Sensing a Need for Demand-Control Ventilation

June 12, 2000
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Mike O’Brien loves demand-control ventilation (DCV). The building automation manager with Temperature Engineering Corp., Sterling Heights, MI, a $20 million commercial-industrial design-build mechanical contracting firm, says he’d like to use it on every job the company goes on.

In fact, they do include it automatically with energy management systems (EMS) on all new design-build projects.

O’Brien says that the energy savings and improved indoor air quality (IAQ) make it attractive to most building owners — not to mention the fact that DCV usually pays for itself within a year.

“In an office building, it just makes sense. Those buildings don’t typically reach full occupancy until 9 or 10 in the morning. If the building starts at 7 a.m., they’re running their outdoor air dampers full open when they’re not needed. DCV provides tremendous savings there,” says O’Brien.

He also says it comes in handy for tenants who may be concerned about IAQ. “With a building automation system, building owners can trend log it and say, ‘Yes, we had good air quality on that day.’ Then they have proof if someone says that the building is making them sick.”

What it Does

There’s no question that many more people became more concerned with ventilation rates and IAQ when ASHRAE Standard 62-1989 (now 62-1999) was being formulated and later revised.

With the increased attention on proper ventilation in buildings, some started to wonder whether the old way of doing things — typically constant air volume — was really the best way to provide proper ventilation in a building.

In a constant-volume system, outdoor air dampers are set based on the maximum occupancy for that building. So what happens is whether there is one person or 100 people in that space, the same amount of air flows through the space. This can lead to over-ventilation and wasted energy.

A DCV system uses an air quality sensor (carbon dioxide) to determine the occupancy level for the building space. “When you have one person in that space, you can cut the outdoor dampers down so you’re only bringing in the amount of air that you need for one person,” says Adrienne Thomle, product management team lead for hvac commercial components, Honeywell Home and Building Control, Golden Valley, MN.

“What that does is saves energy for the building owner. It also helps out with the indoor air quality in the building by bringing in fresh air when it is needed by the occupants,” she explains.

“The combination of energy savings and fresh air control when needed is a winning proposition for both the building owners and the building occupants.”

How to Achieve DCV

There are different ways of achieving demand-control ventilation. Some people prefer to use a CO2 sensor that drives an outside damper motor and that damper modulates based on the CO2 sensor output.

Honeywell states that the best way to save energy and give occupants the best air quality is to use an economizer with an integrated DCV system. This type of system can be used on anything from a built-up ddc system to a small, stand-alone air handler.

O’Brien says that the economizer with an integrated DCV system is the best package available for customers. He installed his first one about 14 months ago on a two-story building, where the first floor was a bus depot and the second floor was the dispatch area for the state police.

“The second floor was heavily populated all the time, while the first floor was really loaded at times, and then there would be nobody there. The economizer with CO2 package worked really well,” says O’Brien.

He adds that the nice part about the integrated packages is that costs have dropped dramaticals in recent years. They’re also more reliable.

“Several manufacturers now offer sensors that don’t require calibration for five years. That’s really attracting building owners,” notes O’Brien.

Can Be Installed Anywhere

An economizer can be used just about anywhere in new construction or retrofit applications.

Bob Sundberg, market manager for commercial products, Honeywell, notes that there are probably five million buildings in the United States and not even half of them have economizers on them. “There are several million buildings that do have economizers, but those that do may have a technology that’s 30 years old,” he says.

He adds that some rooftop units may have economizers that have the equivalent of a horsehair for humidity control, or a nylon sensing element that has stretched and broken after a season or two, rendering it nonfunctional. These situations are perfect for a contractor to recommend a new economizer.

“For most rooftop units, retrofitting an economizer doesn’t involve more than two or three maximum hours of actual labor, and the controls that are involved are very simple and very straightforward,” says Sundberg.

The economizer portion of the package is basically there to determine whether or not free outside air can be used for cooling, rather than running the compressor. A solid-state enthalpy sensor is placed outdoors to determine if that outside air is cool enough to use. An air quality sensor can also be added outside to determine if the outdoor air is clean enough to bring inside.

A CO2 sensor is then placed in the indoor space. When the sensor detects a rise in occupancy, it increases the amount of ventilation to the space. If the sensors determine that both the outdoor and indoor air are of poor quality, an alarm goes off, alerting the building owner and/or occupants.

O’Brien notes that he’s had very little trouble with economizers with DCV. His only concern is on very large buildings that have a main air handler. In these situations, there may be an increase in CO2 in, say, a conference room, but by the time the air reaches the main air handler, it’s already diluted to a point where the sensor can’t detect it. “In those cases, you need to put a CO2 sensor on the vav box, or whatever is handling the immediate space.”

Other than that, O’Brien highly recommends offering the system to customers. “Offering DCV has generated more work as far as installing them and getting the maintenance contracts on the building. Even though the sensors don’t need to be recalibrated for five years, building owners want us to check them periodically anyway. That’s helped our service department.”

In addition, there are no other contractors in his area who are offering DCV at the same level as O’Brien. That makes him something of a hot commodity, especially as more buildings become concerned about IAQ and proper ventilation rates.

That’s OK with O’Brien.

Sidebar: Economizer Software Soon to Be Available

If you’re unsure about the type of payback a customer might expect to see with an economizer (with or without integrated demand-control ventilation), Honeywell will soon be coming out with software to help.

The software will take four common building types, identify climate data in 239 cities, and simulate the actual energy usage by those building models.

“The building model’s energy usage will probably vary from a given customer’s building, but the percentage of costs and savings for the building model will give a very accurate measure of how much they are spending and how much might be saved if they employed those control strategies,” says Bob Sundberg.

The software will give users the ability to put in square footage, city, peak demand rates, electrical rates, gas rates, efficiency of the equipment, cost of the retrofit, and occupancy rate in order to determine the payback period.

Honeywell says the software will be available later this year. Check the company’s website for updates at www.honeywell.com/building/components/. There is also more information located there about the company’s economizers.

Joanna R. Turpin

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