, I hope you've gotten some ideas about it. Thanks a ton and take care.
