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Trends in HVAC Control
And bucking the trends via the BAS…?
Funny how temperature control specifications have evolved over the
years. Was a time, of course before my time, when each and every system
was unto itself, completely stand-alone and, with the exception of some
global supervisory controls (OAT changeovers, time-of-day scheduling),
there wasn’t much in the way of centralized control. Fast forward a
generation or two, and the era of digital control is ushered in with
the notion that everything can and will be controlled digitally and
supervised by a Building Automation System (BAS).
Nowadays so much of that is still the case, however recently (and recently meaning over the last ten years or so) I’ve noticed several trends in HVAC monitoring and control, trends that in a sense, have taken away from the BAS and given back to the equipment manufacturer or some third party product line. In this column I’ll discuss just a few of these trends, the how’s and the why’s behind them, and the prospects of the BAS contractor “taking back” these systems of monitoring and control.
Control of Multiple Boilers
Was a day when controlling multiple boilers within a common hot water heating system required some trickery. Electromechanical controls and relay logic were employed to operate boilers sequentially to maintain a common hot water temperature setpoint. Boiler lead/lag alternation was either done manually, or performed with some additional, complicated ladder logic.
Then came the packaged boiler sequencer panel, which could control up to four boilers (and maybe more with additional “cascaded” panels), including boiler staging and burner modulation. These were pre-engineered “specialty” products that were manufactured by a third party, designed to control a boiler plant centrally, albeit in a stand-alone manner.
With the advent of distributed direct digital control, the need for these panels became supplanted, with digital controllers being able to accomplish virtually the same functionality as the manufactured boiler sequencer panel, all within the networked BAS.
Nowadays, with the various communication protocol standards in place, these boiler sequencers are back “in vogue”, at least as far as the consulting engineers are concerned. Oftentimes these days I run across the line “Provide manufacturer’s boiler sequencer panel” to centrally control the boilers, followed by a statement regarding BAS interface to the panel via a standard communication protocol. So a manufacturer-specific boiler sequencer panel, designed specifically for centralized control of multiple boilers, coupled with the requirement of being able to communicate to the BAS via network connection.
Monitoring BTU consumption is a relatively new concept, again relative meaning over the past ten years or so! Required components include a flow meter that measures gallons per minute (GPM), and a pair of temperature sensors, one installed on the main supply and return water lines, be it a hot water (boiler) or chilled water (chiller) plant.
Products manufactured specifically for this task include the package of parts (flow meter, sensors, and separate control panel with display), and the fully self-contained model, that builds the supply temperature sensor into the same unit that houses the flow meter, which also has an on-board display.
To get the BTU information into the BAS would traditionally require an output signal from the meter that would feed an input on a digital controller. From there, the BAS would have the BTU information, however would not necessarily have any of the other information that the outboard BTU meter is collecting for its internal BTU calculation.
In the present day, we’re now more apt to connect to the BTU meter by any one of a number of communication protocols, as most of these devices are now offered with that option. In doing so, we get not only the BTU calculation, but also the flow rate and temperature readings of the supply and return mains, which incidentally can be used in other calculations and for other control strategies, thus effectively eliminating the requirement for additional (redundant) sensors that wire in directly to the BAS.
A refrigerant monitoring system is often required in water cooled chiller applications, wherein the chiller bundles are located indoors, and hence all the refrigerant as well. In the old days you were likely to find a single point refrigerant detector that would annunciate a refrigerant leak and also shut down the chiller. Simple, yet effective.
Nowadays we have these fancy “systems of components” consisting of a central panel, remote sensors, on-board and remote visual and audible alarm annunciation (strobes and horns), and all that, packaged and quoted as a single system by a single-source vendor. Add to that the option to communicate directly with the BAS via standard protocol, and you have yourself a nice, yet pricey, refrigerant monitoring/alarm system that conforms to local and national codes and guidelines. Let the manufacturer’s purpose-designed control system do all the dirty work, and communicate status and alarm conditions to the BAS via the network cable.
Bucking the Trends – Value Engineering
So why can’t we handle any of the afore-mentioned control system scenarios entirely within the BAS? As a controls contractor, my company asks these questions on a regular basis, especially when we’re bidding a project and are offering “value engineering” alternatives. “Hey, we’ll control that boiler plant via the BAS”, or “We’ll monitor water flow and temperatures, and perform the BTU calculation within the BAS”. Or finally, “Yeah, we can eliminate the purpose-built refrigerant monitoring control panel, and perform all of the monitoring and alarm functions through the BAS!”
There is a myriad of reasons that these self-contained systems have propagated the industry and are commonly specified. Traditionally, purpose-built control systems have all of the engineering and R&D time put behind them, and so they’re proven to work. Consultants like that these systems are “pre-engineered” and have been tested and certified. And now that with most of these types of systems you have the option, or even as a standard offering, of a communication interface, it’s almost a slam dunk.
As for me personally, I straddle the fence on the subject; on the one hand, I believe that there is value at looking into having the BAS perform these functionalities (if done correctly), and there’s money to be saved by offering this up as an alternate. On occasion, the consulting engineer will even build that into the specification, stating that the BAS contractor “provide an alternate to perform all facets of specified monitoring and control via the BAS”, or something to that effect.
On the other hand, having a pre-engineered solution on hand is not a bad way to go. The manufacturers have gone the distance, to ensure a design that’s proven, and maybe more importantly, not readily subject to intervention. And the consultants have bought into it, and quite frankly for good reason feel comfortable with it.
Tip of the Month: Design a primary/standby pump sequencer panel! The intention is to build a purpose-specific control panel that will alternate two full-sized pumps automatically on a run-time basis, and bring the standby pump on in the event of primary pump failure. In the old days I’d use relay ladder logic to get it done, and wire in a single flow switch to the panel to determine failure of the primary pump. Nowadays use a digital controller with inputs to accommodate current sensing switches on each pump motor, and outputs to control the pump motor starters. Build the logic within the controller, to perform all facets of pump sequencer operation. Or just go out and buy one that’s got a communications option!
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