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Common Routines in DDC
Part 2 of 2
As we left off from last month, we continue with the time-based strategies, those that are dependent on operating schedules and time-of-day functionality. Seeing as time is not always on our side, let us not waste any more of it!
This routine employs a “learning strategy” in order to optimize itself over time. The goal of the routine is to start the HVAC equipment (air handlers, zoning equipment) prior to the occupied mode, such that the occupied setpoint is achieved precisely at the time the system transitions from unoccupied to occupied mode.
So how does it do that? Well, the routine is typically built into the BAS software, and all you need to do is set it up and implement it. Once invoked, the routine “trends” data such as outside air temperature, space temperature at the start of the routine, and space temperature at the transition from unoccupied mode to occupied mode. The routine actually “learns” from this data, and adjusts the start time each day in order to get closer to its ultimate goal. While perhaps never able to reach setpoint precisely at the transition point, at least on a consistent, daily basis, the algorithm will dial in to a rather precise band of control with little error. Of course as the seasons change, so too will the dynamics of this routine. And as you can probably surmise, the routine needs to be in effect for an entire year before it’s “fully vested”, so to speak. This is especially the case for climates that truly experience all four seasons!
The last of the time-based strategies to discuss, timed override is a feature that allows occupants of a building to override the HVAC system(s) from unoccupied to occupied mode, for a preselected period of time. This usually takes the physical form of a pushbutton at the zone level, incorporated into the zone temperature sensors. Pushing the button of course does nothing when the HVAC systems are in the occupied mode. But during the off-hours, press that button and the systems come to life! All so that you in your tiny little zone can be comfortable whilst you work in the wee hours of the night or on a Saturday morning! Seriously though, with centralized systems such as VAV systems powered by central air handlers and terminal units at the zone level, a push of the button will not only wake up your VAV box, but will also bring on the air handler. It is for this reason that the subsequent strategy (Tenant Billing) has been developed. Of course the benefit to this strategy is flexibility for the occupant, and if after-hours occupancy is commonplace, then the tenants should be willing to pay for this feature.
Implementation of timed override is straightforward. Set up via the BAS, the override buttons at the zone sensors are designed to be acknowledged as a request for after-hours operation of the HVAC equipment. The duration of the override period is also set via the BAS, and is typically 1-3 hours. Once the override period has lapsed, the system will revert back to the unoccupied mode, if it indeed is still scheduled to be in that mode. Pressing the button again will invoke another override period, and pressing the button during an override period will, with many systems, terminate the override period.
This strategy is directly related to the previously discussed strategy. Set up via the BAS, the routine automatically tracks and logs tenant-initiated timed overrides, and generates bills for the after-hours energy usage. Applications include multi-tenant commercial office buildings, and the benefits come in the form of additional revenues for the building owner or facilities management company.
The routine itself is built into to the BAS and simply needs to be set up and invoked. Afterward, the BAS administrator can track the occurrence of timed overrides, log these occurrences, and decide whether or not to bill for the after-hours energy usage. Billing for after-hours activity would certainly need to be negotiated and agreed-upon prior to that first bill, or the tenant could be in for one rude awakening!
To economize is to use outdoor air for “free cooling”. The term, specifically as it applies to packaged rooftop units and air handling units, means modulating the outside and return air dampers to maintain a mixed air cooling setpoint whenever the outside air is “suitable” for economizer operation. Suitable meaning that the air outdoors is both cool and dry. The benefits to this routine is…anyone, anyone? You guessed it…free cooling!
The key to this strategy is to be able to monitor both outside air temperature and humidity. You don’t necessarily want to bring in air that’s suitable for cooling purposes, if only to exacerbate an indoor humidity problem. On the other hand, you can economize with outside air that’s warmer than your mixed air cooling setpoint, provided that it’s dry. If outside air conditions permit, then the outside and return air dampers are modulated to maintain a mixed air temperature setpoint of typically 55 degrees (adj.). If the outside air temperature is below 55, then the economizer has no problem achieving it’s goal. However if the outside air temperature is greater than 55, then what happens is that the outside air damper modulates fully open, and the mixed air “is what it is”: 100% outside air at whatever temperature. Under these conditions, in which the economizer is invoked yet cannot achieve cooling setpoint on its own, the mechanical means of cooling is normally pressed into operation to help out.
When more than one piece of equipment is provided to serve a single system, the equipment is sized in one of two ways: either 1) each piece of equipment is sized to handle the entire system load (primary/backup), or 2) the equipment is sized so that the sum capacities of the equipment will handle the entire system load (lead/lag). Get it?
Okay, simple example is two pumps. If one pump can handle the system load while the other pump sits dormant, then this is the “primary/backup” sizing criterion, where one pump acts as the primary while the other is the backup, in case of failure of the primary. On the other hand, if each pump is sized so that both pumps need to run when there is a demand for full system capacity, then this is the “lead/lag” sizing criterion. In essence, the lead pump is in constant operation under all load conditions, while the lag pump only gets pressed into operation when the system load becomes greater than what the lead pump can handle.
What does any of this have to do with the title of this section? Well, in either case, there is always one pump running. In the case of the primary/backup scenario, the primary pump is always running, whereas in the case of the lead/lag scenario, the lead pump is always running. This pump is logging runtime hours while the other pump sits back and relaxes. Unless some form of “alternation” is implemented, the backup or lag pump will never get the exercise that the primary or lead pump gets.
The goal of automatic alternation is to even out the runtime hours of each piece of equipment (pumps in this example). It is a simple routine that basically changes the status of the equipment (from primary to backup, or from lead to lag), every once in a while. This routine is applied to pumps, boilers, chillers, etc. It is set up via the BAS, such that when the primary or lead piece of equipment reaches a predetermined amount of runtime, then the alternation takes place and the runtime counter resets. And the whole cycle repeats itself. This is good practice for operating such types of equipment for many reasons, and is easily implemented through the DDC system.
|Tip of the Month:
You can do the reset schedule graphically, but you can also do it more
precisely using linear interpolation. The equation is as follows:
Y2 = Y1 + [(X2-X1)(Y3-Y1)] / (X3-X1)
Using the example above under the Reset heading, X1, X2, and X3 are the given outside air temperatures (-10, 25, and 60 respectively), Y1 and Y3 are the given hot water temperature setpoint values (180 and 140, respectively), and Y2 is the hot water temperature setpoint value that we’re trying to find. So did you come up with 160 degrees for Y2? Congratulations, you’re now an official expert on the concept of reset control (not to mention linear interpolation)!
As the term implies, this strategy is aimed at adjusting, or resetting, a setpoint of a control process, based on the measured value of a related variable. Perhaps the most popular application, and the one most commonly implemented, is hot water temperature setpoint reset based on outside air temperature. Other applications include: reset of discharge air temperature setpoint based on space or outside air temperature, and reset of (VAV) supply air pressure setpoint, based on zone-level demand. The benefit to this strategy is improved controllability, which will tend to lead to energy savings, especially in systems in which efficiency losses are inherent due to poor (duct or piping) insulation or loosely constructed ductwork.
Set up via the BAS, a reset schedule is crafted, defining the upper and lower bounds and establishing a linear relationship between the two variables. For example, a hot water reset schedule may be defined as follows: when the outside air temperature is -10 degrees, the hot water temperature setpoint is 180 degrees, and when the outside air temperature is 60 degrees, the hot water temperature setpoint is 140 degrees. For all outside air temperature values between the upper and lower bounds, the hot water setpoint is determined as a linear function of outside air temperature. Get it? Okay, let’s take it one step further with a specific example:
Using the reset schedule described above, what’s the hot water temperature setpoint if the outside air temperature is 25 degrees? If you draw the reset schedule as a graph, with outside air temperature as the X axis, and hot water temperature setpoint as the Y axis, you can then interpolate graphically to find the answer to the question. Okay, so what is it? The answer is in this month’s Tip of the Month.
So to summarize, implementation of reset control, where applicable, results in providing a better match for the HVAC load, which in turn allows for improved controllability, and ultimately results in energy savings.
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