It is well established that air-side economizers save energy in the Pacific Northwest. And this stands to reason when you look at a graph of where the bulk of Seattle weather bin data lies:
(click for larger image)
The majority of the bin hours per year lie to the left of the 55º line, indicating that an economizer system would eliminate the need for mechanical cooling altogether during these hours. And nearly all of the hours are located to the left of the 75º line, where ambient temperatures would be lower than return air temperatures in a cooling system--allowing the system to offset some mechanical cooling load by using outside air.
When you consider that the use of outside air also brings IAQ benefits, it is clear why air-side economizers are such a compelling strategy for Northwest mechanical systems.
But there is a problem with air economizers in small packaged units: Too many of them don't work properly in the field. The reason for this is that for most small rooftop packaged cooling units do not have factory installed economizers. The standard of the industry is a bolt-on option that is shipped as a separate assembly to the jobsite for installation by the installing contractor. In some cases, they may not even be available at all for some duct configurations.
Typical small packaged unit economizer instalation
In practice, these economizers have a high rate of failure. The issue of non-functional economizers for small rooftop packaged units is significant enough that Puget Sound Energy includes re-commissioning of these devices in their Commercial HVAC Rooftop Unit Premium Service Rebate (program developed with the assistance of NEEC). And the Califorina Public Interest Energy Research program (PIER) goes further, recommending to owners and designers:
Specify reliable, factory-installed and -tested economizers with direct-drive actuators and low-leakage dampers.
That's exactly what Aaon provides on all of their units down to 1 ton.
(click for larger image)Aaon's rooftop unit design provides inherent energy advantages over the competition. And factory-installed economizers are just one of many.
Extra: PIER software to estimate economizer savings.
Want free psychrometric software? See our offering here.