"What is the Correct Application for an Economizer?"
The Comfort Zone by Maury Tiernan, Geary Pacific Corp.

I know this is a question that has been on your mind all week. Probably more irksome than who will win in November.

An economizer is found in many requests for bid, but is it being applied correctly? Did it just get picked up from some other specification, and is now being requested on a 10' x 8' Guard Shack bid? Well yes, that is exactly what happens sometime. So what is an economizer, and when should it really be used?

An economizer is thought to provide "free" heating and cooling, however that is not the case. An economizer only provides free cooling and only when two conditions are met simultaneously. Included in an economizer is a device called an enthalpy control. This is a control that measures both the temperature and the humidity of the ambient condition. If the air is dry enough and cool enough, then and only then will the economizer open to provide free cooling. Although the enthalpy control is adjustable, it would not allow the economizer to open in Phoenix's high heat even though it is dry, or Hawaii's high humidity even though it is cool. Remember both conditions must be met at the same time, not just one.

So what jobs are correct for an economizer? Specify an economizer on those jobs with a high internal heat load, at times when the outside ambient conditions are both dry and cool. Some examples are: restaurants, churches, auditoriums, computer rooms, equipment shelters, etc. All these applications have the potential for high internal heat loads at times when the outdoor conditions are favorable.

Free cooling, sure we all want that, especially if we are a restaurant owner who's mainly open in the evening. Wow, pay for the electricity to heat the ovens, then pay for the electricity to remove the heat from the kitchen with the HVAC unit. How about the need to meet the new ASHRAE 62-89 requirements for the ventilation of an auditorium? These are when an economizer's free cooling are really applied correctly.

Is it practical then, on a commercial office building? Probably not. Generally, a commercial office building does not generate enough internal heat to warrant the initial expense of an economizer. Further, an economizer is a mechanical device that requires regular service and a knowledge of how it works. Is an economizer trouble free? Yes, but only if they are maintained! In an industry that has a hard time getting the end user to change the filters, is it practical to believe it will properly maintain a device like an economizer? Make sure the end user has facility people on staff, or provide them with a maintenance contract on the project.

It might sound as if I am suggesting against bidding, or using an economizer. Not so! An economizer is a magnificent ventilation tool when applied correctly. What I am against is the misapplication of a product, and the costs passed on through the "food chain" because we don't question the specification. Why send a product to the end user that is not correct, and charge them for it. It would make much more sense to give them 2' x 6' walls with R19 insulation, than misapply an economizer because we are unclear on how it functions. Lastly, the economizer is not the only HVAC ventilation device available to provide solutions to your customer's applications. Collect as much information on the end user's requirements, then check with your HVAC supplier, or Mechanical Engineer for the proper selection.