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Originally published in Interiors & Sources

02/22/2013

Economizers and Energy Recovery Wheels: Heads and/or Tails?

Several factors dictate whether one or both of these technologies fits your facility

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    Assessing Optimal System by Climate
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    An economizer utilizes outdoor air to provide free cooling, so it works best in cool, dry climates. It requires an economizer box, sensors, special fan arrangements, and more holes in the building’s exterior.
    courtesy of Richard Lord
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    An energy recovery wheel captures valuable heat and moisture from exhaust air instead of sending it outside. It requires the wheel, proximity to exhaust and ventilation streams, and additional ductwork. Both technologies have enormous potential for energy savings.
    courtesy of Richard Lord
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Economizers and energy recovery wheels are alternative energy strategies, but choosing between them isn't like flipping a coin — the two components are actually two sides of the same coin.

Both technologies have enormous potential for energy savings: economizers because they utilize outside air, and energy recovery wheels (also called "heat wheels") because they don't waste exhaust air. But that potential will be maximized depending on the status of your existing HVAC system, the climate of your location, and your building's efficiency.

"Energy recovery wheels and economizers go hand in hand," says David P. Callan, senior vice president of consulting engineering firm Environmental Systems Design, Inc. "But every building is different. They're like people – no two are the same. To implement either system, there's a lot to be done."

To Retrofit or Replace?
The energy savings alone of either technology can justify a retrofit, although doing so gets complicated. Retrofits require significant space considerations for either type of equipment. Economizers also need special fan arrangements and new holes in the exterior, Callan explains, and energy recovery wheels would necessitate more ductwork.

Both likely require more sophisticated controls, such as a digitally controlled system with a computer front end to perform calculations that indicate when to turn the equipment on or off, Callan adds.

"If you want to implement either system, you should look for an opportunity when you're at the end of your HVAC system's useful life," he says. "If you already have to replace that system, then the cost of integrating these technologies is much less, the payback is maximized, and it's less complicated than retrofitting an existing system."

Pick Low-Hanging Fruit First
Conduct an energy audit or evaluation to begin the decision process of whether to implement these technologies. That will tell you whether it's feasible to take such a significant step toward energy efficiency or if there are smaller hurdles to jump first.

"For new buildings, ASHRAE 90.1 and many codes already require economizers and to investigate and install energy recovery in a lot of cases," Callan says. "For existing buildings that don't have these technologies and are considering them, it's a serious commitment."

Your HVAC load depends on several other building systems, such as how efficient the lighting or envelope is. The first step is likely making sure your entire building system is up to snuff. A consulting engineer can tell you whether there is low-hanging fruit to pick first.

"An engineer will know if the system has five years of usable life left and if you should replace ballasts, sensors, or a couple of valves first," Callan says. "Engineers can perform an evaluation for little or no cost, spend an hour with the owner, look through the building and at some climate charts, and determine if the case warrants further investigation."

Once your building is tweaked and fine-tuned, this investigation will analyze your existing HVAC system and perhaps take an energy model of the building. After that, you can determine if an economizer or an energy recovery wheel – or both – is right for your facility.


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