Economizers: The Love/Hate Relationship on the Roof

What’s an Economizer?


An economizer is a simple, elegant idea intended to save businesses money by delivering cooling to their buildings with much less energy consumed and much less cost. In a nutshell, an economizer is a mechanical vent attached to a roof top unit (RTU) that opens to draw-in outside air for cooling a building when the outside air temperature is cool and dry enough.

How do Economizers help?

Economizers can save a bunch of money for businesses. The RTU compressors – the part that pulls heat out of the air for cooling a building – do not need to run if the RTU has an economizer and the outside air conditions are favorable.

To put things in perspective, a single RTU compressor for a small commercial building can draw 5kW to over 20kW while it’s running and cost several hundred to over a thousand dollars per month to operate during cooling season. The RTU’s fan, by contrast, draws only several hundred Watts to perhaps 2kW, so when the compressor can be turned off and the economizer opened, the cost to cool a building is cut by well more than half. These savings can add-up over the course of a year – leading to hundreds, and in some cases, thousands of dollars per year in savings.

small-commercial-RTU-faultsSo what’s not to love about economizers?

In short, they break. Often.

According to a New Buildings Institute analysis of studies examining over 500 RTUs, nearly two out of three had malfunctioning economizers.

If your building has economizers, there is a very good chance that your economizers are broken right now.

There are numerous reasons why economizers fail: air temperature sensors can fail or fall out of calibration, other aspects of the control algorithm can fail, and the damper can become physically stuck.

economizer-failure-graph-300x134.pngThe challenge with broken economizers – and maybe the reason why failures are so common – is that it’s difficult to detect when there is a problem. In the absence of active monitoring and diagnostics, there are no obvious symptoms of problems with economizers.

If the ambient temperature inside a building is comfortable and consistent with thermostat set-points, the general assumption is that everything is working correctly, even if your business is losing thousands of dollars due to unnecessary use of the RTU compressor during cooler days.

So what’s a business to do?

Addressing challenges with Economizers

Given that economizers cost a few thousand dollars, it’s a shame so few businesses that have installed them are reaping the rewards that they are capable of delivering.

To address the prevalence of failures, make sure your HVAC service providers are routinely checking the economizer during their regular preventive maintenance service calls. They should be capable of catching and correcting the most common failure modes with just a small effort.

You should also consider installing technology that can proactively identify and alert on economizer faults. With just a few data points – for example, real-time RTU energy consumption and outside air temperature – some technologies, like the eMonitor, can pro-actively notify you when a system is not working properly.

With just a modest investment in time and/or technology, your business can recapture all those profits your broken economizers have been consuming.

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