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Hello and welcome back!  This week, we have some great topics planned for you.  So, we had planned to continue our HVAC controls series by going through chilled water systems, but we're going to flip things around a little bit and cover hot water systems. 

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Right now, at the time I'm writing this, it is January 21st, and it is a balmy 20 degrees outside, which means we are in heating season - at least, most of you are in heating season - and you may be wondering how to control hot water systems. Well, my friends, that's what this week's post is going to be about.  

We're going to begin by discussing hot water concepts.  From there, we're going to go through a single boiler sequence, then a multi-boiler control sequence, and finally we're going to go through steam control.

If you find this post valuable and you'd like to learn how to control HVAC systems with your building automation system then check out our Control Sequence Fundamentals Course.   

Hot Water Concepts 

Hot water is a just another mechanism for heat transfer.  Ultimately water itself is a mechanism for transferring and absorbing energy. We have to realize that energy, mainly heat, is rated in BTU’s.  BTU's or British Thermal Units are a measurement of energy. 

When we are either removing heat from or transferring heat to a space, we are transferring energy.  Whether it's the air stream absorbing energy from our heating hydronic system, or it's our cooling hydronic system, absorbing energy from our air stream.  

Hot water systems utilize multiple forms of heat generation and then transfer that heat to the hot water system.  That hot water then is distributed throughout the building either through radiators, coils, or heat exchangers.  There's a variety of different distribution methods and we'll talk through all of those in this post.  

So heat, as I mentioned, is a form of energy, and this energy is measured as British Thermal Units.  I’ll use the terms BTU’s and British Thermal Units interchangeably throughout this post, so if you are not aware, a BTU/British Thermal Unit is the amount of heat required to raise the temperature of one pound of water by one-degree Fahrenheit.  

Why water?  Why do we want to use water?  Well, if you've ever worked on air-side systems and tried to transfer heat via electric reheat, you'll notice it's very difficult.  It takes a lot of energy to transfer heat to an air stream.  

Now imagine if you were transferring heat to an air stream and then sending that air stream across a campus from a central utility plant to a building that's maybe half a mile away.  You'd send this heated air on it's merry way, but how much heat do you think would be lost by the air stream?  Probably a lot.  

The beauty of water and steam are that they can contain massively more energy, in the form of BTU’s, than air.  Air is the least efficient transfer mechanism and steam is the most efficient.  Stream unfortunately creates corrosion and requires special equipment and special control sequences. This makes steam more useful as a heat distribution source rather than an actual control mechanism.  In this case, you'll have a central utility plant that produces steam and it will distribute that steam from the central utility plant to individual buildings.   

Hot water is more commonly used for temperature control. The hot water will be supplied to coils and your control systems will control the temperature in the air stream utilizing hot water.  So where do boilers come into play and what are boilers? 

As we'll discover in just a second, boilers are mechanisms that transfer BTU's from a fuel source into the water or steam, that is then delivered throughout the campus or building.  

Basically, you create heat by combusting fuel, gas, coal, or wood, or you create heat via electrical heat.  Regardless of what kind of heat you use, the heat transfer process is the same. Lower temperature return water enters the boiler, it's heated up, and then it's supplied out to the buildings and/or to the building systems themselves.   

Typically the boiler has a burner that will combust its fuel source and create heat.  The hydronics loops water flows through tubes and the heat raises the temperature of the water inside the tubes.  The tubes then supply the water to the main pipe where the pump then pumps the water into the building or into coils. 

When we're working with boilers, we need to be cognizant of several different constraints. Don't worry we'll cover these later in the post.  

For example we need to be aware of the minimum outlet temperatures and maximum outlet temperatures.  We also need to maintain a temperature drop across the supply and return. And a very important point is the need to slowly warm up boilers which affects secondary control, morning warm up, multiple boiler setup ups, running things in unison, doing lead-lag with peak control, and many other tasks.   

Speaking of secondary hot water temperatures,  secondary hot water temperature should be managed via piping, not via boiler control.  That's something that a lot of people tend to misunderstand, and we'll talk about that once again, later in the post.  Lastly, lead-lag is different than chillers, but how you do lead-lag with boilers, is a little bit different.

In some cases, it’s similar and in others it’s different.  

Single-Boiler Sequence 

We're going to start with a single hot water boiler by focusing on the three things we need to have control of.  This is really important to understand because when you're troubleshooting or programming boilers, if you don't have control of these three things, you're not going to have very precise control and, in some cases, you will not be able to get the system to run.  So, the three things are: 

  1. Flow 
  2. Fuel/power 
  3. Control 

Flow

Flow is critical because without flow, we can really mess things up.  We want to know, where does flow come from?  In order for us to heat up the water, we have to have water, and if we keep heating the same water and we don't have flow, then we can actually hit our high limit on our boiler and cause problems, especially if that limit safety fails for some reason. In order to have flow we need our isolation valves open and our pumps turned on.  

Fuel/Power

Now that our pumps are turned on and our isolation valves are open, we need a way of heating the water, and this is where a combustion source comes into play.

Several years ago, I was working at a hospital in Dallas when a rare freeze came through.  Everyone in the city had their heat turned all the way up and because of this, the overall gas pressure had dropped.  However we didn't notice this because we're getting enough gas pressure to enable the boiler on low fire, but as soon as it kicked on high fire, it would trip off.  It was only after a couple hours of troubleshooting that we thought to look at the incoming pressure.  

This is why I say understanding flow, fuel, and control is so important because you need to think through these processes and troubleshoot in that perspective. 

Control

Next we move to control.  There are really three things we have to control with a single hot water boiler: 

  1. Boiler enable (make sure the boiler is on) 
  2. Ensure the pump is on 
  3. Ensure we have a supply setpoint 

Sometimes we will have a setpoint and sometimes we won’t.  Boiler enable's typically only require a RIBU1C or similar device to pass the start signal to the boiler via set of contacts.  We tend to interlock flow status with the boiler enable. This becomes important because we will enable the pump and  as long as we get flow status we will enable the boiler.  Sometimes the boiler has its own flow status as well as its own circulating pump.   

Finally, we have the hot water setpoint.  Sometimes setpoint control is maintained via a two-wire control setpoint, and other times the setpoint will be controlled via a BACnet or Modbus interface card. We also can manipulate the hot water setpoint by controlling the fire rate of the boiler.  

There are typically two hot water setpoints. We have the primary hot water loop which we try to maintain at around 180 degrees.  Then, if we have a secondary loop, we will mix that hot water into the secondary loop, and the secondary loop setpoint will be controlled either based off space temp reset or an outdoor air reset.  Sometimes the primary loop temperature will also be controlled off an outdoor air reset. It just depends on your building profile is, how the loads affect the building, and how much heat loss you're going to have in the building envelope itself.  

In regards to secondary loop control, there's really two ways we're going to do that.  It will either be through supply temperature reset or mixing valve reset.  So, we're going to look at those two.   

Single Hot Water Boiler Control

First in our hot water sequence with a single hot water boiler. We will enable the boiler whenever the outside air temp is below 60 degrees.  Programmatically this is a simple comparative logic function where we compare outdoor air to this arbitrary adjustable setpoint.  Once that happens, the boiler is enabled, and the primary pump is scheduled to run.   

You’ll notice this is kind of inferring that there's a lead-lag scenario, which is typical of boilers and chillers. You’ll have a primary pump that runs as lead, and then you have a lag pump that will come on in the case that primary pump fails.  The lead and lag are both individually sized for the GPM requirements of the boiler or chiller and are rotated either on runtime or based on start count.   

Once the pump is running, if it fails, the backup pump will start, the boilers will operate based on proof of flow and an enable command. The hot water setpoint will be reset based off outside air.  As outside air increases the hot water set point will decrease, and vice versa.  As I mentioned, this is typically across a 180 to 220 degrees reset.  

Multi-Boiler Control 

Now we move into multiple boilers, which is pretty much the exact same sequencing as single boiler control.  The primary difference is how we enable the boilers.  You have three main enable methodologies unison controlstage control and lead-lag control.   

Unison Control

Unison control is where both boilers, or all the boilers, are run in unison.  This really helps you to not have to worry about bringing on a boiler and having to have that boiler run and hit low fire before it can be added to the loop. 

If you enable a boiler you don't want to be running cold water through a boiler, or introducing cold water into a loop, or even introducing very hot water into a boiler and causing sweating.  

Unison control largely avoids those concerns, but it assumes that the boilers are the same size and that the water is the same flow rate. Unison control does not work well when boilers are different sizes and different flow rates. And unless the system is designed to handle that you could start to lose some efficiencies. 

Staged Control

Staged control, which should be fairly familiar to any of you who have ever controlled DX coils or controlled staged reheat.  Programmatically you're using a sequencer logic block that is driven by a PID loop to sequence on boilers.  This is a “last on, first off” sequence, and the output of the PID loop, will drive on boilers as this threshold exceeds certain enable thresholds.  There is also a min/max, off/on, and min/max run time. You'll gradually stage on boilers, and then gradually stage off boilers based on these timings.   

Lead Lag Control

Finally, you have lead-lag control which is very similar to pump lead-lag control except for we're doing this with the boilers.  Now, if we fail to meet setpoint in a purely lead-lag scenario, nothing will happen.  I want to be clear on this, as it may confuse some people.  

There is another version of lead-lag, which is peak-load conditioning.  With peak-load conditioning if you're not meeting setpoint, the lag device can match the control conditions of the lead device, which is very similar to unison control. 

This is mainly meant to help on really cold days.  Where I’m located, we have extremely cold days that could drop to –50 degrees Fahrenheit.  In that case, you’ll need all boilers running completely to ensure you are meeting your heating setpoints. 

Let's take a look at a sequence to drive all this home.

In this sequence, the hot water system is enabled for operation when the outside air temp is below 60 degrees.  Each boiler has a circulating pump associated with it that circulates water inside the boiler.  It's primary pump will then draw water from the circulating loop into the primary loop.  

These boilers are going to be focused on maintaining a hot water temperature setpoint.  If the load exceeds the capacity of the lead boiler, then the lag boiler is pushed into operation.  Once it's enabled, and the circulation pump is running, it will be operated to maintain the main boiler setpoint.   

Even though, we're driving off the same setpoint, but each boiler is going to have its own individual control.  It'll control its own fire rate, driving to the primary loop setpoint.  Once the demand drops below the capacity of a single boiler, the lag boiler is disabled.  

To control this way requires a form of capacity calculation. Capacity can be determined via a BTU calculation or by simply looking at how far below setpoint are we.   

I personally like to use BTU calculations for capacity control as I feel it's more accurate.  That being said driving boilers off BTU load calculations is harder than simply driving to a setpoint.  Temperature setpoint is always going to be easier because there are less calculations.  

In a primary secondary loop control sequence we feed the hot water into a three-way control valve, also known as a mixing valve.  The mixing valve is going to mix the primary loop hot water with the secondary loop hot water.  The secondary loop is going to use the mixing valve to control to a setpoint between 140 and 180 degrees based on outside air temperature.   

The reason why we want to use around 140 to 180 degrees is because, just like with steam, we could potentially overshoot our setpoint and overheat our spaces because of how many BTU’s we are able to transfer into that air stream. 

In multiboiler applications our primary pumps are typically going to be sized for full capacity on the primary side, and on the secondary side we will typically have a differential pressure sensor at the end of the pipe run.  We will have VSD’s or VFD’s that are controlling to that differential pressure sensor which is going to increase as valves close and decrease as valves open.  Thus, as more valves open, the differential pressure is going to decrease, and our pumps are going to increase their flow rate, in order to account for the pressure drop across the coils.  

Another thing that you'll see in some hydronic hot water loops, that you don't normally see in chilled water loops, is air intake dampers being interlocked with operations of boilers because most boilers utilize combustion as their primary source of heat production, whereas chillers are utilizing the refrigeration cycle, compressors, and evaporators. 

What do we need for combustion? 

In most cases, we need some form of oxygen in order to combust the combustible material and create heat.  Thus, we need to make sure that we have opened the  air air intake dampers so that the boiler can then draw air into itself for the combustion process.  

Steam 

 Steam can be very dangerous as I've seen it cut through brooms, and even burn people just by getting their hands too close to uninsulated piping.  However, steam has amazing heat transfer and capacity capabilities.  

The reason why is has to do with the capacity of different states of matter to contain energy.

It takes 180 BTU’s to heat 32 degrees water to 212 degrees.  If you take freezing water and you heat it up to boiling, it takes 180 BTU's to do that, one BTU per pound of water.  It takes 970 BTU’s just to change one pound of 212-degree water into 212-degree steam.  That's four times the amount of energy to perform a state change from water to steam at the same temperature, and therefore you see steam utilized on a lot of large campuses, because you can pump so much energy into the same mass of medium.  

Steam contains 970 BTU’s in one pound pf steam.  That's awesome, but it's also why it's so dangerous because it transfers so much heat and it's also why it's very hard to control.   

Steam comes in two forms: 

Dry steam and Wet steam.  We want to avoid wet steam as it will diminish the latent heat that we need for the heat transfer process.  Moisture makes it very difficult to transfer energy and since we are using steam to transfer that energy we want dry steam.  Dry steam is going to contain the most latent heat for us, so that we can utilize heat exchangers to transfer heat.  

We also are going to be focused on low pressure.  We do not want high pressure.  High pressure is used for motorization, steam turbines, etc.  We're looking at low pressure, as we want our steam pressurized, but we only want it a low pressure because then we don't need crazy valves, coils, or heavily reinforced systems.  We’re not using steam to drive things; we're using steam to transfer energy.  

Heat transfer is either done through a heat exchanger, typically a tubular heat exchanger, not a plate heat exchanger, or it's utilized directly via a coil in an air stream.  Coils are more for pre heat on large 100 percent outdoor air units in very cold climates.  

For both heat exchangers and coils we will use one-thirds and two-thirds valves. As you can infer from the name these valves provide one-third of the steam, and then the other is two-thirds.  Now, due to steam containing so many BTU's, you will get most of your heat transfer when you open that one-thirds valve.  Steam is pressurized, so you're going to get a lot of heat transfer right away.   

Programmatically the one-thirds valve will open, based on the PID loop, then it'll close if the setpoint is not being met, and the two-thirds will open.  If the setpoint is still not being met, the one-thirds will open after the two-thirds is fully open. That's how we typically do valve control with one-thirds, two-thirds coils.

A typical steam loop is either going to have a scaling strategy or a separate loop strategy.  A scaling strategy is where you have a single-process loop and you're going to scale the one-thirds, two-thirds valves off that.  Or, you can have a separate loop strategy where you can have the one-thirds valve that is driven off a PID loop, and then you will have the two-thirds that will open afterwards.  

Personally, I don't prefer the separate loop strategy as it's just very difficult to control.  I prefer the scaling strategy. 

One of the things we deal with, with steam, is condensate because when we transfer the energy out of steam, it's going to change state back to water. 

We will typically have a condensate pump at the bottom of the steam coil to pump out any condensate, and that condensate will typically be reclaimed and then utilized either for heating water or for domestic water heating.  There's a variety of ways you can utilize condensate, or it may just be returned to the steam loop via a condensate piping.  The most important thing is that we have a condensate pump and we're removing condensate from the coil or the heat exchanger.  

Let's discuss a steam sequence of operations and then we'll close out this post.  What's going on here is we have a tubular heat exchanger. What happens is, the heat exchanger is going to be enabled whenever an air handler is on. 

The heat exchanger is being utilized for a building, typically, it'll be sitting in a building and then the steam will come into it and you'll have a primary hot water or secondary hot water loop leaving the heat exchanger.  The tubular heat exchanger will have one-thirds, two-thirds valves that will open based on a hot water supply temperature for the hot water loop.  

So, we're basically doing the exact same that we would do with a steam coil setup, but we're doing that with a hot water loop.  

Conclusion

There you have it, a pretty deep dive into hydronic hot water systems. I hope you enjoyed the post and if you found this post to be good and beneficial, then I encourage you to check out our Control Sequence Fundamentals Course.  Thanks a ton, and I look forward to next week when we dive through chilled water systems.  Take care!

Phil Zito

Written by Phil Zito

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