December 2009

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Common Routines in DDC
Let the system work for you!

Part 1 of 2

Steven R. Calabrese
Steven R. Calabrese
Control Engineering Corp.

Contributing Editor

Recently I was doing some thinking about how I can better serve my customers. I wholly believe that knowledge is power, and I also believe that a knowledgeable customer is one that is better informed as to the decisions he or she must make in terms of purchasing my goods and services. Therefore it is my responsibility, and thus part of my role in the sales process, to bestow knowledge upon my customers to the best of my abilities, in that they may make an educated decision that will ultimately lead to a win-win deal between purchaser and seller (them and me). 

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So I got to thinking about how my products and services, namely the Direct Digital Control (DDC) systems that we furnish and install, can truly benefit the end-user, aka, my customer. Hence I put together a spreadsheet encompassing all of the common DDC routines that we customarily offer and often implement with our systems, extolling the value of the installed DDC system, and offering suggestions on how to fully exploit the utilities that come “part of the package” so to speak, yet oftentimes aren’t invoked or even thought about, for that matter.

This column, the first of a two-part series, lists those routines that I included in my spreadsheet, but of course does so in text form, such that I might add some “words of wisdom” here and there, and even attempt to inject some light-hearted humor, to keep the subject matter interesting and readable (I shall do my best!). 

Demand Limiting 

As the name implies, the focus of this strategy is to limit energy use by monitoring the energy demand, and then automatically adjusting equipment operation to limit the demand and reduce operating costs. Applications are far-reaching, from commercial office buildings to manufacturing facilities and warehouse structures. Benefits of course are energy savings, which mean reduced operating costs, as long as occupant comfort isn’t compromised too much and operating processes aren’t jeopardized.

Set up via the DDC or Building Automation System (BAS), the routine is to recognize an event, such as a signal from a power meter. The BAS, in response to this event, sheds its electrical load by turning off certain equipment, limiting the capacity of the equipment, or letting temperatures drift from setpoint.

Demand Response 

This is kind of a new thing, but the concept is picking up steam and soon enough will be like any other idea that starts out “pie in the sky”, but is then realized upon introduction, implementation, and finally acceptance, of the burgeoning technology that supports and facilitates the idea behind it.

Similar to demand limiting, this strategy utilizes the current advances in technology to take advantage of the electrical “spot market”. Specifically, Demand Response (DR), is the process by which a facility will automatically avoid peaks and shed its electrical load in response to an “energy cost signal” from the Internet. Its applications range from commercial office buildings to process-heavy manufacturing facilities, and its benefits include energy savings, reduced operating costs, and even rebates from the utility, if in fact the local utility has such a rebate program.

So like demand limiting, the DDC system is set up to shed its electrical load, in response to an event, by limiting the equipment capacity or by adjusting operating setpoints, hopefully without compromising process efficiencies or occupant comfort. Unlike demand limiting, the event that DR acts upon is not a change in demand as might be evaluated from the facility’s power meter, but actual real-time data gathered from the Internet, relating to the current energy costs. So the routine is set up to use this data, take advantage when energy costs are low, and shed electrical usage when prices are high. A very dynamic strategy, and one whose time has come (or we certainly hope so!).

Demand Controlled Ventilation 

Demand Controlled Ventilation (DCV) is a strategy that’s been around for awhile, however its applicability (is that a word???) has increased in recent years, due to advances in technology and the ever-decreasing costs of carbon dioxide (CO2) sensors. In a nutshell, DCV is controlling the volume of outside air delivered into zones by monitoring the demand for fresh air at the zone level, typically by monitoring the CO2 levels in the zones. It works well when there is occupancy diversity among spaces within the same facility, such as with schools and theatres. The main benefit to implementing DCV is energy savings, resulting from introducing and conditioning less outside air, during periods of reduced occupancy. The perception sometimes is that DCV will also improve Indoor Air Quality (IAQ), however the real story in many cases is that, prior to the implementation of DCV, the IAQ is adequate due to the air-delivery systems bringing in an over abundance of outside air, so invoking a DCV strategy, while certainly improving system efficiency and reducing energy usage, won’t necessarily bring about an improvement in the overall IAQ. 

The key to this strategy is monitoring the CO2 levels in the spaces. This can be done zone by zone, or by monitoring the return air CO2 level, application permitting. CO2 is a human by-product, and a reliable indicator of human occupancy. When levels are low, the opportunity exists to cut back on the amount of outside air being brought in by the air handling equipment. 

Automatic Time Scheduling 

The first to be discussed of the “timed-based” family of strategies, this is nothing more than scheduling operating modes of equipment based on time-of-day and occupancy. In its simplest form, this means turning equipment on during occupied modes, and turning it off during unoccupied modes. There’s typically more to it than that, as we’ll explore under the next few headings. Applications include all manner of HVAC equipment, particularly airside and zone level equipment, as well as lighting and other building systems. Benefits include energy savings and optimized building control. 

Reliable Controls Night Setback/Setup 

Another timed-based strategy, this concept expands on the simple time-of-day, turn-on-and-turn-off strategy described above. An unoccupied mode routine applied with air handling systems and zoning equipment, the strategy entails automatically changing the heating (setback) and/or the cooling (setup) setpoints based on the time-of-day schedule. Upon transition to the unoccupied mode, supply fan operation changes from continuous to intermittent operation (meaning that it only comes on when there is a call for heating or cooling), and the heat and cool setpoints are “spread out”. This routine takes advantage of shutting down equipment during unoccupied periods, but builds in the “safety feature” of not allowing the spaces to become too warm or too cold during these modes.

Implemented via the BAS, the routine is typically built in to the software, and simply needs to be set up. The heating setpoint is decreased (to 60 degrees adj.) and the cooling setpoint is increased (to 80 degrees adj.). The fan mode is set to cycle on upon calls for unoccupied heating or cooling, and will cycle off when the call for heating or cooling is satisfied. 

Morning Warm Up 

For multizone air handling equipment, particularly VAV and reheat system air handlers, those of which serve many zones and function to provide cool air to the zoning equipment (VAV boxes, reheat coils) during normal occupied modes, the Morning Warmup cycle is a tradition that dates back many, many years. Nowadays more optimized routines exist for transition between occupied and unoccupied modes, but the good old MWU is still a viable strategy. The basic idea behind it is to bring on the air handling unit, prior to the occupied mode, in a full-heat capacity, in order to warm up the general spaces served, before going into normal occupied “cooling” operation. 

This is a routine reserved for the cold weather months, and its benefit is primarily occupant comfort. Set up through the BAS, the air handler (if equipped with heat) receives a command to turn on and go to full heat. VAV boxes are commanded to go fully open, and those equipped with heat may be allowed to engage. The system typically operates in this mode until the return air temperature, or an average of zone temperatures, reaches the morning warm up setpoint (70 degrees adj.). 

It is notable to mention that for single-zone equipment, such as packaged rooftop units operated by conventional thermostats, no special morning warm up mode is really needed. If the temperature in the morning is too cold, then the unit, upon transition to the occupied mode, will enter a full-heat mode, provided that the temperature in the space is far enough from heating setpoint. If the general space temperature is not at setpoint by the time the occupants start coming in, then simply adjust the time-of-day schedule so that the transition from unoccupied to occupied mode occurs a little earlier…or tune in next month and learn about Optimal Start! 

Tip of the Month: Do what I do! Keep a little “cheat sheet” of these routines, for quick reference when you’re “on the go”. Program them into your cellular device for easy access, and add to the list as ideas come to you. Stay tuned as next month I continue this series with another half-dozen strategies to cover!

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