What’s an economizer? If you’re asking that question, you are not yet an energy efficiency fanatic. That’s okay. This post is for you. Economizers are nifty energy saving systems. They are not a device, like a motor or a lamp. And they do not pontificate about Gross Domestic Product. Rather an economizer is a system of actuators and dampers that regulate air conditioning systems based on outdoor temperatures.
What does that mean in English? A damper is just a metal plate that sits in front of a vent and controls how much air comes into or out of it. They often look like Venetian blinds for your vents. An actuator is a device, often hydraulically controlled, that adjusts how open or closed the dampers are, thereby controlling how much air flows in or out.
Why are economizers important? An economizer enables a rooftop air conditioning unit to decide whether it wants to suck in fresh outdoor air or use return air from inside the building. Return air is the stale, warm air that sits inside a building and gets sucked out through exhaust vents. When it’s cool outside, using fresh outdoor air instead of mechanically cooling return air saves a huge amount of cooling energy!
How does the economizer “decide”? Economizers are hooked up to a range of temperature and other sensors outside the building and inside the return air ducts. If the building is requesting air conditioning, the economizer compares temperatures (technically it compares enthalpies, but we’ll ignore that for now), inside and outside the building.
In such a situation how does the economizer actually work? If it’s cold enough outside to cool the building with outdoor air (e.g. 60°F outside), then instead of mechanically cooling warm, stuffy 70°F air from inside the building, the economizer opens the outdoor air dampers wider so that more air can come in (like opening your venetian blinds to let more natural light through your window). An example of a working economizer is shown below. The economizer below sees outdoor air above 70°F, so it has closed the outside air dampers and it is recycling return air from inside the building:
But why would a building need air conditioning if it’s only 60°F? For many reasons. South facing rooms may trap large amounts of solar radiation that cannot escape (think of a greenhouse). Interior cores of buildings build up thermal mass. Human beings radiate heat and moisture, especially when you pack them into cubicles. Computers, printers, and copiers suck up electricity to operate convert it into heat in the process. Close your eyes – can you hear your computer’s fan whirling? That’s because it’s generating waste heat.
What happens if it’s hot outside? If, for example, it is 85°F outside, then the outdoor air dampers are set to the minimum required outdoor air (20%) and the 70°F return air is re-used, cooled, and pumped back into the building. Minimum outdoor air requirements exist to make sure enough fresh air is being pumped into a building.
Why doesn’t everyone use economizers? Economizers are great for cool and temperate zones. They save lots of energy and money when the temperature is between 55 and 70 degrees and buildings need cooling. They’re not as useful in hot and humid climates. In Florida or Arizona, most of the air conditioning season is well above 70°F. In California, where much of the climate is more moderate, Title 24, the building energy efficiency code, requires economizers on all HVAC units above 75,000 BTU/hr and 2,500 cubic feet per minute (by way of comparison, the average home HVAC unit produces anywhere from 5,000-20,000 BTU/hr of cooling).
What’s the problem with economizers? How did you know there was a problem? Who told you? As it turns out, economizers break all the time. Because they consist of so many moving parts that may change positions dozens of times in a single day, economizers often break after five years. It has been estimated by a variety of studies that about two-thirds of all economizers in the field are broken.
What does a broken economizer look like? Funny you should ask. We happen to have a picture from a recent energy study below. Here, the outside air damper has fallen off completely and is blocking the return air damper.
The consequences of this are many and all negative. For one, since the return air duct is blocked, that air is now trapped in the building and pressurizing it; it will find its way out through doors, windows, or meander its way to other return vents. Or it may make it difficult to open doors inside your building. More importantly, lots of energy is being wasted. In the summer, when it’s 80°F outside, the unit is cooling 80°F air all the way to 55°F (this is called the “cooling supply air temperature”), instead of cooling the 70°F return air. In the winter, when the building is heating, the unit is now sucking in 45°F outdoor air and heating that to 85°F (this is called the “heating supply air temperature”), instead of using 70°F return air. Fixing the broken economizer on even a small unit will save over $1000 annually.
How do you calculate the savings from economizers? Unfortunately, a full answer will have to be saved for another day. The short answer is through something called bin analysis in which one analyzes the typical temperature and humidity data throughout the year and applies some basic thermodynamics equations to determine the savings.
What’s the take home message? Economizers are great tools that save energy and keep building air fresh. They are often extremely cost effective. And most people have never heard of them. So now you can go impress a friend and ask: “Speaking of that local sports team, did you know that 64% of the economizers on their corporate office building are likely broken?” “What’s an economizer?”