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39 min read

Airside Unit Control Fundamentals

By Phil Zito on Nov 18, 2020 8:00:00 AM

Hey folks! Welcome back! This week, we are going to be continuing our series on HVAC Control Sequences. In our previous post, we went through airside space control and how we use conditioned air. We also talked about VAV boxes and how they're controlled as well as all the different point types. 

Short on Time Listen to the Audio Instead of Reading


This weekwe're going to be covering: 

We're also going to be covering pressurization, supply versus exhaust, volumetric offsets and volumetric matching and we're going to dive deep into the topics of airside control.  

 This week's post, like last week'sis brought to you by our Control Sequence Fundamentals Course If you are looking to learn how HVAC systems work, and you want to know how to interpret sequences of operations and implement them within your building automation projects, then this course is for you.  This fully online course will teach you everything you need to know when it comes to HVAC and controlling HVAC with a building automation system.  Click on the link above to find out more! 

Airside Control 

 So, let's dive in to airside control.  As I mentioned, in our last post, we talked about space control.  We talked about individual zones, and we're still going to be talking about individual zones in the forms of fan coils and rooftop units.  

From there, we'll jump into multi-zone control.  Now, multi-zone control is exactly what it sounds like. It's multiple zones that we're controlling.  Before we even get there, though, we need to understand what we're controlling and why we're controlling it.  

Now, as I mentioned in our previous post, our primary control variable is going to be air.  We are most concerned with controlling air It doesn't matter what temperature the air is or the humidity level of the air if we can't get the air to the right spot.  So, our primary focus point is making sure we have airflow.   


Now within a building, there’s this concept called pressurization.  Iyou've ever walked into an office building and the doors blow open as you start to open them or they're already open, basically they're cracked open just a little bit and you can feel air whistling through.  Well, that is an over-pressurized building.   

What that means is, there's more air being pumped into the building than is being exhausted out of the building.  That air has to find a way out I want you to picture air the same way you picture water... now, I think a lot of folks get this mixed up, they picture air as air, and they picture water as water, and they don't realize that they both have mass.  If you were to put water into a cup and keep filling that cup, it would overflow, right?  Well, it's the same with air. If you keep putting air into a building, and you fill that building up with air, eventually that air is going to find a way out of that building.  So that's pressurization. 

Now pressurization is really important for us to understand because it's how we control all of our zones.  We need to make sure that we're delivering the right amount of air volume to a space in order to change the environmental variables within that space.  We do that by pressurizing the building.  

Typically, we will have a building be positively pressurized and the reason behind that is that we are keeping the hot air or cold air outside of the building, depending on the time of the season.  In spaces, we can have both negative or positive pressure.  Some spaces, we may not want air in that space to get out.  These are typically rooms like isolation rooms, and so we will have a negative pressure in that space.  In other spaces, we will want to external air out, so we'll have positive pressure.  

We do this with our airside units by modulating supply and exhaust fans.  A fan, quite simply, is a device that is driven by motor and it moves masses of air depending on the fan type, using a variety of methods.  Iwill move air and that air will then go down ductwork to be delivered to diffusers which then will allow the air out into the space.   

Now, that's approach is typically only found in fan coils and rooftops.  In air handling systems, like air handlers, especially VAV air handlers, the air will be delivered via the ductwork to a VAV box, which then will regulate it even more.  Now we're going to come back to this because there's some important things that have to happen in order for that to work.  


Before we get to that, let's go through a fan coil. The purpose of a fan coil is to have an individual zone system that can control a space independent of any other system.   

A Fan Coil is like a rooftop, basicallyexcept that it’s typically going to be inside the building.  It’s also typically zoned for a space where you want to have the cooling source right there.  You'll typically see this in spaces like IDF's  

Data closets have a high potential for heat, so you’ll want to keep the space cooled.  So you'll usually have a fan coil unit serving that space.  Now I hesitate to say this because it's not always true, but most fan coils will just utilize plenum air for themselves and won't be using outside air.   

Now I know as soon as I say that, I'm going to start getting messages from folks saying, “Well, I've got a fan coil that uses outside air,” and you're absolutely right!  There are fan coils that use outside air, but most of the time when you need outside air for ASHRAE 62.1, you're going to be utilizing a rooftop unit. 

When you have a IDF for MDF space that is not persistently occupied, and you're not terribly concerned with outdoor air quality because you're not bringing in outdoor air, you can typically use a fan coil and that'll be just fine.  Fan coilsmost of the time  are using hydronic coils for heating and cooling, (hydronic is another word for water coils) .…Once again, this is one thing that I'm saying most of the time .  People, love to email me and I even say, “Most of the time, I don't say absolute. So, hey, you're hearing it.    

As I mentioned their are hot water coils and there are chilled water coils.  Typically fan coils will  have a condensate pan and that condensate pan will allow the condensate to drip in it.  It’ll have a float alarm, and if the condensate gets too high, meaning it's not draining, then it'll trip and it’ll turn off the fan. 

Essentially what will happen is, the fan coil will look at the space temperature.  Based on the space temperature, if it is above setpoint, the fan coil will go and drive the cooling valve to maintain supply or zone temperature, depending on setup.  

Now, typically the fan is going to be running, but this isn't always.  Sometimes the fan will only run when it's in cooling or heating mode.  Sometimes the fan will be designed to run all the time.  So, it just depends on the sequence that the engineer set up.  

Okay, so you have this fan coil that’s running and the cooling comes on.  Then, it meets setpoint, and the cooling will close down, and the heating can open up if it goes below setpoint.  That's how a fan coil controls to space temp. Usually the fan coil will have a little bit of deadband between the cooling and the heating setpoint, so you have two separate set points. 

Those set points are typically controlling the zone temp, but sometimes controlling to supply or discharge air depending on what you want to call it.  Personally, I like to call everything discharged air if it's being discharged, and I like supply air to be the incoming air, especially at VAV boxes from an air handler.  It's clearer that way.  Supply air is being supplied to a unit; discharge air is being discharged from a unit. That's how I like to word things.  

As I mentioned, you have a condensate pan, you have a condensate alarm, and potentially a condensate pump.  As the fan coil runs, and as the cooling coil opens up, that cold water is going to cause the discharge air to reach saturation.  Iwe're familiar with psychometrics, the air is going to hit the point at which saturation is achieved, aka one hundred percent relative humidity, and you cannot can't put any more moisture in the air.  

The air stream will start to produce moisture in the form of condensate Condensate will roll down the coil into the drain, and drain out into a pan, which then drains out into another drain. If that pan starts to fill up, the float, or the condensate alarm, will start to float up.  It'll trip, and it will turn off the unit and generate an alarm.  That's typically the sequence of fan coil units.  

Rooftop Units

Moving right along to rooftop units, we begin to notice that rooftop units have a very similar sequence to fan coils, but not necessarily the exact same sequence.  

As I write this I'm reading a single-zone rooftop unit sequence I found on the web and it starts with the wording “A programmable thermostat controls the operation of the rooftop unit.”  

Wow, that's amazing!  [Enter Sarcastic Grin]

“Desired, occupied and unoccupied heating and cooling set points are programmed via the thermostat." This kind of wording is common for rooftop unit sequences.

So if you're about to have a mental lock on thisI'm going to help you outReplace programmable thermostat with controls. There you go.  

Ok, so we have this "programmable thermostat" which really is a BAS controller and the operation of the rooftop unit is controlled by it.  

Then we have occupied and unoccupied heating and cooling set points being programmed via the thermostat, or the controller.  Fan operation is determined by the position of the fan on-auto switch.  

So, if a fan, has on/off/auto switch most of the time, for us, we are going to be in Auto mode.  

Often we will also see the heating and cooling operation described as heat/auto/cool/off.  Most of the time we are going to be an Auto mode as well because we're controlling this with a building automation controller.  

So here's the big difference in rooftop unit control.  With a single zone, rooftop unit, most of the time, not always, but most of the time, these are considered packaged units.  Everything's self-contained and packaged together.  As a packaged unit, it has DX cooling and it also hastypically, gas heat.   

Common Setpoints

Now, what's going on in a packaged unit is that we're driving our unit, once again, based on zone temperature.  Zone temperature is going to drive our heating and cooling, but we also have a supply fan.  

So, if we talk through the unit, we have a heating-cooling system, typically direct expansion cooling.  This is very similar to, if you're familiar with chillers, using compressors.  We basically have a condenser and evaporator side, and we remove the heat.  Then that heat gets transferred to the refrigerant and then the refrigerant transfers that heat out of the air stream into the outside air. 

So, we're removing heat from the return air and transferring it to the outside air that's basic direct-expansion cooling.

We also have either gas or electric heat. Gas heat is simple; we turn on the gas, ignite it, it heats up, and then it heats the air stream, straightforward usually this is done through a single contact point 

Electric heat, is pretty much exactly the same; we introduce an electric signal across some wires that have resistance built into them.  Resistance causes heat because the electrons can't flow through and they start to heat up which then creates heat.

The important thing is that we have airflow when heating or cooling, most systems have an airflow switch interlocked with the heating and cooling systems.

Now obviously, we have supply fan and occasionally we will have an outside air damper and return air damper. It is important to note that sometimes we will only have an outside air damper.  

If we do have an outside air damper and return air damper, they are normally interlocked and opposed.  So, if the outdoor air damper100% open, the return air damper is zero; if the return air dampers 100%, the outdoor air dampers zero. 

We also tend to have a discharge air temp sensorsometimes we will have some safeties in the form of static safeties, as well as low temperature safeties.

Those are typically all the points we have.  

Fan Control

With that out of the way let's talk about the sequence. The sequence is pretty much exactly the same as a fan coil.  The fan might run all the time during occupied, or it might only cycle on when we're in a heating or cooling mode.   

Typically what will happen is the outdoor air damper on the 100% outdoor air roof tops will open up fully.  But prior to engaging the fan, the damper end switch is made (meaning the contact closes to complete the safety circuit or state) to prove that we have the damper open. Only then will the fan command be allowed to trigger. Typically the fan command will pass through that end switch electrically and it will energize the fan contactor and turn on the fan. 

Heat and Cooling Control

The fan runs and the unit will control to space temp.  The unit will drive the heating or cooling either through staged control, in the case of most electric, except for SCR and some DX systems

The heating or cooling will be sequenced on, so it'll be stage one, stage two, stage three, etc.  In the case of gas, it will typically just be an on/off command.   

In addition to controlling temperature we now start to introduce this concept called economizer, which is a way of using outdoor air that is cold so that we don't have to cool down the air.  

Damper Control

There is another way of using the outside air damper, but it isn't called economizing.  It is called demand ventilation. Demand Ventilation is where we open up the economizer to a point that allows the appropriate amount of air fresh air in so that our occupants aren't breathing CO2 and passing outbecause we want them to have fresh air so that they stay awake and they get enough oxygen.  

There's a variety of different ways we can use outside air to control our systems so, I'm going to take you through a lot here.   

One way is by looking at the outdoor air temp versus the return air temp.  If the outdoor air temp is less than the return air temp, we will open up the economizer and let that outdoor air in.  We'll use that in lieu of the cooling coil to actually cool down the space.  However, there's a danger with doing that because we’re not factoring in humidity.   

So what we'll typically do in those kind of scenarios is to use return and outside enthalpy. Enthalpy is the potential moisture energy contained in the air, basically it's a combination of temperature and humidity

Enthalpy tells us the amount of energy in the air because, and I'm just picking numbers out of the air here and not even looking at a psychrometric chart, 80 degrees at 40% relative humidity is going to have less moisture in the air than 72 degrees at 90% humidity.  I'm pretty sure that works out on a psychrometric chart and that’s why I chose some extreme numbers there.  But again, I may be wrong, like I said I don't have a chart in front of me as I write this.

If we look at it from that perspective, it may be more efficient for us to use outdoor air than return air. Then again it may not, ultimately it depends on our setpoint.

Those are the two main ways of enabling outside air control.  

Now on mixed air units outdoor air dampers will either be used to control the air temp, similar to the cooling coil sequence or DX coil sequence or control the mixed air temperature.

Either way on mixed air units you'll go and drive the damper open and shut to control the temperature.  Typically you will control to zone temps, although sometimes you will control to discharge air or mixed air temp, it depends on the sequence.

As I mentioned earlier this assumes that the outdoor air damper is enabled.

Now, in climates that are relatively mild, not very humid, using a outside damper for cooling can work well.  This obviously would not work well in like, Louisiana, where it's super humid.  This would not be a good idea to use this strategy. 

In addition to temperature control, we also have CO2 control, which we will talk about in the air-handling section.  You will see CO2 control occasionally in rooftop units if that rooftop unit is serving a cafeteria, or an auditorium, spaces like that.

Common Control Strategies

Now that we talked about fans, coils, dampers, set points, I'm going to go through some sequence language and help you understand some common control strategies. In this section I assume the Rooftop Unit has a return duct and damper.

As I mentioned, we are going to control our rooftop unit based on achieving a set pointtypically a room setpoint, and the unit will typically be in an occupied and unoccupied. 

If the outside air is not suitable for free cooling, as determined by the outside air enable control mode, which can be dry bulb temperature or enthalpythe outside air damper remains at the minimum position.  

Note the damper will maintain minimum during heating mode as well. 

Why are we remaining at minimum position?  Well we're there because we need to provide that minimum outdoor airflow to satisfy ASHRAE 62.1.  At this point if there is a call for cooling the first stage of DX cooling is engaged.  This is where we're using sequenceblock in our program.   

So up to this point, we have used a comparative pattern to determine if we're in cooling mode.  We're driving that into a PID loop, which is feeding into a sequencer, and we also have a comparative pattern to enable our outdoor air damper.  We're taking a minimum select of whatever the minimum position is because we're not in a state that is suitable for free cooling.  

Upon a further rise in temperature, above the occupied cooling setpoint, the second stage of cooling is engaged if the rooftop has two stages.  Then it just continues on and so forth.  Then, as the temperature falls back down, we disengage the stages in the reverse order of what they are turned on.  

So if stage two was the last one on, stage two is the first one off.  We have minimum on and off times in  our sequencing block and we have minimum and maximum runtimes as well because we don't want to short cycle our compressors.

If the outdoor air is suitable for free coolingas determined by the aforementioned factors, then the outside air and return air dampers are going to modulate to maintain a discharge air temperature, typically of 55 degrees (although on some RTU's we will reset DAT setpoint based on zone temp or we will directly control to zone temp).

When we talk about air handlers, we will talk about why we use 55 degrees as a common setpoint.  We will talk about why that setpoint is used so often and what that means from a psychometric perspective.

If the outside air damper is enabled for free cooling, then we are going to drive our damper to a discharge air temperature, mixed air temperature, or to zone temperature, whatever it’s supposed to be driving to

If the outside air damper is fully open and still cannot satisfy the cooling needs of the space, then it will call for a second stage of cooling which will be the first stage of DX cooling, and hopefully space temperature will fall back towards setpoint.  If not you could potentially see a shift to mechanical cooling mode where the DX continues to control and the economizer will disengage and close down.   

It's really important that you understand not just what the low point for your economizer enable is, but also what the high point is for your economizer disable.  You don't want your economizer open when it's 90 degrees with 60% relative humidity outside and then you're bringing in hot, muggy air.  There is no way your building's going to be able to remove that with your compressors; and you're just going to destroy your units and the inside of your building. I want you to really be aware of that.  

Moving on to the heating portionwhen the space temperature drops below the occupied heating setpoint, the unit is put in heating mode.  

The outdoor air damper remains at the minimum position and the first stage of heat is engaged.  Upon further drop in temperature below the occupied setpoint, more stages of heat are engaged.  If you can't meet the setpoint, then then you're either in a design-day scenario, or you have an undersized unit, or you have some sort of mechanical issue going on.  

This is often why you will see these kind of units actually paired up with perimeter heat, like radiators.  Those will actually form a heat curtain on the exterior side of the building to keep heat loss from happening, usually through the windows and things like that. 


In our next post we will cover air handling units.  We will dive through constant volume units and we will talk about VAV units I believe we will also get to the difference between mixed air and hundred percent outdoor air, also make-up air units. 

As always, if you have any questions or comments, feel free to reach out.  Please ask any questions you have in the comments section below this post.

And as always, I really recommend you check out our Control Sequence Fundamentals course if you are finding this information interesting, especially if you wish it was going a little bit deeper and you really need to know this stuff.  That course will take care of you.  Our course will literally, in a matter of weeks, take your HVAC knowledge from zero to  a level at which you understand pretty much everything you need to know in order to do your job as a building automation professional or as a building automation engineer.  

Alright folks, look forward to hearing from you in the comment section, and look forward to seeing you here again next week. Take care! 

Phil Zito

Written by Phil Zito

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